the abdominal skeleton of lizards in the family iguanidae

Herpetologists' League

The Abdominal Skeleton of Lizards in the Family IguanidaeAuthor(s): Richard EtheridgeSource: Herpetologica, Vol. 21, No. 3 (Sep. 24, 1965), pp. 161-168Published by: Herpetologists' LeagueStable URL: http://www.jstor.org/stable/3891103 .

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VOL. 21 September 24, 1965 NO. 3

THE ABDOMINAL SKELETON OF LIZARDS IN THE FAMILY IGUANIDAE

RICHARD ETHERIDGE

INTRODUCTION

In certain iguanid lizards the costal cartilages of the abdomen form continuous chevron-shaped arcs posterior to the xiphisternum. In their characterizations of iguanid genera Boulenger (1885) and Cope (1886, 1892, 1900) used the term "abdominal ribs" to refer to these continuous chevrons; lizards in which the costal cartilage pairs fail to meet one another midventrally were said to lack "abdominal ribs." Variation in the abdominal skeleton of iguanid lizards is somewhat more complicated than the simple dichotomy "'abdominal ribs present or absent" would indicate. The postxiphisternal cartilages exhibit four distinct patterns of attachment to the bony dorsal ribs, and for the most part the pattern of attachments is constant within genera and generic groups. It is the purpose of this work to describe the patterns of postxiphisternal costal cartilage attachments in the family Iguanidae, and to indicate some of the ways in which these patterns may be taxonomically useful.

TERMINOLOGY

The terms "abdominal ribs" and, later, "parasternalia" have found wide usage in reference to the abdominal skeleton of lizards. The history of their usage, and of their phylogenetic significance has been extremely confused. The best review of the entire subject of the abdominal skeleton in vertebrates appears to be that of De Vos (1938). Parts of this review that have a bearing on the problem of terminology of the abdominal skeleton of iguanids are sum- marized below.

On the basis of paleontological, comparative anatomical, and embryological studies, the skeletal elements in the abdominal mus- cles of vertebrates may be divided into two distinct types: (1) elements derived from the dermal armor of primitive vertebrates, and which develop by the direct ossification of connective tissue without an intermediate cartilaginous phase, and (2) elements of cartilage or of bone that have passed through a cartilaginous phase, and that originate as chondrifications within the myocom-

161



162 HERPETOLOGICA Vol. 21, No. 3

mata. An abdominal skeleton of the former type occurs in Spheno- don, Crocodilia, Archaeopteryx, many fossil reptiles, and in the Stegocephalia. An abdominal skeleton of the latter type is found in lizards and in Liopelma.

The term "abdominal ribs" has been most widely applied to the dermal abdominal skeleton of Sphenodon, Crocodilia, etc., but it has also been used to designate the cartilaginous inscriptional ribs of lizards. Boulenger (1885-1887) appears to be the first to have used "abdominal ribs" in referring to the abdominal skeleton of lizards. He did distinguish between these and the abdominal skeletons of Sphenodon and Crocodilia, for he referred to the latter under the name "plastron" (Boulenger, 1889). It should be noted that Boulenger used the term "abdominal ribs" to refer only to those postxiphisternal cartilages that form continuous chevrons. Because of this, and because the term has found even wider usage in desig- nating the dermal abdominal skeletons of other reptiles, "abdominal ribs" should be abandoned as a term for the abdominal skeleton of lizards.

The term "parasternum," or "parasternalia" for the discreet elements was coined by Gegenbaur (1898:307) for the dermal skeleton in Stegocephalia, Sphenodon, Crocodilia, and many fossil reptiles. Unfortunately many morphologists have applied the term to cartilaginous abdominal elements, including those of lizards, which cannot be of dermal origin. Camp (1923) applied the term parasternum to all of the serially arranged ventral cartilaginous elements in lizards, but he also considered them to be homologous with the abdominal skeleton of Sphenodon and Crocodilia. The existence of bone in the abdominal skeleton of any lizard has never been proven and therefore, as far as we know, an abdominal skeleton of dermal origin does not exist in these animals. Thus, "parasternum" is also a term that should not be applied to lizards.

The term "gastralia," by which Baur in 1897 first designated the abdominal skeleton of Sphenodon (see Furbringer, 1900:280) has since been widely used for all vertebrate abdominal skeletal elements of dermal origin. It has been used in reference to the abdominal cartilages of certain lizards, but only by those who believed these elements to be homologous with the dermal elements of other reptiles. Thus the same argument for not using "parasternum" with reference to lizards applies to the term "gastralia."

Having rejected all of the terms that have been widely applied to the adbominal skeleton of lizards, I do not wish to propose another. In this work I shall follow De Vos (1938) in using the term "inscriptional ribs," which merely indicates their topographic position in the myocommata. It should be understood that although the primary concern here is with those abdominal cartilages posterior to the xiphisternum, all of the inscriptional ribs from the cervi-



1965 ABDOMINAL SKELETON OF LIZARDS 163

cal region through the postxiphisternal region are serially homologous.

THE ABDOMINAL SKELETON IN IGUANIDS

In every iguanid lizard there is a series of slender, riblike elements of calcified cartilage ventral to the distal ends of the bony dorsal ribs. All of these cartilages are imbedded in the myocommata, and are therefore serially correspondent to the bony dorsal ribs. Each dorsal rib in the cervical region has a short rib of cartilage at its distal end, and the dorsal ribs of the thoracic region are tied to the sternum and to the xiphisternal rods by similar, but longer cartilages. Posterior to the xiphisternum the riblike cartilages may or may not join their corresponding dorsal ribs, and the members of a pair may or may not join one another at the ventral midline to form a continuous, chevron-shaped arc.

The most common pattern of attachment is one in which all of the inscriptional ribs posterior to the xiphistemum are attached to their corresponding dorsal ribs and end free without reaching the ventral midline (Fig. IA). The number and length of the inscriptional ribs is variable. The bony dorsal ribs to which they are attached are usually as long, or almost as long as the preceding dorsal ribs attached to the xiphisternum. The dorsal ribs which follow are usually abruptly shorter than those bearing cartilaginous inscriptional ribs.

This pattern of attachment occurs in Sceloporus, Uta, Urosaurus, Sator, Petrosaurus, Uma, Callisaurus, Holbrookia, Leiocephalus, Liolaemus, Stenocercus, Tropidurus, Uranoscodon, Plica, Urocen- tron, Platynotus, Proctotretus, Ctenoblepharis, Phrynosaura, Phryno- soana, Crotaphytus, Strobilurus, Morunasaurus, Phymaturus, Cory- thophanes, Laemanctus, Basiliscus, Sauromalus, and in all but a few species of Ophryoessoides. It also occurs in some individuals of Conolophus, Amblyrhynchus, Cyclura, and Ctenosaura.

A second pattern of attachment is similar to the preceding except that one or two of the inscriptional rib pairs, either the first or second, or both, may join one another at the ventral midline to form continuous chevrons (Fig. IB). This sequence has been found in all specimens examined of Iguana, Brachylophus, and several species of Ophryoessoides, and occurs in some individuals of Cono- lophus, Amblyrhynchus, Cyclura, and Ctenosaura.

In the third pattern of attachments (Fig. 2A) the anterior inscriptional ribs extend from the distal ends of the dorsal ribs to the ventral midline, where they join one another to form continuous chevrons. Posterior to these none of the dorsal ribs bear cartilagi-




A~~~~~

S

S

x

x

P

P

~ Sa

FIG. 1.-Ventral view of the sternum, xiphisternum, and inscriptional ribs (solid black) and their dorsal rib attachments in Stenocercus chrysopygus (A) and Amblyrhynchus cristatus (B). Abbreviations: S, sternal ribs; X, xiphisternal ribs; P, postxiphisternal ribs; Sa, sacral diapophyses.

nous elements. The attached chevrons may be followed by one or more that are similar but are not attached to their corresponding dorsal ribs. Rarely, these floating chevrons are in turn followed by a splint-like element on one or both sides, attached neither to their corresponding dorsal ribs nor to one another midventrally.




FIG. 2.-Ventral view of the sternum, xiphisternum, and inscriptional ribs (solid black) and their dorsal rib attachments in Anolis carolinensis (A) and Chalarodon madagascariensis (B). Note: dorsal ribs are lacking on the last few presacral vertebrae in Anolis.

This sequence occurs in the following genera, each of which is followed by the numbers of fixed and floating chevrons that have been found in that genus: Anolis (including Audantia, Deiroptyx, Diaphoranolis, Mariguana, Norops, Tropidodactylus, and Xiphocer- cus) 5:2, 5:1, 4:2, 4:1, 4:0, 3:1, 2:2, 1:3; Polychrus (including




Polychroides) 10:0, 9:2, 9:0, 8:0; Anisolepis 4:3; Aptycholaemus 4:3; Chamweleolis 4:2; Chamaelinorops 4:2, 3:3; Phaenacosaurus 4:3; Enyalius 2:3; Enyalioides 4:0, 2:1; Leiosaurus 5:0; Hoplocercus 2:3; Urostrophus 3:2, 2:3, 4:1.

A fourth pattern of postxiphisternal inscriptional ribs consists en- tirely of paired elements that are free of attachment either to their corresponding dorsal ribs or to one another at the ventral midline (Fig. 2B). Members of a pair may approach one another very closely, touch, or even overlap midventrally, but never join one another to form a continuous chevron. This sequence has been found only in the Malagasy genera Chalarodon and Oplurus. In both there may be two or three pair of inscriptional ribs.

DIsCUSSION

Because of its complexity and variability within the family Iguanidae the abdominal skeleton is of greater potential taxonomic value than the simple dichotomy "abdominal ribs present or absent" would indicate. In large groups of related genera-the iguanines, sceloporines, tropidurines, basiliscines, anolines, the Malagasy genera-all, or almost all of the genera possess the same pattern of attachment. There is a high degree of conservatism within most species and genera in such details as inscriptional rib length, number of chevrons, and so on. It may be possible to determine evolutionary trends within genera and generic groups, and in one group of iguanids, the anoles, this has been done (Etheridge, unpublished thesis, Univ. of Michigan, 195,9). This group will illustrate the manner in which the inscriptional ribs may be taxonomically useful.

The great majorty of anole species show little or no intraspecific variation in the number of attached and unattached cartilaginous chevrons. Usually less than one percent of a sample possesses a number other than that typical for the species. The primitive forms have a high total number of chevrons and a high ratio of attached to floating chevrons. Evolutionary trends within the anoles involve a reduction in the total number of chevrons and a loss of attachment of progressively more of the posterior chevrons. The number of attached and floating chevrons is therefore useful for the determination of phylogenetic relationships within this group. In a few species (e.g., Anolis nebulosus, A. nebuloides, A. schmidti) the number of attached and floating chevrons exhibits a high degree of variation unrelated to sex or size within each population. In at least one species (Anolis humilis) the number of attached and floating chevrons exhibits geographic variation. Thus, not only the number




of attached and floating chevrons, but also the extent and patterns of variation in the numbers may be taxonomically useful.

Studies of the patterns of variation in the postxiphisternal inscriptional ribs of other iguanids should also provide information that will be helpful in the determination of phylogenetic relationships within the family, especially at the generic and generic group levels.

MATERIAL EXAMINED

Lists of the genera and species of sceloporine and tropidurine iguanid lizards examined for this study are published elsewhere (Etheridge, 1964:630-631; 1965: in press). The species of anoles examined are listed in my unpublished doctoral thesis (University of Michigan, 1959:200-219). Other species examined are:

Amblyrhynchus cristatus; Conolophus pallidus, C. suberistatus; Iguana iguana, I. delicatissima; Ctenosaura acanthura, C. clarki, C. hemilopha, C. pectinata, C. quinquecarinata, C. similis; Cyclura baelopha, C. cornuta, C. macleayi, C. ricordi, C. rileyi; Brachylophus fasciatus; Dipsosaurus dorsalis; Sauromalus ater, S. hispidus, S. obesus, S. shawi, S. slevini, S. varius.

Basiliscus galeritus, B. plumifrons, B. vittatus; Corythophanes cristatus, C. hernandezii, C. percarinatus; Laemanctus serratus.

Anisolepis grilli, A. iheringi, A. lionotus, A. undulatus; Aptycholaemus longicauda; Polychrus gutturosus, P. liogaster, P. marmoratus, P. peruvianus, P. spurrelli.

Oplurus cyclurus, 0. quadrimaculatus, 0. torquatus; Chalarodon madagascariensis.

Enyalioides laticeps, E. praestabilis; Enyalius bibroni, E. catinatus, E. fitzingeri; Hoplocerus spinosus; Leiosaurus darwini; Morunasaurus groi; Phymaturus palluma; Strobilurus torquatus; Urostrophus vautieri.

LITERATURE CITED

BOULENGER, GEORGE ALBERT. 1885-1887. Catalogue of the lizards in the British Museum (Natural History), London.

. 1889. Catalogue of the Chelonians, Rhyncocephalians and Croco- diles in the British Museum (Natural History), London.

CAMP, CHARLES LEWIs. 1923. Classification of the lizards. Bull. Amer. Mus. Nat. Hist., 48:289-435.

COPE, EDWARD DRINKER. 1886. On the species of iguaninae. Proc. Amer. Philos. Soc., 23(122): 261-271.

. 1892. The osteology of the Lacertilia. Proc. Amer. Philos. Soc., 30:185-222.

1900. The crocodilians, lizards and snakes of North America. Ann. Rep. U. S. Natl. Mus., 1898 (1900) 2:151-1294.

ETHERIDGE, RICHARD. 1964. The skeletal morphology and systematic relationships of sceloporine lizards. Copeia, 4:610-631.

. 1965. West Indian and South American lizards of the iguanid genus Leiocephalus. Copeia. In press.

FURBRINGER, M. 1900. Zur vergleichenden Anatomie des Brustschulderap- parates und der Schultermuskeln. Part IV. Jena. Ztschr. f. Naturwiss, 34:215-7.18.




GEGENBAUR, C. 1898. Vergleichende Anatomie der Wirbeltiere mit Beruick- sichtigung der Wirbellosen. Engelmann, Leipzig.

ROMER, ALFRED SHERWOOD. 1956. Osteology of the reptiles. Univ. Chicago Press, xxi + 772 pp.

DE VOS, C. M. 1938. The inscriptional ribs of Liopelma and their bearing upon the problem of abdominal ribs in Vertebrata. Anat. Anz., 87:49- 112.

Department of Zoology, San Diego State College, San Diego, California.

NEW OR UNUSUAL AMPHIBIANS AND REPT'ILES FROM OAXACA, MEXICO. I

JOHN D. LYNCH AND HOBART M. SMITH

Between November 1962, and June 1963, Mr. Thomas Mac- Dougall collected for the University of Illinois Museum of Natural History (UIMNH) 83 specimens of amphibians and reptiles rep- resenting 39 species, in the state of Oaxaca, Mexico. Eight of these species are especially noteworthy. An account of these eight follows the appended list of all species obtained at 11 localities visited (Fig. 1). (1) San Felipe del Agua: Rana pipiens, R. pustulosa, Bufo occidentalis. (2) Cerro San Felipe: Elaphe triaspis intermedia, Thamnophis chrysocephalus, T. e. eques, T. cyrtopsis cyclides, Rhadinaea vittata, R. aemula, Toluca megalodon, Tri- morphodon tau, Micrurus ephippifer, Crotalus i. intermedius, Abronia oaxacae, Barisia i. imbricata x i. planifrons, Barisia viridiflava. (3) San Andres Chicahuastla: Bufo occidentalis, Hyla euphorbiacea, Scelo-porus mucronatus omiltemanus, S. f. formosus, S. subpictus, Eumeces brevirostris. (4) Oaxaca-Tuxtepec Road, near km. 110 (about 30 mi. S Tuxtepec): Hyla euphorbiacea, Eleutherodactylus mexicanus. (5) Nueva Raza: Drymobius m. margaritiferus, Ninia s. sebae, Bothrops atrox. (6) West of "La Cumbre," Oaxaca, 10,000 ft. +: Crotalus i. intermedius. (7) Rlo de la Y Sola ("El Vado"): Stenorrhina freminvillei lactea. (8) North of Zanatepec, 1,500 ft. ?: Sceloporus m. melanorhinus, S. malachiticus internasalis, Eleu- therodactylus macdougalli, Ptychohyla e. euthysanota. (9) North of Zanatepec: Drymobius chloroticus. (10) Sierra Madre, North of Zanatepec: Drymobius chloroticus, Bothrops nigroviridis aurif er, B. godmani, Leptodeira annulata cussiliris, Leptophis ni. mexicanus, Leptotyphlops p. phenops, Pliocercus e. elapoides x e. diastemus, Sceloporus malachiticus internasalis, Scincella g. gemmingeri. (11) "Lagunas," Zanatepec: Ameiva u. undulata.

We are indebted to Dr. D. F. Hoffmeister and Mr. Thomas



the abdominal skeleton of lizards in the family iguanidae

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