The Osteology of the Reptiles/Introduction

by Samuel Wendell Williston

Layout 2

 

PART I

THE SKELETON OF REPTILES

 

INTRODUCTION

THE PRIMITIVE SKELETON OF REPTILES

That the reptiles were evolved from the Amphibia, and more specifically from that order known as the Temnospondyli, seems now assured. The earliest as also the most primitive reptiles that we know belong to the order called the Cotylosauria. With the exception of Eosauravus from the middle Pennsylvanian of Ohio, of which, unfortunately, the skull is unknown, our knowledge of them goes no further back than the late Carboniferous and early Permian. At that time there was a considerable diversity of known forms, belonging to at least four well-differentiated groups and twenty or more families; from which we may very properly conclude that their earliest ancestors, the beginning of their stock, lived much earlier, certainly at the beginning of the Upper Carboniferous, and very probably in Lower Carboniferous times. We therefore never can expect to find in the rocks of the Permian any real connecting link between the two classes.

Both the reptiles and the amphibians had changed in this interval, an interval perhaps of millions of years, retaining in varying degrees their ancestral characters, while losing or adding others in various ways. The reptiles, by the acquirement of a new mode of life, the loss of gills in their youth and entire emancipation from the water, became more progressive than the amphibians, and their evolution was more rapid. Characters that are common to many amphibians became more and more rare among the reptiles, and the amphibians, handicapped by inherited habits, were restricted more and more to subordinate rôles, and only a few of the more progressive continued to develop. They, for the most part, lost those characters and adaptations that brought them into immediate competition with the reptiles, and by the close of Triassic times had become restricted to habits and habitats no longer invaded by them. The modern toads, frogs, salamanders, and blindworms differ far more from the higher amphibians of Paleozoic times than did the latter from their contemporary reptiles.

Fig. 1. Primitive Cotylosaur Seymouria, from nearly complete specimen, from above.
A little less than one third natural size.

Nevertheless, there were still so many inherited characters among both the amphibians and reptiles of early Permian times that nothing distinctive of either class can be found in the skeleton, except in the atlas and feet, with a considerable gap in the structure of their vertebrae. In the vertebral column there was a general change among the Temnospondyli from the embolomerous to the rhachitomous type, that is, from the more simply divided centrum of two disks to the tripartite centrum composed of wedges; while all reptiles had acquired a reduced embolomerous form with one disk, the centrum, and one wedge, the intercentrum.
Fig. 2. Seymouria (Cotylosauria). A, from above; B, from side. One third natural size. Doubtless all amphibians of Lower Carboniferous times had embolomerous vertebrae, but only a very few of their stock persisted as late as the Permian. In general literature the Amphibia are distinguished from the Reptilia by the possession of two occipital condyles. The earliest amphibians doubtless all had a single occipital condyle, an inheritance from their ancestral fishes—all that we know from the Lower Carboniferous had—of which only one known descendant with that character survived to the Permian. The reptiles, however, retained the single condyle until the beginning of their evolution into mammals, when they too developed a double condyle. We relied, until recently, upon the widely open palate of the Amphibia as a final distinguishing character of their class, but we now know that some, if not all, of the earliest amphibians had a [closed] palate like that of the [earliest] reptiles, but of these none is known at the beginning of Permian times. In other words, a single condyle and a closed palate are more primitive characters of the tetrapods than those we had assumed as characteristic of the Amphibia. We know no amphibians with as many bones in the digits as the early reptiles possessed, and no reptiles with as many bones in the tarsus as the early amphibians had, but doubtless when we discover the feet of the earliest reptiles we shall find them not different from the feet of the contemporary amphibians.

Every known bone in the skull of the Temnospondyli, except the interfrontal of a very few, has been found in the skull of early reptiles, and all, indeed, in a single genus Seymouria (Figs. 1, 2) from the Lower Permian of Texas. And there is no bone in the skeleton of reptiles that is not known in these same amphibians, except the preparietal of the Anomodontia, the supraorbitals of various Squamata and the predentary and rostral bones of certain dinosaurs, and doubtless the last two, if not all, are simply dermal bones which became temporarily attached to the skull. The girdles and limbs of the two classes are distinguishable only by minor characters. And thus, while we do not know from these later rocks, and probably never shall from rocks later than the Lower Carboniferous, all of the characters common to the two classes in any one animal, from the comparison of all it is not difficult to decide what were the primitive characters of the reptilian skeleton in almost every detail. They may be summarized as follows:

The Primitive Skull of the Reptilia

Rugose, with five openings in roof:

⁠A.⁠ Paired, divided, terminal nares.

⁠B.⁠ Paired orbits beyond middle [i.e., in front of the middle of the skull].

⁠C.⁠ Median parietal (pineal) foramen.

An emargination of the occipital border, between tabular and squamosal, for the ear [the "otic notch"].

Seventeen pairs of roof bones; four pairs of palatal bones; eight pairs of cranial bones; eight pairs of mandibular bones; three unpaired cranial bones; one unpaired palatal bone—seventy-eight in all.

A. Paired Bones
1.	Premaxillae (px)	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	Dentigerous
2.	Maxillae (mx)
3.	Septomaxillae (sx)		Nasal bones
4.	Nasals (na)	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Median roof bones
5.	Frontals (fr)
6.	Parietals (pa)
7.	Interparietals (ip)
8.	Lacrimals (la)	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Surrounding orbits
9.	Prefrontals (pr)
10.	Postfrontals (pf)
11.	Postorbitals (po)
12.	Jugals (j)
13.	Intertemporals (it)	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Temporal bones
14.	Supratemporals (st)
15.	Tabulars (t)
16.	Squamosals (sq)
17.	Quadratojugals (qj)
18.	Dentaries (d)	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Mandible
19.	Coronoids (cor)
20.	Splenials (sp)
21.	Postsplenials (psp)
22.	Angulars (an)
23.	Prearticulars (pa)
24.	Surangulars (sa)
25.	Articulars (art)
26.	Prevomers (pv)	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Palatal bones, dentigerous
27.	Palatines (pl)
28.	Pterygoids (pt)
29.	Ectopterygoids (ec)
30.	Quadrates (qu)		Articulation of mandible
31.	Exoccipitals (eo)	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	Cranial bones
32.	Paroccipitals (poc)
33.	Proötics (pc)
34.	Postoptics (al)
35.	Stapes (stp)
36.	Epipterygoids (ep)
37.	Ethmoids (se)
⁠
AA. Unpaired Bones
38.	Parasphenoid (ps)		Palate
39.	Supraoccipital (so)	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}$	Cranial bones
40.	Basioccipital (bo)
41.	Basisphenoid (bs)

The Primitive Postcranial Skeleton

A. Membrane Bones, Exoskeletal

1.⁠ Sclerotic plates in orbits.

2.⁠ Paired clavicles, cleithra and elongate interclavicle.

3.⁠ Dermal plates or scutes.

 

AA. Cartilage Bones, Endoskeletal

1.⁠ Notochordal vertebrae; two or three cervicals, about twenty-three presacral; one sacral; moderately long tail; proatlas; atlas embolomerous; all vertebrae to tenth or twelfth caudal with free, holocephalous ribs, articulating continuously with intercentrum and diapophysis.

2.⁠ Intercentra between all vertebrae.

3.⁠ Slender and numerous parasternal ribs.

4.⁠ Scapular girdle composed of paired scapulae, procoracoids and metacoracoids, fused in adult life, the three forming the glenoid socket; a supracoracoid and a supraglenoid foramen.

5.⁠ No sternum.^[1]

6.⁠ Pelvis plate-like with small obturator foramen only; acetabulum formed by the three bones, closed.

7.⁠ Legs short and stout.

8.⁠ Humerus dilated at extremities, with entepicondylar foramen.

9.⁠ Carpus with four bones in proximal row; two (three?) in middle row; five in distal row; all well ossified.

10.⁠ Hand pentadactylate, the fourth finger strongest and longest; phalangeal formula 2, 3, 4, 5, 3.

11.⁠ Tarsus composed of nine bones: two in first row^[2]; two in second; five in distal row; all well ossified.

12.⁠ Feet pentadactylate, the fourth toe strongest and longest; phalangeal formula 2, 3, 4, 5, 4.

 

1. [But see footnote 1, on page 122 below.—Ed.]
2. [But see footnote on page 187 below.—Ed.]