NEOGENE SLOTH ASSEMBLAGES (MAMMALIA,

PILOSA) OF THE COCINETAS BASIN (LA GUAJIRA,

COLOMBIA): IMPLICATIONS FOR THE GREAT

AMERICAN BIOTIC INTERCHANGE

by ELI AMSON1,2 ,3 , JUAN D. CARRILLO2 and CARLOS JARAMILLO1

1Smithsonian Tropical Research Institute, Box 0843-03092, Balboa-Ancon, Panama; jaramilloc@si.edu2Palaontologisches Institut und Museum der Universitat Zurich, Karl Schmid-Strasse 4, Z€urich, CH-8006, Switzerland; juan.carrillo@pim.uzh.ch3Current address: AG Morphologie und Formengeschichte, Bild Wissen Gestaltung – ein Interdisziplin€ares Labor & Institut f€ur Biologie, Humboldt-Universit€at,

Philippstrasse, 12/13, D-10115, Berlin, Germany; eli.amson@hu-berlin.de

Typescript received 19 February 2016; accepted in revised form 8 May 2016

Abstract: We describe sloth assemblages from the Cocin-

etas Basin (La Guajira peninsula, Colombia), found in the

Neogene Castilletes and Ware formations, located in north-

ernmost South America, documenting otherwise poorly

known biotas. The tentative referral of a specimen to a small

megatherioid sloth, Hyperleptus?, from the early–middle

Miocene Castilletes Formation, suggests affinities of this

fauna with the distant Santa Cruz Formation and documents

a large latitudinal distribution for this taxon. The late Plio-

cene Ware Formation is much more diverse, with five dis-

tinct taxa representing every family of ‘ground sloths’. This

diversity is also remarkable at the ecological level, with sloths

spanning over two orders of magnitude of body mass and

probably having different feeding strategies. Being only a few

hundred kilometres away from the Isthmus of Panama, and

a few hundred thousand years older than the classically

recognized first main pulse of the Great American Biotic

interchange (GABI 1), the Ware Formation furthermore doc-

uments an important fauna for the understanding of this

major event in Neogene palaeobiogeography. The sloths for

which unambiguous affinities were recovered are not closely

related to the early immigrants found in North America

before GABI 1.

Key words: Cocinetas Basin, Great American Biotic Inter-

change, Neotropics, palaeobiodiversity, Pilosa, sloth.

SLOTHS (Tardigrada) are represented today by two gen-

era that are ecologically restricted to a peculiar suspen-

sory mode of life, and geographically restricted to the

Neotropical region (Nowak & Paradiso 1983). However,

this modern distribution is depauperate when compared

to the recent past, as their fossil record includes a wide

range of body sizes (up to several metric tones; Fari~na

et al. 1998; Raj Pant et al. 2014), various feeding ecolo-

gies (Bargo & Vizca�ıno 2008), and a broad distribution

from Alaska (Stock 1942) to southernmost South Amer-

ica (and possibly Antarctica; Carlini et al. 1990; Gelfo

et al. 2015 and references therein). Their geographical

distribution was particularly broad during the Pleis-

tocene, which is partly the result of a major event in

Neogene palaeobiogeography: the Great American Biotic

Interchange (GABI; Wallace 1876). As South and North

America were isolated during most of the Cenozoic,

very distinct faunas populated the two landmasses

(Simpson 1980). This isolation was ruptured by the

northward migration of South American taxa and

southward migration of North American taxa (Webb

1985; Cione et al. 2015). In the classical understanding,

this process started with single-taxon dispersals (Wood-

burne 2010), involving the herald taxa sensu Webb

(2006) as early as 20.9 Ma (Bloch et al. 2016). This was

followed by the main phases of the GABI (or pulses,

GABI 1–4; sensu Woodburne 2010), occurring from 2.6–2.4 Ma, and involving the legion taxa sensu Webb

(2006). These migrations have mostly been assumed to

have been driven by the closure of the Central Ameri-

can Seaway and the formation of the Isthmus of

Panama, the most likely route of migration. However,

habitat and climatic changes probably played a much

more important role in mammal migration patterns

(Bacon et al. 2016). Indeed, the timing of the formation

of the Isthmus is nowadays challenged by new geologi-

cal data (Montes et al. 2015) and the onset of GABI by

palaeontological (Bloch et al. 2016) and molecular data

(Bacon et al. 2015), that suggest a complex process that

started at the Oligocene–Miocene transition.

© The Palaeontological Association doi: 10.1111/pala.12244 1

[Palaeontology, 2016, pp. 1–20]

All four families of ‘ground sloths’ took part in all key

steps of the interchange, pre- and post-GABI 1 sensu

Woodburne (2010). The earliest immigrants are Thi-

nobadistes (Mylodontidae) and Pliometanastes (Megalony-

chidae), which are present as herald taxa in North

America by c. 9 Ma (Webb 1989). Two other herald

taxa, Megalonyx (Megalonychidae) and ‘Glossotherium

chapadmalense’ (Mylodontidae; and the subsequent

Paramylodon; see McAfee 2009) are in North America by

c. 7 and 5 Ma respectively (Woodburne 2010). Finally,

subsequent sloths are involved in the main phase of

GABI, including Eremotherium (Megatheriidae) and

Nothrotheriops (Nothrotheriidae).

Intensive fieldwork recently undertaken in the Cocine-

tas Basin of the La Guajira peninsula of Colombia (the

northernmost continental region of South America)

resulted in the collection of thousands of specimens from

the Jimol, Castilletes and Ware formations (Moreno et al.

2015). At latitude 12°S, the Cocinetas Basin stands out in

delivering rich tropical South American palaeobiotas (Jar-

amillo et al. 2015) in the most biodiverse region of the

world (Wallace 1876), otherwise characterized by a poor

fossil record. Moreover, the Ware Formation in particular

is critical for the understanding of the GABI (Jaramillo

et al. 2015). Firstly, it is geographically near to the Isth-

mus of Panama (only 700 km away). Secondly, it is just

slightly older (c. 2.8 Ma) than GABI 1 (2.6–2.4 Ma). The

Ware Formation and its tardigradan assemblage have

therefore the potential to improve our understanding of

the dynamics of GABI in the Neotropics.

GEOLOGICAL SETTING

The Neogene stratigraphy of the Cocinetas Basin, includ-

ing the Jimol, Castilletes and Ware formations, was

recently revised (Moreno et al. 2015). Of those forma-

tions, only the Castilletes and Ware formations contain

mammalian assemblages. Both crop out along the West

coast of the La Guajira peninsula (Colombia) at various

localities, of which one from the Castilletes Formation

and several from the Ware Formation delivered specimens

ascribed to sloths (Fig. 1). The environment of deposition

of the Castilletes Formation ranges from shallow marine

to fluvio-deltaic, and it has been dated as 16.7–14.2 Ma

(late early Miocene to early middle Miocene, upper Bur-

digalian–Langhian, SALMA Santacrucian/Colloncuran;

Hendy et al. 2015; Moreno et al. 2015). The base of the

Ware Formation is dominated by fluvio-deltaic environ-

ment deposits, while the top is dominated by open-ocean

shoreface and nearshore deposits. Its age ranges from 3.40

to 2.78 Ma, late Pliocene (Piacenzian, SALMA Chapad-

malalan/Marplatan; Hendy et al. 2015; Moreno et al.

2015).

MATERIAL AND METHOD

We follow the recommendations on open nomenclature of

Bengston (1988). For dental terminology, we follow Gaudin

(2004), that is, the mesialmost tooth is designated as the

caniniform (C1 and c1 for the upper and lower respec-

tively) even if its shape departs from that of a canine-like

tooth. The following (more distal) teeth are designated as

molariforms (M1–4 and m1–3 for the upper four and three

lower molariforms respectively). Standard measurements

were taken with digital calipers when lower than 15 cm and

with a tape measure otherwise. In addition to standard

digital photographs, stereophotographs were taken using a

custom-built seesaw.

A phylogenetic analysis was conducted to suggest a

hypothesis regarding the affinities of one specimen (MUN

STRI 36643). The data matrix of Gaudin (2004) was used,

pruning all non-mylodontid taxa except Bradypus and

Megalonyx, which are used in the present analysis as out-

groups. Along with MUN STRI 36643, another additional

terminal taxon, Kiyumylodon, was included. This recently

described mylodontine was coded based on Rinderknecht

et al. (2007). The initial ordination of the characters of

Gaudin (2004) were kept. The edited data matrix is avail-

able in the Dryad Digital Repository (Amson et al. 2016a).

The algorithm employed was the exhaustive search (Branch

and bound) of PAUP 4.0b10 (Swofford 2002).

All specimens have STRI catalogue numbers and are

associated with a locality number and stratigraphic data.

F IG . 1 . Geographical distribution of tardigradan specimens in

the Cocinetas Basin. The magnified region is indicated on the

inset map by a rectangle. The cross marks the locality of the tar-

digradan specimen from the Castilletes Formation. The arrows

point to the Ware Formation outcrops. Scale bar represents

5 km. Colour online.

2 PALAEONTOLOGY

These are accessible through the STRI PaleoDatabase

(http://biogeodb.stri.si.edu/jaramillo/fossildb). Specimens

are deposited at the palaeontological collections of

MAPUKA (MUN), Universidad del Norte, Barranquilla,

Colombia.

In order to estimate body mass of several specimens, we

used the published regression equations of Janis (1990) (in

Fari~na et al. 1998). One of these equations involves the

occlusal area of first lower molariform (herein designated

as c1 for Pliomegatherium lelongi, see below). This area was

measured on a photograph (with scale) of the specimen

with the program Fiji (Schindelin et al. 2012).

Institutional abbreviations. F:AM, Frick Collection of the Ameri-

can Museum of Natural History, New York, USA; MCL, Museu

de Ciencias Naturais da Pontif�ıcia, Universidade Cat�olica de

Minas Gerais, Belo Horizonte, Brazil; MMP, Museo Municipal

de Ciencias Naturales ‘Lorenzo Scaglia’, Mar del Plata, Argen-

tina; MUN, Mapuka Museum, Universidad del Norte, Barran-

quilla, Colombia; MUT, Museo Universitario, Tarija, Bolivia;

STRI, Smithsonian Tropical Research Institute, ciudad de

Panam�a, Panam�a.

Other abbreviations. AP, anteroposterior; CC, craniocaudal; DV,

dorsoventral; GABI, Great American Biotic Interchange; LL, labi-

olingual; MD, mesiodistal; ML, mediolateral; PD, proximodistal.

Teeth: C1/c1, upper/lower caniniform; M1–4/m1–3, upper four/

lower three teeth distal to the caniniform; ‘A’ preceding one of

the former, the corresponding alveolus.

SYSTEMATIC PALAEONTOLOGY

MAMMALIA Linnaeus, 1758

XENARTHRA Cope, 1889

TARDIGRADA Latham & Davies, 1795

[= PHYLLOPHAGA Owen, 1842; = FOLIVORA Delsuc et al.,

2001]

Gen. et sp. indet.

Figure 2

Referred material. MUN STRI 20108, locality 390022, Ware For-

mation.

Description. MUN STRI 20108 consists of a left patella (maxi-

mum PD length = 66.7 mm; maximum ML width = 53.8 mm;

AP depth at centre of articular facet = 22.8 mm; apex PD length

(taken from distal border of articular facet) = 26.9 mm; facet

ML width = 52.7 mm). It represents a small-sized sloth,

approaching the dimensions of Nothrotherium for instance. The

articular surface is strongly convex mediolaterally (Fig. 2C). This

indicates a strongly developed medial trochlear ridge of the

femur. The medial portion of the articular surface is much smal-

ler than the lateral one, which is reminiscent of Hapalops

(Fig. 2B). The proximal side bears two grooves (Fig. 2C), as in

Mionothropus (De Iuliis et al. 2011). The apex is strongly tapered

(Fig. 2A, B), as in Thalassocnus (Amson et al. 2015), but it dif-

fers from that of the latter in being bent laterally.

Remarks. We will conservatively not consider MUN STRI

20108 to represent a taxon of its own in the diversity

count, as the possibility that it represents one of the taxa

recognized below cannot be excluded.

MYLODONTIDAE Gill, 1872

Gen. et sp. indet.

Figure 3

Referred material. MUN STRI 38047, locality 470062, Ware For-

mation.

Description. MUN STRI 38047 consists of an eroded posterior

dentary bearing three partial molariforms here referred to as m1–3 (m2 MD length = 8.1 mm; m3 MD length = 16.5 mm; DV

height at coronoid process = c. 56 mm; DV height at condyloid

process = 45.9 mm). The ontogenetic stage cannot be precisely

assessed. Based on the mesiodistal length of the last molariform, it

is larger than the holotype of the small-sized ‘Brievabradys’ laven-

tensis (Villarroel 2000; this species is probably a synonym of Glos-

sotheriopsis pascuali or another species of Glossotheriopsis, H. G.

McDonald, pers. comm. 2015), and approaches the dimensions of

a juvenile Glossotherium chapadmalense (Anaya & MacFadden

1995; mentioned therein as Glossotheridium chapadmalense, see

McAfee 2009). The m1 only preserves a rounded portion of distal

outer layer of the tooth (Fig. 3B). The m2–3 only preserve the lin-gual half of each tooth. The outline of m2 was most likely

F IG . 2 . Tardigrada gen. et sp. indet. from the Ware Formation,

left patella (MUN STRI 20108). A, anterior view (proximal towards

top). B, posterior view (proximal towards top). C, proximal view

(anterior towards top). Scale bar represents 2 cm. Colour online.

AMSON ET AL . : NEOGENE SLOTHS FROM THE COCINETAS BAS IN 3

subcircular. The m3 is bilobate, and the relative size of the lobes is

hard to assess due to their preservation. Such a disposition is typi-

cally found in mylodontids. It is noteworthy that geologically

older forms of uncertain status, such as Orophodon, also feature

such a disposition (Pujos & De Iuliis 2007).

The coronoid process is small and rounded dorsally (Fig. 3A).

The condyloid process has a short neck and a ventral position,

roughly at the level of the tooth row (diagnostic of the Mylodon-

tidae; Saint-Andr�e et al. 2010). The posteromedial opening of the

mandibular canal is located ventral to the anterior edge of the

dorsal tip of the coronoid process. The smaller posterolateral

opening of the mandibular canal is located at the level of the ante-

rior root of the coronoid process. Although eroded, the angular

process appears regularly rounded, differing from the posteriorly

pointed process usually seen in mylodontids.

Remarks. We will conservatively not consider MUN STRI

38047 to represent a taxon of its own in the diversity count,

as the possibility that it represents a juvenile individual of

the Lestodontini recognized below cannot be excluded.

MYLODONTINAE Gill, 1872

LESTODONTINI Ameghino, 1889 sensu Gaudin (2004)

Gen. et sp. nov.

Figure 4

Referred material. MUN STRI 36643, locality 470060, and tenta-

tively MUN STRI 20424, locality 390023 and MUN STRI 34353,

locality 470060 (see remarks below), all Ware Formation.

Description. MUN STRI 36643 consists of an edentulous partial

horizontal ramus of the dentary (Fig. 4A, B; Ac1 MD

length = 24.4 mm; Ac1 LL width = 17.8 mm; Am1 MD length

= 19.7 mm; Am1 LL width = 18.2 mm; Am2 MD

length = 24.6 mm; Am2 LL width = 19.5 mm; ramus DV depth

between c1 and m1 = c. 54 mm). The complete alveoli of the

three most mesial teeth and the mesial half of a more distal alve-

olus are preserved. Based on the depth of the horizontal ramus,

MUN STRI 36643 is roughly 20% smaller than Thinobadistes

segnis (Webb 1989), hence much smaller than Lestodon armatus

(Lydekker 1894) and much larger than Glossotheriopsis pascuali

(McDonald 1997) or ‘Brievabradys’ laventensis (the latter species

probably being either a synonym of the former or another spe-

cies of Glossotheriopsis, H. G. McDonald, pers. comm. 2015).

The c1 alveolus is slightly deflected labially, as in Thinobadistes

and Bolivartherium urumaquensis (Carlini et al. 2006a; Fig. 4A,

B). This deflection is accompanied by a labial bulge of the den-

tary, and, in occlusal view, by the presence of concavities on the

labial side of the dentary directly distal and mesial to the alveo-

lus of the caniniform, as is Thinobadistes. However, the mesial

concavity is more marked in Thinobadistes, an almost right angle

marking the posterolateral edge of the spout. In Lestodon and

Lestobradys (Rinderknecht et al. 2010), the caniniform is more

strongly deflected labially. There is no marked diastema in MUN

STRI 36643, as in Thinobadistes, Glossotherium (Owen 1842),

Kiyumylodon (Rinderknecht et al. 2007), and Pseudoprepotherium

(Hirschfeld 1985), and differing from Lestodon (in which a dia-

stema is present). In occlusal view, the outline of the c1 alveolus

is triangular with rounded edges. It bears a low longitudinal

ridge on its lingual wall, as in Lestodon and Lestobradys. In the

latter genera, however, the alveolus projects dorsally and is

somewhat twisted medially. In MUN STRI 36643, the c1 alveo-

lus is larger than those of m1 and m2, as in Lestodon and Thi-

nobadistes. An enlarged and triangular caniniform is considered

to be diagnostic of the clade formed by Thinobadistes and Lesto-

don (Lestodontini sensu Gaudin (2004); Lestodontinae sensu

Webb (1989)). However, the caniniform is relatively not as

enlarged as in the latter taxa. The m1 of the specimen F:AM

102658, referred to Thinobadistes segnis (Webb 1989) is 69% as

large as the c1 (mesiodistal length), while in MUN STRI 36643

it is 81% as large.

In occlusal view, the alveoli of m1 and m2 are oval, with their

main axis directed mesiolabially. A very slight longitudinal ridge

is found on the labial wall of m1 alveolus. The alveolus of m3,

although fragmentary, preserves part of a strong labial longitudi-

nal ridge, indicating that the tooth was lobate. A lower dentition

comprising a caniniform, mesial molariforms with subcircular

cross-section, and a lobate distal molariform, is a disposition

typically found in some mylodontines. The cross-section of m1

and m2 is oval in Kiyumylodon and Lestodon, but subtriangular

in Thinobadistes (Webb 1989).

A large mental foramen (dorsoventral height = 11.3 mm) is

found just anterior to the alveolus of the caniniform.

MUN STRI 20424 consists of a proximal fragment of the left

tibia (Fig. 4C–E; ML width = 115.2 mm; AP depth between

condyles = 63.8 mm; length from posterior edge of lateral con-

dyle to anterior edge of medial condyle = 93.8 mm). The speci-

men is from an adult, as shown by the epiphyseal closure.

F IG . 3 . Mylodontidae gen. et sp. indet. from the Ware Forma-

tion, partial left dentary with fragmentary m1–3 (MUN STRI

36643). A, lingual view (occlusal towards top). B, occlusal view

(labial towards top). Scale bar represents 5 cm. Colour online.

4 PALAEONTOLOGY

Based on width at proximal end, it is 81% as large as Thi-

nobadistes segnis (mean of four specimens measured by Webb

(1989)).

In proximal view (Fig. 4D), the relative size of the condyles

and their shape are reminiscent of Glossotherium robustum (Owen

1842) and Paramylodon harlani (Stock 1925). As in the latter two

taxa, the medial condyle is not circular but oval, and its long axis

is directed anterolaterally to posteromedially. A similar condition

is found in Megalonyx jeffersonii (Leidy 1855). In Thinobadistes

segnis and Lestodon armatus, the medial condyle is not as elongate.

In Catonyx cuvieri and Valgipes bucklandi, the long axis of the

medial condyle is directed anteroposteriorly, and the condyle is

relatively smaller. The intercondylar space is narrower, especially

posteriorly, than in most sloths. However, rather close medial and

lateral condyles are found in Thinobadistes segnis and ‘a bridge of

bone connecting the posterior margin of the two facets’ is

described in Pseudoprepotherium confusum (Hirschfeld 1985). In

proximal view, the tibial tuberosity barely protrudes anteriorly

from the level of the medial condyle. A similar condition is found

in Lestodon armatus, and differs from Thinobadistes segnis

(and most other sloths) in which the tuberosity is more developed

anteriorly at the level of lateral condyle.

There is a well-developed crest joining the lateral condyle to

the shaft, which forms a deep fossa on the posteriolateral side

distal to the proximal facet for fibula (Fig. 4E). This is also

found in mylodontines, such as Glossotherium robustum, but

not in scelidotheriines such as Catonyx cuvieri or Valgipes

bucklandi.

F IG . 4 . Lestodontini gen. et sp. nov. from the Ware Formation. A–B, edentulous partial left dentary (MUN STRI 36643); A, occlusal

view (lingual towards top); B, labial view (occlusal towards top). C–E, proximal fragment of left tibia (MUN STRI 20424); C, anterior

view (proximal towards top); D, proximal view (anterior towards top); E, lateral view (proximal towards top). F–G, right M4 missing

part of the intraalveolar part of the tooth prism (MUN STRI 34353); F, photograph and drawing in occlusal view (lingual towards

top); G, lingual view (occlusal towards top). Scale bars represent 5 cm for A–E and 1 cm for F–G. Colour online.

AMSON ET AL . : NEOGENE SLOTHS FROM THE COCINETAS BAS IN 5

The proximal facet for the fibula faces posterodistally. It faces

slightly more laterally in Glossotherium robustum and, conversely,

it faces slightly more distally in Catonyx cuvieri. There is a well-

developed facet for the cyamo-fabella as in other sloths, such as

mylodontids and megatheriids. In MUN STRI 20424, this facet

faces posteriorly and has a triangular outline.

On the anterior side of the bone, there is a crest issuing from

the tibial tuberosity that is directed mediodistally (Fig. 4C). A

similar crest is found in Glossotherium robustum (Owen 1842).

In Lestodon armatus, it is more distally directed (Gervais 1873).

MUN STRI 34353 consists of a bilobate tooth formed by con-

centric layers of orthodentine (Fig. 4F, G; MD length = 17.3 mm;

Max LL width of mesial lobe = 13.7 mm; Max LL width of distal

lobe = 6.5 mm). The inner nucleus of vasodentine is almost

entirely missing (Fig. 4F). Only patches of cementum are pre-

served at various heights along the tooth. The larger mesial lobe is

compressed mostly mesiodistally, but with a diagonal direction as

well, giving the tooth an asymmetrical shape in both the labiolin-

gual and mesiodistal directions. The smaller distal lobe is

strongly compressed labiolingually. Well-marked labial and lin-

gual sulci at the level of junction between the lobes are found

along the whole length of the tooth (Fig. 4G). The resulting

morphology is reminiscent of the right M4 of Pleurolestodon

dalenzae (Saint-Andr�e et al. 2010). The long and constricted

distal lobe of M4 is a diagnostic feature of this species, as the

other valid species of the genus, P. acutidens (Saint-Andr�e

et al. 2010), lacks the strong labiolingually compression of the

distal lobe (Rovereto 1914). However, the distal lobe is longer

and less constricted in P. dalenzae than in MUN STRI 34353,

forming a ‘secondary’ rounded nucleus. The labial side of the

mesial lobe forms a somewhat more acute curve in P. dalen-

zae than in MUN STRI 34353. In addition, the holotype of

P. dalenzae (MNHN-BOL V 3348) is larger (mesiodistal length

1.27 times greater).

It is noteworthy that an approaching morphology is featured

by the most distal lower molariform (m3) of Pseudopre-

potherium confusum (Hirschfeld 1985), but the latter bears a

mesiolabially directed facet on its mesial lobe. Moreover, the

whole vasodentine nucleus is relatively smaller in the m4 of

P. confusum.

The occlusal surface is not planar, the lingual part of the

mesial lobe being a bit less worn down than the rest of the

tooth. The lingual part of the mesial lobe bears a minute wear

facet that mostly faces occluso-lingually.

Remarks. We performed a phylogenetic analysis including

as the ingroup MUN STRI 36643 coded as a terminal

taxon and the mylodontids included in Gaudin (2004).

Three equally parsimonious trees were recovered. Their

topology is the same as the one recovered by Gaudin

(2004), as their difference lies in the position of MUN

STRI 36643. The latter is indeed either sister-group of

Lestodon, or Thinobadistes, or of the clade formed by the

two latter genera (= Lestodontini), as shown by the poly-

tomy of the strict consensus of these three trees (Fig. 5).

In a second analysis we included the mylodontine

Kiyumylodon, also known from an isolated mandible. A

total of 18 equally parsimonious trees were obtained.

Their consensus is much less resolved than the first analy-

sis, with the Mylodontinae, except for Pseudopre-

potherium, forming a polytomy. The only relationship

that is resolved within the latter polytomy is the

Lestodontini, recovered as in the previous analysis com-

prising MUN STRI 36643. As a result, MUN STRI 36643

should be considered as a Lestodontini sensu Gaudin

(2004). Differing from Lestodon and Thinobadistes, MUN

STRI 36643 most likely represents an undescribed taxon,

nov. gen et sp. but we refrain from formally erecting a new

taxon, pending the discovery of more complete material.

MUN STRI 20424 and MUN STRI 34353 can only be

safely ascribed to a medium-sized Mylodontinae gen. et sp.

indet. We tentatively ascribe them to the same taxon as

MUN STRI 36643, as no features appear be incompatible.

SCELIDOTHERIINAE Ameghino, 1904

Gen. et sp. indet.

Figure 6

Referred material. MUN STRI 16535, locality 390025, Ware For-

mation.

F IG . 5 . Hypothesis of phylogenetic affinities of MUN STRI

36643 among Mylodontidae. Strict consensus tree obtained with

PAUP 4.0b10 (Swofford 2002) that has a length of 550, a consis-

tency index of 0.59, and retention index of 0.55.

6 PALAEONTOLOGY

Description. MUN STRI 16535 consists of a complete left ulna

(see 3D model in Amson et al. 2016b). The specimen is from an

adult, as shown by the epiphyseal closure (PD length = 280 mm;

AP depth at coronoid process = 93.3 mm). MUN STRI 16535 is

shorter than all scelidotheriines measured by McDonald (1987),

which span from 313 to 476 mm. The four specimens of Sceli-

dotherium leptocephalum measured by Mi~no-Boilini et al. (2014)

are also longer, the smaller measuring 370 mm. It is however

longer than that of Pseudoprepotherium confusum (c. 20 cm;

Hirschfeld 1985).

In lateral view (Fig. 6C), the posterior end of the olecranon

process does not reach the level of the anterior end proximally,

which is reminiscent of Glossotherium robustum (Owen 1842)

and Scelidotherium leptocephalum (Mi~no-Boilini et al. 2014) and

differs from those of Catonyx tarijensis (McDonald 1987), Cato-

nyx cuvieri, Valgipes bucklandi (Cartelle et al. 2009) and Paramy-

lodon harlani (Stock 1925). The most reminiscent olecranon

process is found in Scelidotherium leptocephalum, which also fea-

tures a characteristic pointed proximal end.

The anconeal process is very slightly broken, but is obviously

weakly developed (Fig. 6C), which differs from Glossotherium

robustum, Catonyx cuvieri and Valgipes bucklandi (Cartelle et al.

2009), and is reminiscent of Scelidotherium leptocephalum,

Paramylodon harlani and Catonyx tarijensis.

The radial facet is restricted to a small area continuous with

the most lateral part of the humeral facet (Fig. 6A).

The ulna is more elongate (depth at coronoid process/proxi-

modistal length = 0.33) when compared to Glossotherium robus-

tum (0.45; Owen 1842) and Paramylodon harlani (0.41; Stock

1925), and slightly stouter than Valgipes bucklandi (0.26; MCL

22464) and Pseudoprepotherium confusum (0.26; Hirschfeld

1985), its proportions being more reminiscent of those of Sceli-

dotherium leptocephalum and Scelidodon chiliense (both 0.32;

MMP 549S and MUT 1510 respectively; McDonald 1987).

A generic attribution is precluded by the paucity of postcra-

nial data for Neogene taxa (Mi~no-Boilini et al. 2011).

MEGATHERIOIDEA Gray, 1821

Hyperleptus? Ameghino, 1891

Figure 7

Referred material. MUN STRI 37413, locality 130024, Castilletes

Formation.

Description. Bone fragment with three poorly preserved molari-

forms (M2 MD length = c. 6 mm; M2 LL width = c. 10 mm;

M3 MD length = 6.8 mm; M3 LL width = c. 8 mm; M4 MD

length = 6.8 mm; M4 LL width = 6.2 mm). The different types

of dental tissues are not discernable. There is no diastema

(Fig. 7A). On the lingual side of the middle and mesial teeth,

F IG . 6 . Scelidotheriinae gen. et sp. indet. from the Ware For-

mation, left ulna (MUN STRI 16535). A, anterior view (proxi-

mal towards top). B, posterior view (proximal towards top). C,

lateral view (proximal towards top). Scale bar represents 10 cm.

Colour online.

AMSON ET AL . : NEOGENE SLOTHS FROM THE COCINETAS BAS IN 7

the bone projects medially. Attributable to a small portion of

the posterior end of the palatine, this indicates that the specimen

represents upper teeth and maxilla. The occlusal surface of the

teeth does not reach the most ventral level of the bone, possibly

suggesting a very immature individual or a very abrasive diet

(Fig. 7B). Furthermore, each tooth is surrounded by a large

space, which would tend to corroborate the first hypothesis. The

M2 is poorly preserved. Its outline and that of M3, although not

entirely preserved, are clearly compressed mesiodistally. The M3

preserves mesial and distal ridges, a synapomorphy of megatheri-

oids (Gaudin 2004). Conversely, the outline of M4 (the only one

entirely preserved) is subcircular, a singular trait in sloths. The

mesial half of its occlusal surface is covered by a labionlingual

ridge. Although mostly broken, there was most likely a lower dis-

tal ridge as well. Hyperleptus garzonianus is characterized by a

rounded M4 and mesiodistally compressed M1–3 (Ameghino

1891). However in Hyperleptus garzonianus the M4 is not as

circular as in MUN STRI 37413, being slightly wider labiolin-

gually than long mesiodistally (see measurements of Scott

(1903–1904)).

Remarks. A sub-circular outline of M4 seems to be a

derived trait found of Hyperleptus garzonianus. As the M4

of MUN STRI 37413 also departs from the plesiomorphic

strongly elliptical condition of megatherioids, a close

affinity with the latter species is likely. But the fragmen-

tary nature of MUN STRI 37413 and the fact that its M4

seems even more circular than that of Hyperleptus garzo-

nianus preclude a definitive attribution to this species.

Hyperleptus is known from the Santa Cruz Formation

(Scott 1903–1904), and is one of a number of small

megatherioids of uncertain affinity, which also includes

Pelecyodon, Schismotherium, Analcimorphus and Hapalops

(Gaudin 2004).

MEGALONYCHIDAE Gervais, 1855

Gen. et sp. nov.

Figure 8

Referred material. MUN STRI 16601, locality 390077 and MUN

STRI 34226, locality 470060, both Ware Formation.

Description. MUN STRI 16601 consists of a left patella repre-

senting a relatively large-sized taxon (Fig. 8D–E; maximum PD

length = 112.3 mm; maximum ML width = 92.7 mm; AP depth

at centre of articular facet = 32.2 mm; apex PD length (taken

from distal border of articular facet) = 47.8 mm; facet ML

width = 86.4 mm). It is slightly smaller than the specimen of

Megalonyx jeffersonii measured by Leidy (1855), but much larger

than Pliomorphus mutilatus, as shown by the width of the patel-

lar trochlea of the femur (Brandoni 2009). The tapered apex

gives the patella a teardrop shape in anterior view (Fig. 8D), as

commonly seen in sloths. The articular facet is only very weakly

convex mediolaterally (Fig. 8E), precluding the attribution to a

pedolateral sloth (Mylodontidae or Megatheria) or a glyptodont.

Moreover, the lateral half of the facet is shorter proximodistally

than the medial half, as in Megalonyx jeffersonii (Leidy 1855).

However, in anterior view, the general shape differs from that of

Megalonyx jeffersonii, in that both its proximal border and apex

are more tapered.

MUN STRI 34226 consists of an ulna preserving the proximal

epiphysis and part of the diaphysis (Fig. 8A–C; AP depth at

coronoid process = 83.0 mm; PD length from proximal end to

level of coronoid process = 62.2 mm). The specimen is from an

F IG . 7 . Megatherioidea Hyperleptus? (MUN STRI 37413) from

the Castilletes Formation. A, photograph and drawing in occlu-

sal view (labial towards top). B, lingual view (occlusal towards

top). Scale bar represents 2 cm. Colour online.

8 PALAEONTOLOGY

adult, as shown by the epiphyseal closure. The most distal part

of the preserved diaphysis is anteroposteriorly deep, suggesting

that roughly at least a fifth of the diaphysis is missing.

The olecranon process is directed posteroproximally, forming

an angle of roughly 45° with the proximodistal axis of the bone

(Fig. 8A–C), as in Megalonyx jeffersonii (see for instance Leidy

1855, pls IX, X). However, it differs from the latter genus by lack-

ing the strong medial inflection that affects the whole tuberosity

(Fig. 8A). Indeed, the olecranon process only slightly protrudes

medially in MUN STRI 34226. The presence of a small tuberosity

on the medial side of the tip of the olecranon process is reminis-

cent of Megalonyx jeffersonii, and Nothrotheriops shastensis as well.

The articular surface for the humeral trochlea is not entirely

preserved, but one can observe that it is weakly expanded laterally

and posteriorly (Fig. 8A, C), a characteristic feature of Megalonyx

jeffersonii, and differing from Nothrotheriops shastensis for

instance. As in both latter genera, the anconeal process is weakly

developed anteriorly. It appears even narrower mediolaterally than

in these two genera. Although fragmentary, it can be discerned

that the radial facet is continuous with the humeral facet.

Intraspecific variation regarding this trait is described in Mega-

lonyx jeffersonii (McDonald 1977). A large rounded pit is present

distal to the radial facet. A strong crest, issued from the process

for the radial notch, joins rapidly the anterior edge of the shaft.

The shaft is slightly bent medially (Fig. 8A, B). Its lateral side

bears a strong muscular crest issued from the posterior end and

directed anterodistally.

Remarks. MUN STRI 16601 and MUN STRI 34226 were

found isolated and each can be ascribed to an undescribed

Megalonychidae. We conservatively ascribe both specimens

to the same Megalonychidae gen. et sp. nov. in order to

avoid overestimating the taxonomic diversity of the Ware

Formation.

MEGATHERIA sensu Gaudin (2004)

MEGATHERIIDAE Gray, 1821

MEGATHERIINAE Gray, 1821

Pliomegatherium lelongi Kraglievich, 1930

Figure 9

Referred material. MUN STRI 36685, locality 390024, and tenta-

tively MUN STRI 19747, locality 390017 and MUN STRI 20446,

locality 390026, all Ware Formation.

Description. MUN STRI 36685 consists of a partial mandible

with c1 (mesialmost tooth, molarized in megatheriines),

eroded below the level of the alveolar plane (Fig. 9A–B; maxi-

mum MD length of c1 = 32.3 mm; maximum LL width of

c1 = c. 36 mm; LL width at distal end = 33.8 mm; maximum

MD length of alveolus of c1 = 37.4 mm). It is an adult

F IG . 8 . Megalonychidae gen. et sp. nov. from the Ware Forma-

tion. A–C, left ulna (MUN STRI 34226); A, anterior view (proximal

towards top); B, posterior view (proximal towards top); C, lateral

view (proximal towards top). D–E, left patella (MUN STRI 16601);

D, anterior view (proximal towards top); E, posterior view (proxi-

mal towards top). Scale bar represents 10 cm. Colour online.

AMSON ET AL . : NEOGENE SLOTHS FROM THE COCINETAS BAS IN 9

F IG . 9 . Pliomegatherium lelongi from the Ware Formation. A–B, partial mandible with c1 (MUN STRI 36685); A, photograph and

drawing in occlusal view (anterior towards top); B, right lateral view (occlusal towards top). C, partial caudal vertebra (MUN STRI

19747) in dorsal view (cranial towards top). Scale bar represents 5 cm. Colour online.

10 PALAEONTOLOGY

individual, as indicated by the thorough closure of the symph-

ysis, and the very short distance between the labial side of c1

alveolus and lateral side of horizontal ramus (De Iuliis 1996).

Based on the tooth dimensions, it is a medium-sized megath-

eriine, similar to the holotype of Pliomegatherium lelongi

(Brandoni 2006), and smaller than Pyramiodontherium brevi-

rostrum (Carlini et al. 2002; and, by extension, also smaller

than Megatheriops rectidens, see Pujos et al. 2013). The cross-

section of c1 is trapezoidal, with the mesial side slightly nar-

rower labiolingually than the distal side (Fig. 9A). This is

characteristic of Pliomegatherium lelongi (Brandoni 2006), and

differs from the mesiolabially constricted teeth of early

megatheriines such as Megathericulus, and square cross-section

of the teeth of Megatherium americanum, Eremotherium lauril-

lardi and Eremotherium eomigrans. The dorsal edge of the pre-

served portion of the spout is not preserved (Fig. 9B), which

prevents us from precisely assessing the relative development of

the ventral bulge of the dentary (diagnostic within the Megatheri-

inae subfamily; De Iuliis 1996). The posterior end of the symph-

ysis is preserved (Fig. 9A). It forms a partially closed notch

(c. 80°). This is reminiscent of Pliomegatherium lelongi (Brandoni

2006) and Eremotherium laurillardi, and differs from the more

open symphysis of Megathericulus patagonicus, and more closed

one of Megatherium americanum. It is located approximately at

the level of the mid-mesiodistal length of c1, roughly as in Ere-

motherium laurillardi, Pliomegatherium lelongi and Pyramiodon-

therium bergi (Brandoni 2006), but differs from Megathericulus

patagonicus, Eomegatherium andinum, and Anisodontherium

halmyronomumin, in which it is more anterior (Pujos et al. 2013;

Tejada-Lara et al. 2015). In adult Megatherium americanum, it is

more posterior (De Iuliis 1996).

MUN STRI 19747 consists of a centrum with a transverse

process from a caudal vertebra (Fig. 9C; CC length of cen-

trum = 59.8 mm; DV height of centrum at cranial side = 57.3

mm; ML of centrum at caudal side = 77.6 mm). The dimen-

sions would fit a medium-sized Megatheriinae, such as the sev-

enth caudal vertebra of Megatherium urbinai. The cross-sectional

shape of the centrum is slightly flattened dorsoventrally, indicat-

ing proximity to the cranial region of the series. The cranial and

caudal facets of the centrum are only slightly oblique. The trans-

verse process is oriented laterocaudally, forming an angle of

roughly 40° with the sagittal plane. It ends with a globular

tuberosity that is located caudal to the level of the centrum.

Moreover, the body of the transverse process is triangular in

cross-section. Such a condition is found in Megatherium (Owen

1861; Pujos & Salas 2004). In Megalonyx, the transverse pro-

cesses are also strongly oriented laterocaudally, but they are

more dorsoventrally flattened. In glyptodonts, the transverse

processes are either oriented only laterally or laterocranially (e.g.

Glyptotherium texanum, Gillette & Ray 1981).

MUN STRI 20446 consists of a centrum of caudal vertebra

(CC length of centrum = 57.9 mm; DV height of centrum at

cranial side = 56.2 mm; ML of centrum at caudal side =c. 71 mm; maximum ML width (one side doubled) = 170 mm).

The cross-section of the centrum is slightly flattened dorsoven-

trally, indicating proximity to the cranial region of the series.

The cranial and caudal facets of the centrum are only slightly

oblique to each other (i.e. the cranial is set slightly dorsal to the

caudal). The four hypophyseal processes and their facets for the

haemal arches are well developed.

Remarks. Pliomegatherium lelongi is otherwise recorded in

the ‘conglomerado os�ıfero’, Ituzaing�o Formation, Entre

R�ıos Province, Argentina (upper Miocene – Pliocene; Bran-doni 2006). Pliomegatherium sp. is also reported in the

‘Barrancas de San Gregorio’, San Jos�e member of Raig�on

Formation (Perea et al. 2013), San Jos�e Department, Uru-

guay (Mones 1988). These localities are at a latitude of

c. 32–34°S, and are c. 5000–5500 km from the localities of

the Ware Formation. The geographical range of P. lelongi is

hence drastically expanded, with specimens being found

across the northern two-thirds of South America. The

broad geographical distribution seems to be common in

the subfamily, as exemplified by the Panamerican Eremoth-

erium laurillardi (Cartelle & De Iuliis 1995). Based on the

dimensions of the c1 of MUN STRI 36685, the body mass

of this individual is estimated to be between c. 1000 and

4400 kg, with a mean value of 2400 kg (Table 1). This is

consistent with the 2500 kg estimate given for another

medium-sized megatheriine, Pyramiodontherium scillatoy-

anei (De Iuliis et al. 2004).

MUN STRI 19747 and MUN STRI 20446 can only be

safely ascribed to Megatheriinae indet. and Xenarthra

indet., respectively. But, given the similar morphology

and dimensions of the centra, and the fact that no fea-

tures appear to be incompatible, we tentatively ascribe

them to the same taxon as MUN STRI 36685.

TABLE 1 . Body size estimations for the two size extremes of the tardigradan assemblage of the Ware Formation.

Measurements (cm/cm2) Prediction (kg)

MUN STRI 36685

c1 length (FLML) log mass = log FLML * 3.263 + 1.337 3.23 996

c1 width (FLMW) log mass = log FLMW * 2.909 + 2.03 3.60 4449

c1 area (FLMA) log mass = log FLMA * 1.553 + 1.701 10.042 1806

Mean of predictions 2417

MUN STRI 12924

Occipital height (OCH) log mass = log OCH * 2.783 � 0.42 5.37 41

The equations are based on Janis (1990) (in Fari~na et al. 1998). First lower molar (FLM) corresponds to c1 in Pliomegatherium lelongi

(MUN STRI 36685).

AMSON ET AL . : NEOGENE SLOTHS FROM THE COCINETAS BAS IN 11

NOTHROTHERIIDAE Ameghino, 1920

NOTHROTHERIINAE Ameghino, 1920

cf. Nothrotherium Lydekker, 1889

Figures 10, 11

Referred material. MUN STRI 12924, locality 390024, Ware For-

mation.

Description. MUN STRI 12924 consists of a fragmentary basicra-

nium, with the posterior portion of the skull roof (Fig. 10; see

3D model in Amson et al. (2016b); ML width at postorbital con-

striction = 54.5 mm; ML width at mastoid processes = 82.0 mm;

ML width at root of zygomatic process of squamosal = 95.1 mm;

maximum ML width of occipital condyles = 56.5 mm; occipital

height = 53.7 mm; maximum ML width of foramen mag-

num = 30.0 mm; maximum DV height of foramen magnum =c. 28 mm). Two foramina are present at the posterior edge of the

basioccipital, which indicate the incomplete ossification and hence

immaturity of the specimen. Moreover, some of the sutures appear

thoroughly closed (e.g. interfrontals, interparietals), while others are

not (e.g. supraoccipital-exoccipital). A subadult stage is hence

F IG . 10 . Cf. Nothrotherium from the Ware Formation, partial basicranium (MUN STRI 12924), general views (photograph and

drawing). A, dorsal view (right lateral towards top). B, left lateral view (dorsal towards top). C, occipital view (dorsal towards top).

Abbreviations: ape., anterior process of entotympanic; coa., canal for the occipital artery; eoc., external occipital crest; exc., exoccipital

crest; f., foramen; frag., fragment; nc., nuchal crest; oc., occipital condyle; Occ., occipital bone; p., process; sf., stylomastoid foramen.

Scale bars represent 5 cm. Colour online.

12 PALAEONTOLOGY

inferred for this specimen. The size of the specimen falls within

the range of the adult specimens of Nothrotherium maquinense

measured by Cartelle & Fonseca (1983), and is slightly larger than

Mionothropus cartellei (De Iuliis et al. 2011) and Pronothrotherium

typicum (Patterson et al. 1992).

Only a dorsal portion of the posterior half of the frontals is pre-

served (Fig. 10A, B). There is no sagittal crest (diagnostic of the

Nothrotheriidae; De Iuliis et al. 2011), but this could be due to

the immaturity of the specimen. The frontoparietal suture is at the

level of the root of the zygomatic process of the squamosal. Part of

the alisphenoid is present on both sides. It contacts the parietal

(diagnostic of the Nothrotheriinae except Mionothropus cartellei,

De Iuliis et al. 2011). The temporal line joins the dorsal edge of

the zygomatic process of the squamosal posteriorly, as in

Mionothropus cartellei, Nothrotherium maquinense and Nothrothe-

riops shastensis, but not in Pronothrotherium typicum (De Iuliis

et al. 2011). The parietals are globose and posteriorly depressed

(diagnostic of Nothrotherium maquinense, Paula Couto 1971).

The root of the zygoma is directed anteriorly (diagnostic of

the Nothrotheriinae; De Iuliis et al. 2011). Lateral to it, there is

a bulge (diagnostic of the Nothrotheriinae except Mionothropus

cartellei, De Iuliis et al. 2011), although it does not appear as

developed as in Pronothrotherium typicum, Nothrotherium

maquinense and Nothrotheriops shastensis.

Anterior to the carotid foramen, and medial to the Glaserian

fissure, the bone has an irregular surface and displays a broken

ridge (Fig. 11). Medial to it, and just anterior to the basioccipi-

tal tuber, the basisphenoid shows a rugose surface. This region

most likely represents the location of the posterior part of the

ossified pterygoid bulla, which presumably would not have been

thoroughly sutured yet to the basiphenoid in this immature

specimen. An ossified pterygoid bulla is present in Nothrother-

ium maquinense and Nothrotheriops shastensis.

The ectotympanic is missing on both sides, except maybe for

a small portion of the anterior crus on the left side. The ento-

tympanic is well preserved on both sides. Anterolateral and pos-

teromedial portions can be distinguished, as described in

Pronothrotherium typicum (Patterson et al. 1992) and Mionothro-

pus cartellei (De Iuliis et al. 2011). The posteromedial portion is

much smaller than the anterolateral one, differing in that regard

from Pronothrotherium typicum (Patterson et al. 1992). A similar

disposition is found in Nothrotherium maquinense (MCL 21703),

but in this species the posteromedial portion seems more dorsally

elevated. In MUN STRI 12924, the anterolateral portion forms a

A B C

F IG . 11 . Cf. Nothrotherium from the Ware Formation, partial basicranium (MUN STRI 12924), close-up of the left auditory region

in ventral view (anterior towards top). A–B, left and right stereophotographs. C, same view as B but labelled. Abbreviations: ant. cr. fr.,

anterior crus fragment; ant. p. ent., anterior process of entotympanic; Bo., basioccipital; Bs., basisphenoid; car. f. + mlf., carotid fora-

men and medial lacerate foramen; ent. p. ali., entoglenoid process of alisphenoid; exo. cr., exoccipital crest; f., foramen; gl., Glaserian

fissure; mast. p., mastoid process; pet. fr., petrosal fragment; p., process; post. p. ent., posterior process of entotympanic; th., tympa-

nohyoid; sm., superficies meatus; ssp., scar of suture for pterygoid bulla; sth. fos., stylohyal fossa; stylo. f., stylomastoid foramen; su.

int. car. art., sulcus for internal carotid artery. Scale bar represents 2 cm. Colour online.

AMSON ET AL . : NEOGENE SLOTHS FROM THE COCINETAS BAS IN 13

ventrally expanded crest-like process. It is sigmoidal in ventral

view. This process appears to be particularly vertical, not being

inclined medially. The posterior end of the entotympanic forms

the medial part of the stylohyoid fossa. The junction between the

posteromedial and anterolateral portions of the entotympanic is

marked by a shallow sulcus, presumably accommodating, as in

Mionothropus cartellei, the internal carotid artery. The jugular

foramen (or posterior lacerate foramen) is large, extending poste-

riorly up to the level of the hypoglossal foramen (diagnostic of

the Nothrotheriidae; De Iuliis et al. 2011). Both of these foram-

ina somewhat share the same excavation and are located espe-

cially close to one another, which is very reminiscent of

Nothrotherium maquinense and Nothrotheriops shastensis, and dif-

fers from the more separate foramina of Mionothropus cartellei

and Pronothrotherium typicum. Posteromedial to these foramina,

there is a globular paracondylar process, very reminiscent of

Nothrotherium maquinense, and differing from the more elongate

process of Nothrotheriops shastensis. The stylomastoid foramen is

found at the dorsal end of a sulcus formed laterally by the mas-

toid process and medially by a well-developed ridge extending

from the nuchal crest (possibly the exoccipital crest sensu Gaudin

1995). This sulcus, terminating dorsally by a foramen, may repre-

sent a widely exposed canal for the occipital artery, a synapomor-

phy of the Nothrotheriidae (Gaudin 1995). A groove also leading

dorsally to the mastoid foramen is described in Acratocnus odon-

trigonus and Schismotherium fractum (Patterson et al. 1992).

Most of the preserved articular surface of the stylohyoid fossa is

formed by the posterodorsal side of the entotympanic. The rest

of the preserved articular surface is a narrow strip, presumably

formed by the tympanohyoid, as in Mionothropus cartellei for

instance (De Iuliis et al. 2011).

The occipital condyles are sessile (Fig. 10C; diagnostic of the

Nothrotheriidae; De Iuliis et al. 2011). They have a peculiar (ab-

normal?) morphology, being marked at mid-height by a medio-

laterally oriented depression. There is a distinct external occipital

crest, developed to an extent very similar to that of a juvenile

specimen of Nothrotherium maquinense (MCL 1020). The nuchal

crest, however, is inconspicuous.

Remarks. Nothrotherium is represented by the Pleistocene

N. maquinense and N. escrivanense, which are restricted to

Brazilian localities. While we provisionally ascribe MUN

STRI 12924 to cf. Nothrotherium due to the fragmentary

nature and immaturity of the specimen, it is likely to rep-

resent a closely related new taxon. Based on the occipital

height of this specimen, the body mass of the individual is

estimated to be c. 41 kg (Table 1). We estimate this speci-

men to be subadult, and consider it unlikely that this indi-

vidual would have reached 100 kg at maturity.

DISCUSSION

Mammalian assemblage of the Castilletes Formation

Although not the main focus of the present contribution,

the Castilletes Formation is discussed here because of an

important modification to its faunal composition. The

specimen previously referred to Megatheriidae (MUN

STRI 16777; Moreno et al. 2015), the only sloth formerly

recognized in the formation, is actually an astrapothere.

However, the Castilletes Formation does contain a

tardigrade, as MUN STRI 37413 represents a small

megatherioid. Tentatively referred to Hyperleptus?, this

record could be consistent with the geological age of the

Formation as it is otherwise known from the Santacrucian

fauna. A sparassodont from the Castilletes formation,

Lycopsis padillai (Su�arez et al. 2015), also suggests affini-

ties with the Santacrucian fauna. On the other hand,

crocodilian taxa of the Castilletes Formation are otherwise

known from Laventan and Huayquerian faunas (Moreno-

Bernal et al. 2016). This emphasizes the importance of

the detailed description of the rest of the mammalian

fauna from the Castilletes Formation.

Diversity of the tardigradan assemblage of the Ware

Formation

A greater taxonomic diversity in the lower latitudes is a

global pattern broadly found today among eukaryotes

(Hillebrand 2004), and it is the result of a long and com-

plex history, as shown in particular for the Neotropics

(Jaramillo et al. 2006). Recent studies have shown how

Neotropical Miocene formations can preserve hyper-

diverse vertebrate assemblages (for crocodilians, see

Scheyer et al. 2013; Salas-Gismondi et al. 2015). However,

taxonomic richness of fossil tropical assemblages is still

severely underestimated (Carrillo et al. 2015). The Ware

Formation gives an important insight into the otherwise

poorly documented diversity at low latitudes during the

Pliocene. The conservative diversity count for the tardi-

gradan assemblage of the Ware Formation is of five

distinct taxa. Each of the four families of ‘ground sloths’

(Mylodontidae, Megatheriidae, Nothrotheriidae and

Megalonychidae) is represented. When compared with

assemblages from other Pliocene Neotropical formations

(Table 2), the Ware assemblage has a rather diverse tardi-

gradan composition. The San Gregorio Formation is a

geographically nearby formation that records a roughly

synchronous continental fauna, and yet only one sloth

has been reported (Vucetich et al. 2010), most likely due

to taphonomic and/or sampling biases. When compared

to older formations, the Ware Formation’s tardigradan

diversity is equalled by the Urumaco Formation (late

Miocene; where the taxa might not have been coeval,

Carlini et al. 2006a) and surpassed by the better-known

Laventan fauna. An example of an extensively sampled

Pliocene formation that is found at higher latitudes is the

Monte Hermoso Formation. Although it records five

species in three families, only two to three species are

14 PALAEONTOLOGY

found in the same facies (Tomassini et al. 2013), making

it roughly half as diverse as the Ware Formation. When

compared to other temperate Pliocene assemblages

(Table 2), a similar conclusion can be drawn, except for

the Uqu�ıa Formation. The astonishing 22 species of sloth

of the Late Miocene Ituzaing�o Formation (‘Conglomerado

os�ıfero, mesopotamian’) has to be taken with caution, as

a recent and unfinished taxonomic revision (every family

except the Mylodontidae) has revealed a lower diversity

than previously reported (Brandoni 2013).

Another aspect of the diversity of the sloths of the

Ware Formation dwells in their ecologies. With a body

mass estimated to be c. 2400 kg, Pliomegatherium lelongi

is a megaherbivore sensu Owen-Smith (1987). While the

closely related Megatherium americanum is reconstructed

as a selective feeder (Bargo & Vizca�ıno 2008), broad geo-

graphical distribution is common in the subfamily, which

is usually considered to be indicative of generalist habits.

In contrast, the specimen ascribed to cf. Nothrotherium is

a small-sized sloth weighing roughly 41 kg. Although not

much can be argued about the ecology of the latter, the

range of two orders of magnitude in body size featured

by this tardigradan assemblage suggests that disparate eco-

logical niches were occupied. In addition to intermediate

body size, the other sloths recovered feature other feeding

ecologies. Based on the morphology of its close relatives,

Lestodon and Thinobadistes, Lestodontini gen. et sp nov.

was most likely wide-muzzled. This is argued to correlate

with another feeding ecology, namely bulk-feeding (Bargo

& Vizca�ıno 2008; Czerwonogora et al. 2011). Furthermore,

Scelidotheriinae gen. et sp. indet. was most likely as nar-

row-muzzled as Scelidotherium leptocephalum, which has

been interpreted as a mixed or selective-feeder with possi-

ble uprooting habits (Bargo & Vizca�ıno 2008). Similarly,

niche partitioning was previously argued to explain the

high diversity of Neotropical crocodilian communities

(Scheyer et al. 2013; Salas-Gismondi et al. 2015).

Ware Formation and GABI

Sloths are main elements in the American Biotic Inter-

change, as they are present in North America as early as

c. 9 Ma (Hemphillian; Woodburne 2010), and as they

take part in the main pulses of GABI migration. Addi-

tionally, it was recently emphasized how the Ware Forma-

tion can enlighten our understanding of the GABI, with

the example of the carnivoran procyonid Chapalmalania

sp., found in Ware, which is suggestive that procyonids,

North American immigrants, dispersed into South Amer-

ica in two separate events (Forasiepi et al. 2014).

All four families of ‘ground sloths’ that participated in

GABI (Mylodontidae, Megatheriidae, Nothrotheriidae,

Megalonychidae) were present in the Ware formation. A

taxon closely related to Ware’s Pliomegatherium lelongi

(MUN STRI 36685), the megatheriine Eremotherium, is

found in North America c. 2.6 Ma (Woodburne, 2010).

This taxon is widespread in both South and North Amer-

ica while its sister taxa, Megatherium, is restricted to South

America. The relationships of P. lelongi to both Mega-

therium and Eremotherium are unresolved, and therefore,

it is not possible to assess whether the P. lelongi clade

participated in GABI. Megatheriinae are also represented

in northern South America by Proeremotherium and

Urumaquia (Carlini et al. 2006b, 2008), but the recovered

elements of those taxa do not include a mandible or lower

teeth that would have allowed a comparison with MUN

STRI 36685.

TABLE 2 . Comparison of the sloth diversity in Neotropical and Temperate Pliocene formations.

Epoch Formation Latitude (°S) Environment Taxonomic

diversity Species

(families)

References

Neotropical formations

Late Pliocene Ware 11.8 Fluvio-deltaic 5 (1) Moreno et al. 2015;

present study

San Gregorio 11.3 Alluvial fans 1 (1) Vucetich et al. 2010

Early Pliocene Codore 11.3 Fluvio-deltaic with

marine inundation

3 (3) Carlini et al. 2006a, b

Temperate formations

Pliocene Uqu�ıa �23 to �24 Fluvial 5 (3)* Reguero et al. 2007

Corral Quemado �27 – 1 (1) Reguero et al. 2007

Inchasi �19.7 Fluvial 3 (2) Anaya & MacFadden 1995

Monte Hermoso �39 Fluvial 5 (3)* Tomassini et al. 2013

Montehermosan/

Chapadmalalan

Andalhual�a �27 – 3 (2) Reguero & Candela 2011

*Taxa most likely not coeval.

AMSON ET AL . : NEOGENE SLOTHS FROM THE COCINETAS BAS IN 15

Nothrotheriops is recorded in North America during the

Pleistocene (c. 1.4 Ma, Woodburne 2010). However, coe-

val Nothrotherium species, which share diagnostic features

with the specimen from Ware, are restricted to Brazil.

Therefore, it seems that Ware’s Nothrotherium is a mem-

ber of the South American Nothrotherium that did not

participate in GABI.

Two other sloths of the Formation, Megalonychidae gen.

et sp. nov. and Lestodontini gen. et sp. nov. (Mylodonti-

dae), have ambiguous affinities. Megalonychidae represents

one of the earliest ‘ground sloth’ families to migrate to

North America (c. 9 Ma) and includes the early immi-

grants Pliometanastes, and subsequently Megalonyx and

Meizonyx (Woodburne 2010). The Megalonychidae gen. et

sp. nov. of Ware is reminiscent of Megalonyx (and by

extension Pliometanastes, of which the postcranium is

hardly distinguishable; McDonald & Naples 2007), but also

features clear differences. A comparison with Meizonyx is

impossible due to the incompatibility of the elements

recovered. At this point, it is not possible to assess if the

Megalonychidae gen. et sp. nov. of Ware is more closely

related to North American or South American megalony-

chids, as South American large-sized megalonychids are

still poorly documented (e.g. Megalonychops primigenius,

known by a fragmentary humerus from the Ituzaing�o For-

mation, late Miocene of Argentina; Brandoni 2013).

Lestodontini gen. et sp. nov. belongs to Mylodontinae,

a subfamily of Mylodontidae that includes Thinobadistes,

one of the earliest migrants to North America c. 9 Ma

(Woodburne 2010). The subfamily continued taking

part in GABI with the late Pliocene – late Pleistocene

Paramylodon harlani (and a specimen of ‘Glossotherium

chapadmalense’; McAfee 2009). Unfortunately, Ware’s

Lestodontini gen. et sp. nov. is not complete enough to

provide synapomorphies that could have discriminated

between South and North American affinities. As empha-

sized by the phylogenetic analysis (see above), Lestodon-

tini gen. et sp. nov. is found within a clade with

unresolved relationships with both the early immigrant

Thinobadistes and Lestodon, known from the Pleistocene

of temperate South American. Lestodontinae is also rep-

resented in northern South America by Bolivartherium

(Urumaco and Codore formations; Carlini et al. 2006a).

A subfamily of Mylodontidae, the Scelidotheriinae, did

not take part in GABI (Mi~no-Boilini et al. 2014) but has

been found in Ware. This subfamily had a wide distribu-

tion during the Pliocene and Pleistocene being recorded in

multiple localities (from �20 to �39°, Paleobiology Data-

base and the related references included therein: Rusconi

1954; MacFadden et al. 1993; Rodr�ıguez-Brizuela & Tauber

2006; Reguero & Candela 2011). The non-migrating sceli-

dotheriine contrasts with other taxa in Ware, whose rela-

tives took part in GABI (Woodburne 2010). The record of

Ware, in the vicinity of the Isthmus of Panama, further

raises the question of why Scelidotheriinae was not

involved in GABI. The specialized niche possibly associated

with derived muzzle shape is most likely a contributing fac-

tor (McDonald 2005). Scelidotheriines are indeed charac-

terized by long and narrow premaxillae, also acquired,

although under other modalities, in non-migrant members

of other subfamilies such as Megatherium and Mylodon.

CONCLUSIONS

The Cocinetas Basin comprises two units that record con-

tinental Neogene biotas, the Castilletes and Ware forma-

tions. Both delivered rich mammalian assemblages

including tardigradan taxa. Sloths are represented in the

early–middle Miocene Castilletes Formation by one frag-

mentary specimen. Referred to a small-sized megatheri-

oid, this record could suggest affinities with the distant

Santacrucian fauna.

The sloths from the late Pliocene Ware Formation are

represented by 12 specimens that belong to 5 taxa from

the 4 families of ‘ground sloths’. Their body sizes span

over two orders of magnitude, and they most likely

featured various feeding strategies, documenting a highly

diverse assemblage of sloths from the Neotropics.

Although geographically close to the Isthmus of Panama,

and temporally preceding GABI 1 by only 0.2–0.4 my, the

sloths for which unambiguous affinities were recovered

are not closely related to the early immigrants found in

North America before the first main pulse of the Great

American Biotic interchange.

Acknowledgements. Our gratitude goes to all the other members

of the field campaigns, and in particular to J. W. Moreno-Bernal

(STRI), who found several of the specimens presented here. The

constant help of L. Londo~no (STRI) is also acknowledged. We

thank C. Montes (Universidad de los Andes) and J. Escobar

(Universidad del Norte) for welcoming EA to the collections

under their care. M. R. S�anchez-Villagra is greatly acknowledged

for helping to conceive the study and improve the manuscript.

H. G. McDonald (Museum Management Program, National

Park Service) and A. A. Carlini (Museo de la Plata) are thanked

for their useful insights. B. Schubert (East Tennessee State

University) is acknowledged for providing pictures of Megalonyx’

caudal series. R. K. McAfee and an anonymous reviewer are

warmly thanked for the improvement they brought to the

manuscript. EA was granted a short-term postdoctoral fellowship

of the Smithsonian Tropical Research Institute, and was subse-

quently funded by the Alexander von Humboldt Foundation. JC

was supported by Swiss National Fund SNF 31003A-149605 to

M. R. S�anchez-Villagra. The Smithsonian Institution, the

National Geographic Society, the Anders Foundation, Gregory

D. and Jennifer Walston Johnson, Universidad del Norte, the

Lab of M. R. S�anchez-Villagra at the Pal€aontologisches Institut

und Museum der Universit€at Z€urich, and the National Science

Foundation (Grant EAR 0957679) also helped to support this

16 PALAEONTOLOGY

work. Thanks to Carlos Rosero for managing all the logistics in

the field. Thanks to the communities of Warpana, Patajau, Aule-

chit, Nazareth, Wososopo, Sillamana, Paraguachon, La Flor de la

Guajira, and Ipapura. Thanks to the Colombian National Police

(Castilletes base) and the Colombian Army (La Flor de la

Guajira and Cerro de la Teta). We express special thanks to our

drivers, Grillo, Lalo and Medardo.

DATA ARCHIVING STATEMENT

Data for this study are available in the Dryad Digital Repository:

http://dx.doi.org/10.5061/dryad.57025

Locality and stratigraphical data are available in the Smithsonian

Tropical Research Institute PaleoDatabase: http://biogeodb.stri.si.edu/

jaramillo/fossildb

3D models are available in MorphoMuseuM: http://dx.doi.org/

10.18563/m3.2.1.e3

Editor. Hannah O’Regan

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