gess (2001) - diplacanthus

Ann. PaI~ontol. (2001) 87, 1, 49-60 © 2001/~ditions scientifiques et m6dicales Elsevier SAS. Tous droits r6serv~s.

A new species of Diplacanthus from the Late Devonian (Famennian) of South Africa

R o b e r t W . G E S S *

Department of Earth, Sciences, Albany Museum, Grahamstown 6139, South Africa

(Received 26 June 2000; accepted after revision 11 December 2000)

Abstract - - An advanced diplacanthid (Climatiiformes) is described from a Famennian estuarine environment of South Africa. It is characterized by having exceptionally long thin fin spines and a deep body form. Unusual details of the fins and fm spine insertions are preserved. This is the first record of a diplacanthid from the Southern Hemisphere. © 2001 l~ditions scientifiques et m6dicales Elsevier SAS

Acanthodes / diplacanthid / Devonian / Famennian / South Africa

R~sum~ - - Une nouvelle esp~ce de Diplaeanthus du D~vonien sup.6rieur (Famennien) d'Afrique du Sud. Un diplacanthide (Climatiiformes) provenant d'un environnement estuarien famennien d'Afrique du Sud est d6crit. I1 est caract6ris6 par des nageoires dorsales exceptionnellement longues et minces, et un corps 61ev6. Des d6tails exceptionnels des nageoires et des insertions des nageoires dorsales sont conserv~es. C'est la premi6re fois que l'on rapporte la pr6sence d'un diplacanthide dans l'h6misph~re sud. © 2001 l~ditions scientifiques et m6dicales Elsevier SAS

Acanthodes / diplaeanthides / D~vonien / Famennien / Afrique du Sud

INTRODUCTION

The construction in 1985 of a road bypass to the immediate south o f Graham-

stown (Cape Province, South Africa) exposed sedimentary rocks o f the Dwyka and

Witteberg groups. Cuttings to the west o f the junction with the Port Alfred road all

* Correspondence and reprints.

- 4 9 -

2 R.W. GEss

contain rocks belonging to the Witpoort Formation of the Witteberg group (figure 1). The Witpoort Formation is a predominantly arenaceous unit with minor interbedded shales. The new road cuttings (figure 2) exposed an unusually thick layer of black shale containing fossil algal, plant; arthropod, and fish remains. The plant fossils were similar to those described from a much thinner, well known shale exposed in a road cutting in Howison's Poort, about 10 km south-west of Grahamstown. However, the new material was clearly more diverse and less fragmentary.

The major and continuing investigator on the site throughout has been the author. All material collected from the site and deposited in the Albany Museum (AM) up to 1995 is listed and discussed in a published preliminary catalogue [8]. More detailed treatments of some of the organisms have been published. The material includes charophyte [9] and phaeophyte algae [10] and a diverse array of plant types including members of the Rhyniopsida, Zosterophyllopsida, arborescent and herbaceous Lycopsida and Progymnospermopsida [1, 8]. Arthropods are represented by remains of Ostracoda, Conchostraca, and Eurypterida; fish by antiarch and arthrodire placo- derms (including Bothriolepis; and Groenlandaspis, Africanaspis) [3, 8, 12], acanthodians (probably Gyracanthides) [8], crossopterygians (coelacanths and an eusthenopterid, a dipnoan, and an as yet poorly understood chondrichthyan [8, 13]. Several trace fossil types have also been noted.

Due to lack of funding little follow on work on the site was undertaken until by 1999 when the roadcutting was considered unsafe due to slumping and the National Road Agency tendered for its upgrading. On request, they made funds available for the author to carry out rescue operations with the assistance of the tenderers: Norland; and Jaffares and Green Inc. During this period the author discovered the presently described Diplacanthus.

Stratigraphic setting The Witpoort Formation, which constitutes the middle part of the Witteberg

group, the youngest division of the tripartite Cape supergroup, is a predominantly quartz-rich sandstone unit with minor interbedded black, grey, and maroon shales. In the Grahamstown area it is underlain by the Weltevrede Formation, consisting of shales, siltstones, and minor sandstones and overlain by shales of the Lake Mentz

subgroup (figure l c). Within the typical pale grey to white quartz arenites, quartz wackes and sub-

arkoses of the Witpoort Formation several black carbonaceous shales up to 1 m in thickness occur at several horizons. These are often associated with fine plant fragments and trace fossils, frequently of the ichnogenus Spirophyton. The best known of these shale horizons is that exposed at Howison's Poort. From it a few plant species have been described from fragmentary specimens [2, 15, 16].

The relative stratigraphic positions of the shales at Howison's Poort and at the new bypass site suggest that they are probably neither continuous nor precisely contemporaneous.

- 5 0 -

A DEVONIAN DIPLACANTHUS FROM SOUTH AFRICA 3

* , ..,.- %...,.

GRAIIAMSTDWN

. . . . . . . . . . ' . ' z

% ' . ' % ' / . ' . ' . % ' . ' . *|kid,','.

15Okm II I

LAKE MENTZ SUBGROUP

WITPOt3R'~ FORMATION

WELTEVREOE FORMATION

VISF~AN - TOURNA|SIAN

FAMENNIAN

FRASNtAN - GIVETtAN

WASHOVER DUNES

TIOAL ~ \ \ ° 5 i/~tE~ / ~ \ _..---)~.::

FLOOD IDAL DELTA

LAGOON

IR]RIER iSLAND OMPLEX

Figure 1. A. Distribution of the outcrop of Witteberg group rocks in South Africa (CT = Cape Town, PE = Port Elizabeth, G = Grahamstown). B. Simplified geological map of the Grahamstown area, showing the position of the site discussed. C. Key to A and B showing correspondence with international geological time scale. D. Proposed depositional environment at the site, mean current direction westerly to southwesterly. Four shaded areas from left to right: shoreface facies sediments; barrier island complex sands; anaerobic lagoonal muds; and bed rock.

Figure 1. A. Distribution de l'affieurement des roches du groupe Witteberg en Afrique du Sud (CT = Cape Town, PE = Port Elizabeth, G = Grahamstown). B. Carte g6ologique simplifife de la rfgion de Grahamstown indiquant la situation du site discutf. C. Correspondance des niveaux figurfs en A et B, avec ceux de l'fchelle gfologique intemationale. D. Environnement suppos6 des dfp6ts du site. De gauche ~t droite, sfdiments littoranx, barri+re insulaire sableuse, boues anafrobies lagunaires, et subs- tratum.

- 5 1 -

4 R.W. GEss

Figure 2. Road cut in 1985 through Witteberg strata immediately south of Grahamstown, exposing a Famennian lagoonal site from which fossil material, including Diplacanthus acus sp. nov., was recovered.

Figure 2. Coupe de la route faite en 1985 au travers la couche Witteberg au sud de Grahamstown, mettant au jour un site lagunaire famennien o/t ont 6t6 rdcolt6s divers fossiles dont Diplacanthus acus sp. nov.

Sedimentology and depositional environment The fossil locality occurs in a road cutting approximately 500 m long and 20 m

deep, where the beds dip gently (10 °) northwards. The sequence has been interpreted as representing a brackish coastal back-barrier lagoon system, with significant fresh-

water input, sheltered by a barrier island [11, 17].

The lowermost part of the sequence exposed in the cutting consists of alternating

beds of brown-grey quartz arenite separated by thin shales. The quartz arenites

appear largely structureless but contain some trough cross-bedding and horizontal

lamination. These are overlain by about 50 m of thickly bedded, pale grey quartz

arenites with occasional thin, dark grey shales and quartz wackes. Erosional contacts

are common. Cross-bedding suggests a general westerly to south-westerly transport direction. Some bioturbation is evident in the wackes. This portion of the sequence

has been interpreted as shoreface deposits passing upwards into washover, tidal channel, tidal delta, and back-barrier tidal channel deposits [11].

The middle portion of the exposed sequence, from which most of the fossil mate-

rial has been recovered, consists of laminated black carbonaceous shales up to 6 m thick. These sediments represent a stagnant back-barrier lagoonal environment [ 11].

After an interbed low in this sequence the black shale fines upwards, becoming increasingly carbonaceous. The lower portion is characterized by a massive bed of

spirophyton-type trace fossils overlain by fairly massive beds containing thin red iron nodule layers and large disarticulated fish bones. Fish tracks are also present in this

lower portion. Higher in the black shale sequence the material is more finely laminated, containing a greater abundance of delicate plant structure preservation as well as soft tissue preservation of (generally)juvenile fish (personal observation).

- 5 2 -

A DEVONIAN DmLACANTnUS FROM SOUTH AFRICA 5

The black shales are overlain by creamy-coloured mica-rich quartz wackes

containing several lenses of black shale, and carbonised progyrunosperm tree trunks.

The top of the sequence consists of white quartz arenites and micaceous quartz wackes, probably representing a return to a tidal delta depositional environment [11].

Age and preservation 'Plumstead [t5] concluded that fossil plant fragments from the Witteberg group

were of Late Devonian age. Anderson and Anderson [2] agreed with this estimate at least for the Witpoort Formation. It is supported by the Early to Middle Devonian age assigned to the underlying Bokkeveld group on the basis of marine invertebrate evidence [4]. Cooper [5], in attempting to derive a correlation with Northern Hemisphere Devonian rocks using eustatic sea-level changes, suggested that the Witpoort Formation was of Famennian age.

This estimate is borne out by the presence among the material excavated during this research, of Archaeopteris, a characteristic Late Devonian plant, as well as Groenlandaspis and Bothriolepis placoderm fishes characteristic of this age [ 18].

The fossil material from the black shale is fairly poorly preserved, taking the form of silvery white, near two-dimensional compressions. During low grade meta- morphism, phyllosilicate minerals, such as mica and possibly chlorite, grew in the cavities remaining after reduction of the organic matter.

These minerals subsequently altered to kaolinite during uplift to present levels. There is therefore insufficient difference in hardness and chemical composition, between the matrix and impression to allow three dimensional preparation of incom- pletely compressed structures. It is also not possible to study fme details with the scanning electron microscope or high powered optical microscope.

Much of the organic remains appears to have been in a state of decay at the time of burial and the skeletal elements of the fish are partially, or completely dissociated. Water current activity in the area of greatest concentration is likely to have been very slight, suggesting that dissociation was aided by the gases of decay and the activities of scavengers.

The presently described diplacanthid is unusually entire in preservation, perhaps indicating more rapid burial, resulting in part from its small size.

SYSTEMATICS

Order CLIMATIIDA Berg, 1940

Suborder DIPLACANTHOIDEI Miles, 1966

Family D~PLACANTmDAE Woodward, 1891

Genus Diplacanthus Agassiz, 1844

Diplacanthus acus sp. nov.

- 5 3 -

6 R.W. GESS

Figure 3. Holotype specimen ofDiplacanthus acus sp. nov., AM5739, left lateral impression, recovered from the road cutting through Witteberg strata immediately South of Grahamstown which exposed a Famennian lagoonal site. Scale bar = 5 cm.

Figure 3. Holotype de Diplacanthus acus sp. nov., AM5739, empreinte lat6rale, gauche, r6colt6e dans la coupe de la route (couche Witteberg), imm6diatement au sud de Grahamstown o3 un site lagunaire famermien a 6t6 d6couvert, l~chelle = 5 cm.

Holotype: AM5739, an almost entire whole bodied, left lateral impression, with

partial counterpart, collected by R.W. Gess, December 1999, f igures 3 and 4.

A d d i t i o n a l m a t e r i a l : AM5740, almost complete right lateral impression o f

an isolated posterior dorsal spine, preserved in part and counterpart, collected by

R.W. Gess, March 2000,figure 5.

AM5741, caudal fin, lateral impression, fragments o f anal fin and anal fin spine,

collected by R,W. Gess, 1990~

Repository: Albany Museum, Grahamstown, South Africa.

T y p e locali ty" Road cutting south o f Grahamstown, Eastern Cape, South Africa,

39.19S 26.32E,

- 5 4 -


i E

Figure 4. Line diagram of holotype specimen ofDiplacanthus acus sp. nov. AM5739, left lateral impression, recovered from the road cutting through Witteberg strata immediately south of Grahamstown which exposed a Famennian lagoonal site. Dotted lines represent incomplete and poorly defined features. Scale bar = 5 cm.

Figure 4. ScMma de l'holotype de Diplacanthus acus sp. nov., AM5739.Les pointill6s d61imitent les parties incompl6tes ou mal conserv6es, l~chelle = 5 cm.

Stratigraphic horizon: Late Devonian Witpoort Formation (Witteberg group)

(figure 2).

Etymology: Latin, acus, a noun in apposition, meaning a needle, referring to the distinctive long, thin spines.

Diagnosis: A Diplacanthus with an unusually deep body, laterally flattened and with excep-

tionally long thin ribbed spines.

D E S C R I P T I O N

Shape and proportions. The specimen is a little over 10 cm long and 8 cm from the tip of the anal spine to that of the posterior dorsal spine. The ratio of max-

- 5 5 -

8 R.W. G~ss

Figure 5. Almost complete right lateral impression of an isolated posterior dorsal spine, recovered from the road curing through Witteberg strata immediately south of Grahamstown which exposed a Famennian lagoonal site, photo reconstruction fxom part and counterpart, AM5740. Scale bar = 1 cm.

Figure 5. Empreinte lat6rale droite presque compl6te d'une Spine dorsale postdrieure, r~colt6e daus le site lagunaire famennien (conche Witteberg), imm6diatement au sud de Grahamstown. Composition pho- tographique gt partir de l'holotype et de sa contre-empreinte, AM5740. l~chelle = 1 em.

imum (body) depth to total length, as determined from restoration, is 0.37. It is therefore of an unusually deep body shape.

Head. The anterior portion of the specimen is the most poorly preserved, the

snout being entirely missing. The circumorbital bones, though present, cannot be accurately reconstructed due to their poor preservation and the superimposition of the

elements of the left side over the right. Tissue preserved anteriodorsally o f the eye is

taken as the line o f the head. Likewise the posterior ventral outline is preserved. No other features of the head can be discerned.

S h o u l d e r g i rd le . Both scapulocoracoids, superimposed, are visible through the mineralised skin and although their exact outline is hard to define its general char-

acter is apparent. The scapular is high and slender. The postbranehial lamina, circa 2 m m wide for most o f its length, extends dorsally for circa two thirds of the depth of the body in that region. A posterior flange is suggested. Details o f the ventral portion of the shoulder girdle, and its relation to the pectoral spine are unclear, and appear to have suffered damage prior to burial. Subsequent to photographing the specimen the

- 5 6 -


author excavated preserved fin tissue away from the relevant area, but failed to obtain

further clarity. The position of the pectoral fin in the specimen would suggest that the

pectoral fin spine was either dislocated after death, or was capable of movement whilst living.

Fins, fin spines and squamation. The pattern of uniform rows of squamation is clearly visible on the posterior of the specimen, being continuous over the entire caudal fin, anterior parts of the posterior dorsal fin, portions of the anal, and possibly the second intermediate fin (figure 3). Circa twenty rows of scales can be numerated within the anterior centimetre of the caudal fin, along the dorsal profile. Detail of individual scales is not preserved.

The spines are unusually long and thin; the dorsal spines bear a small anterior- facing, ventrally expanded lip at the point of insertion into a raised mound of tissue. The posterior of these two dorsal spines is slightly thicker and longer (3.0 mm across the lip; 34 mm long above the lip as opposed to 2 mm across the lip and an estimated undamaged length of approximately 32 ram, 29 mm extant). The outline of the posterior dorsal fin spine may be seen on the counterpart to extend to a depth of at least 14 mm below the point of insertion. The spines bear longitudinal ridges (four visible on anterior dorsal spine, and five on posterior) that appear to give way posteriorly to fine striations.

Although the extreme dorsal tip of the isolated posterior dorsal spine (figure 5) is damaged, the lengthy subdermal portion is well illustrated in part and counterpart. The full length of the subdermal portion, 17 ram, and 28 mm of the fin supporting portion are present. A composite illustration based on part and counterpart is given

(figure 5). The posterior dorsal fin is well preserved, and displays an interesting rudder-like

appearance.

The anal and pelvic fin spines are both 20 mm long, the visible intermediate spine being barely exposed.

The pectoral spine is not apparent and could not be located whilst keeping damage to the remainder of the specimen to a minimum. Its dimensions are however suggested by the dislocated pectoral fin, to which it may still be attached. A mineralised shadow of similar proportions, postero dorsally diagonal across the body, may well be caused by the remains of the other pectoral fin. The other mineralised shadow, extending from near the top of the scapula, back and down towards the anal fin, is presumed to result from the alimentary tract.

Tail. The ratio of chordal lobe length to body length is 0.3. The hypochordal lobe, also being elongate, though not quite reaching an equal length, is downtumed, whereas the chordal lobe is clearly upturned.

- 5 7 -

10 R.W. GESS

s'}

t l I /

t . / t

Figure 6. Line diagram of reconstruction ofDiplacanthus acus sp. nov., recovered from the road cutting through Witteberg strata immediately south of Grahamstown which exposed a Famennian lagoonal site. Dotted lines represent postulated portions of the reconstruction. Scale bar = 5 cm.

Figure 6. Reconstitution de Diplacanthus acus sp. nov. Les pointill6s d61imitent les parties reconstitu6es. l~chelle = 5 era.

D I S C U S S I O N

Diplacanthus is characterised by a deeper body (ratio of maximum body depth to total length for previously described species of 0.22-0.25) than most other acanthodians. This is interpreted as a derived character, increasing body depth also being associated with some other groups of acanthodians [6]. The presently described species has a ratio of 0.37, with a deep flattened body, and exceptionally long thin spines. Although outside of the current body depth ratio accepted for DipIacanthus, the author believes that it is best assigned to this genus, therefore increasing the ratio range of the genus to 0.22-0.37.

- 5 8 -


The very high scapula with its distinctive, parallel sided postbranchial lamina is a

feature characteristic o f Diplacanthus.

The length and anterior placing of the anterior dorsal spine, the opposite position of the posterior dorsal and anal fin spines, together with their deep insertion clearly indicate this family [14]. The outward structure and ornamentation of the fin spines is consistant with the genus. Furthermore fin spine number and arrangement strongly echo that of D. crassimus Duff 1842 (syn. D. striatus) [7]. The longer, thinner fin spines and deeper body form, however, distinguish it from this species.

The rudder-like posterior dorsal fin may represent an adaptation to the high body form. It seems probable, however, that acanthodian fins may have been more complex than the evidence has suggested. The presence of scales on the fin webs would suggest that they were morphologically reminiscent of modern true chondrich- thyans, in which it is not unusual for the fleshy fin to extend beyond supportive spines. In AM5741 the caudal fin displays slight undulations of the trailing edge suggesting a high degree of posterior thinning.

As increasingly high body form and longer, thinner spines have been proposed as features of increasing derivation within the acanthodians [6], the described species may be supposed to be a highly evolved form.

Although sparsely represented the author believes that D. acus probably lived within the estuarine environment, as there is little or no evidence of passive import from a fully marine environment at the palaeolagoon studied. This is consistent with similar environmental settings today, though movement of some motile living organisms between the sister environments is to be expected. The hydrodynamics of this species would also require a fairly low energy environment.

Acknowledgements - The author would like to thank: Tina Helin of Jaffares and Green Inc., Andrew Better of Norland and Rob Damhuis of the South African National Roads Agency for their support and assistance with the project; Roy Lubke of the Department of Botany, Rhodes University, Grahamstown for assistance in motivating the project; Billy de Klerk of the Department of Earth Sciences, Albany Museum for his continuing interest; Francois Jacques of the School of Languages, Rhodes University, for the translation into French of the title, author's address, abstract, keywords and figure legends; C6cile Poplin of Laboratoire de paMontologie, Mus6um national d'histoire naturelle, Paris for her constructive review of an earlier version of the manuscript; and Brigitte Badr6, Editorial Secretary, Annales de paldontologie for helpful editing, in particular for revising the French translations.

REFERENCES

[1] Anderson H.M., Hiller N., Gess R.W., Archaeopteris (Progymnospermopsida) from the Devonian of southern Africa, Botanical Journal of the Linnean Society 117 (1994) 305-320.

[2] Anderson J.M., Anderson H.M., The Palaeoflora of Africa: Podromus of Southern African megafloras, Devonian to Lower Cretaceous, Balkema, Rotterdam, 1985, 423 p.

[3] Anderson M.E., Hiller N., Gess R.W., The first Bothriolepis - associated Devonian fish fauna from Africa, South African Journal of Science 90 (1994) 397-403.

- 5 9 -

12 R.W. GEss

[4] Boucot A.J., Johnson J.G., Talent J.A., Early Devonian brachiopod zoogeography, Spe- cial Papers of the Geological Society of America 199 (1969) 1-133.

[5] Cooper M.R., Facies shifts, sea-level changes and event stratigraphy in the Devonian of South Africa, South African Journal of Science 82 (1986) 255-258.

[6] Denison R., Handbook of Paleoichthyology: Acanthodii, Gustav Fischer Verlag, Stutt- gart, 1979, 62 p.

[7] Frickhinger K.A., Fossilien Atlas: Fische, Mergus, Melle, Germany, 1991, 1088 p.

[8] Gess R.W., Hiller N., A preliminary catalogue of the fossil algal, plant, arthropod, and fish remains from a Late Devonian black shale near Grahamstown, South Africa, Annals of the Cape Provincial Museums (natural History) 19 (1995) 225-304.

[9] Gess R.W., Hiller N., Late Devonian charophytes from the Witteberg group, South Africa, Review of Paleobotany and Palynology 89 (1995) 417-428.

[10] Hiller N., Gess R.W., Marine algal remains from the Upper Devonian of South Africa, Review of Paleobotany and Palynology 91 (1996) 143-149.

[11] Hiller N., Taylor, F.F., Late Devonian shoreline changes: an analysis of Witteberg group stratigraphy in the Grahamstown area, South African Journal of Geology 95 (1992) 203-212.

[12] Long J., Anderson M.E., Gess R.W., Hiller N., New placoderm fishes from the Late Devonian of South Africa, Journal of Vertebrate Paleontology 17 (1997) 253-268.

[13] Long J., Anderson M.E., Gess R.W., Hiller N., An unusual new fossil shark (Pisces: Chondrichthyes) from the Late Devonian of South Africa, Records of the Western Australian Museum, Supplement (1999) 151-156.

[14] Miles R.S., The acanthodian fishes of the Devonian Plattenkalk of the Paffrath Trough in the Rhineland with an appendix containing a classification of the Acanthodii and a revision of the genus Homalocanthus, Arkiv foer Zoologi (1966) 147-194.

[15] Plumstead E.E, A general review of the Devonian fossil plants found in the Cape System of South Africa, Paleontologia africana 10 (1967) 1-83.

[16] Rayner R.J., Early land plants from South Africa, Botanical Journal of the Linnean Society 97 (1988) 229-237.

[17] Taylor EE, Hiller N., A new Devonian fossil plant locality in the eastern Cape, South African Journal of Science 89 (1993) 565-568.

[18] Young G.C. In: Crame J.A. (Ed.), Origins and Evolution of the Antarctic Biota, Geolog- ical Society Special Publication 47 (t989) 43-62.

- 6 0 -

gess (2001) - diplacanthus

Documents