THE SKULL STRUCTURE OF DIPLOCAULUS MAG- NICORNIS COPE AND THE AMPHIBIAN

ORDER DIPLOCAULIA

ROY L. MOODIE From the Zoological Laboratory, the University of Kansas

SEVEN FIGURES

The Permian vertebrate known as Diplocaulus magnicornis Cope is one of the most aberrant and specialized of all the extinct Amphibia. The species was first described by Cope from frag- ments of several crania and portions of the vertebral column; material which had been collected in the Permian of Texas prior to 1882. The genus had, however, been established previously on fragmentary material which had been discovered by Dr. J. C. Winslow and Mr. W. F. E. Gurley in the Pennsylvanian of Ver- milion County, Illinois. The genus Diplocaulus was first located by Cope in 1881 among the Pelycosauria, but later researches inclined him to place the form among the stegocephalous Am- phibia with relationships to the Microsauria.

The skull of Diplocaulus magnicornis is very peculiar in the elongation of the posterior elements of the upper surface of the cranium. The anterior elements do not take part in the posterior prolongations which give the skull such a bizarre appearance. This specialization is due to the extreme elongation of the supra- temporal, the squamosal, the parietal, the epiotic and the supra- occipital.

I was unable to dis- cover it on a well preserved skull in the collection of the Univer- sity of Chicago (No. 2, U. of C. Collections). Cope figured it on the skull of this species which he studied in 1895. Broili says nothing of the presence of thk opening and does not figure it in the restoration of the skull which he gave in 1902. Williston

The pineal foramen is apparently absent.

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32 ROY L. MOODIE

was unable to locate it on the skull of Diplocaulus limbatus Cope which he figured in 1909.

The nostrils are located far forward on the anterior edge of the skull which curves slightly downwards so that they look forward. The orbits are small, almost circular, and are situated far forward as is the case in many of the carboniferous Microsauria. Other than these there are no openings on the dorsum of the skull.

The arrangement of the cranial elements is found to be approxi- mately as Cope gave them in 1895 although there were discovered in the complete specimen evidences of the postorbital which had not been previously detected. Williston was unable to locate this element in the excellent skull of Diplocaulus limbatus Cope and concluded that the element behind-the orbit is thepostorbitofrontal (Trans. Kans. Acad. Science, 1908, pl. la) an interpretation which is open to much question. We havc yet to have a definite proof of the union of these two elements, the postorbital and postfrontal, in any vertebrate skull. If there be only one present it is either one element or the other and not both. Further discussion of this will be postponed for a paper on the development of the alli- gator skull. The outlines of the jugal and quadratojugal were also determined. The suture separating the parietal and the supraoccipital was not found to be so erratic as Cope figuredit but it continues directly across the skull. This may casily have been an individual variation. The suture scparating the frontal into two equal parts was not detected although careful search was made for it. This seems to be the only case on record among the Stegocephala in which there has been an actual fusion of two paired elements of the cranium. Maggi has made some interest- ing suggestions as to the origin of the interparietal of mammals and its correlation with the fused epiotics of the Stegocephala. In discussing Maggi’s paper it was stated that there was no case of a fusion of any of the cranial elements of the Stegocephala known. This I believe to be an error as is demonstrated in the present skull. This would not, however, change the decision in regard to Maggi’s conclusions.

On the dorsum of the skull there were detected in several places evidences of the lateral line canals which occur as shallow grooves.

DIPLOCAULUS MAGNICORNIS 33

These are almost universally known as the ‘slime canals of the Stegocephala,’ a decided misnomer since the lateral line canals have nothing to do with the production of slime. The canals were also detected on the mandible which is associated with the present skull. On the right of the skull (fig. 1) there will be seen a distinct groove which runs along the edge of the skull. This canal had been called by Allis in Amia “the anterior portion of the infraorbital.” There were also detected portions of the supraorbital canals but the skull is so badly crushed that it is impossible to follow the complete course of the canals.

The palate of the skull, as here given, is an advance over any- thing heretofore known. There still remains much to be deter- mined in regard to its structure, but the following is offered as a contribution to the more complete knowledge of the subject. Cope, in 1895, gave a figure of the anterior portion of the palate and Broili, in 1902, gave further notes on its structure. Broili, however, had but a small portion of the skull of the animal on which to base his conclusions. Unless there be an extreme varia- tion in the shape assumed by the skull of this species Broili’s restoration is at fault in regard to the posterior curve of the skull, there being no indication of such a condition in the present speci- men, nor in the species D. limbatus Cope. Cope gave very accu- rately the positions of the teeth on the palate but was unable to determine the elements which bore the teeth. In the present skull the tooth-bearing elements are found to be the premaxillae, the maxillae, the vomers, the palatines and the transverse bnnes.

There are five pairs of openings and depressions on the palate of the skull. These are: the internal nares, correctly repre- sented by Cope and Broili; the palatine vacuities; the depressions which Broili calls the ‘ Ohrenschlitzgruben’ or auditory fossae : the infratemporal foramina and depressions along the posterior lateral border of the palate which appear to be partly due to a folding over of the skull elements and doubtless gave place for the attachment of the masseter and temporalis muscles. The arrangement of the palatal openings of Diplocaulus does not differ in any essential respect from the condition found in the larger Stereospondylia.

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34 ROY L. MOODIE

It is worthy of note that the palatal elements in Diplocaulus do not take part in the formation of the prolongation of the dorsum of the skull and we get an idea of the primitive condition of the skull of the Diplocaulidae from the circumscribed area of the palatal elements which are restricted by the quadrates to the anterior portion of the skull so that all of the elongation and expan- sion has taken place in the dorsum.

The internal nares are the most anterior. They are small, oval and transverse in position. They are bounded by the premaxillae, vomers and palatines. The palatine foramina are large, oval openings situated below the orbits on either side of the median line. Their long axis is parallel to the axis.of the skull. They are bounded by the parasphenoid, transverse, vomers and palatines. The infratemporal foramina lie anterior and somewhat medial to the quadrates. The openings have a rounded triangular form and are bordered by the transverse, the maxilla, the quadratojugal, the quadrate and the pterygoids. Posterior to the quadrate there is an elongate groove which is possibly homol6gous with the quadrate foramen of the Pelycosaurian genus Dimetrodon, of Spheqodon, Anaschisma and other stereospondylous forms. Its function here seems to be for the attachment of the masseter and temporalis muscles. It certainly has the position of the quadrate foramen in other forms. Its elongation is due to the backward growth of the epiotic horns. The other opening marked es in fig. 6, is undoubtedly the external auditory meatus. It represents in part the otic notch or ear slit of other Stegocephala so well shown in Metoposaurus, Mastodonsaurus, and Archego- saurus. Broili has called them the ‘Ohrenschlitzgruben’ and he is undoubtedly right.

The mandible of Diplocaulus magnicornis Cope is moderately heavy, though comparatively slight when compared to the size of the skull. The sutures on the mandible have been impossible to determine, with the exception of those bounding the articular. They show the articular to have been a triangular element. The teeth of the mandible consist of about thirty-five to forty short blunt cones. The form of tooth appears to be well adapted to

The palatal openings are all lateral in position.

DIPLOCAULUS MAGNICORNIS 35

crushing shell fish, as Case suggests, and Diplocaulus may have fed on some of the smaller Mollusca of the Permian rivers and lakes. On the lateral face of the mandible, there is a distinct groove, the operculo-mandibular canal of the lateral line system. It would be interesting matter to determine if this canal extended entirely around the mandible.

Dr. Case in 1908 published a restoration of the entire animal, as the structure seemed to him to demand. However, Case neg- lected the insertion of the clavicular girdle which was already known and which would seem to indicate the presence of limbs. As a matter of fact limbs are still unknown in this species although Williston has recorded the discovery of small limbs in the closely related species D. limbatus Cope. That limbs will ultimately be discovered in the present species can not be doubted. The habits of the animal were undoubtedly as Dr. Case has suggested for them (Pop. Sci. Monthly, December, '08).

Paleontology teaches us nothing as yet of the ancestry of this peculiar genus of amphibians nor have we any record of its de- scendants. It is one of those peculiar forms which stands alone. It shows, however, characters which are more nearly those of the Branchiosauria than of the Microsauria in which order it is usually placed. The characters separating the early orders of Amphibia are essentially those of the ribs and vertebrae. The structure of the skull is essentially similar in all of the groups. We are not able, from the structure and composition of the skull, to distinguish a branchiosaurian from a microsaurian. The characters of the ribs and vertebrae are, however, perfectly constant and distinctive. Of course there are certain superficial characters of the skull which hold true for all branchiosaurs and microsaurs such as the absence or presence of sculpture of the cranial elements and the absence of the lateral line grooves from the skulls of the Branchiosauria. Other characters such as the presence of external branchiae in the Branchiosauria, the lack of endochondral ossificatioli in the long bones and absence of clawed digits would seem to be of considerable importance.

Except for superficial characters the skull of Diplocaulus mag- nicornis Cope is essentially similar to those of the Branchiosauria

36 ROY L. MOODJE

and Microsauria in structure and composition. The elongation of the epiotic regions to form the wide, fan-shaped, horns, the fusion of the frontals and the absence of a parietal foramen are individual or ordinal characters the importance of which is open to debate. That the fan-shaped horns were developed for the protection of gills would seem most absurd. If the creatures had gills the horns probably served to protect them but there is no evidence whatever that these forms were branchiate. Horns of a similar character are developed in many of the Microsauria in genera which are otherwise and structuraIly unrelated. Just what the development of these horns may mean is a difficult problem. The solution offered by Beecher of the significance of spines and horny excrescences indicating decadence may be a good one here but we know so very little about these creatures that conclusions would be premature.

When we consider the characters broadly we perceive that they indicate a group separation of the species of Diplocaulus as I have already indicated (Geol. Mag., May, '09, p. 220). The characters which have been discussed ally the prekent genus with the Branchiosauria rather than with the Microsauria. The only character of the microsaurs which the species of Diplocaulus possess is the sculptured nature of the elements of the clavicular girdle and cranial elements. The characters of the ribs and ver- tebrae are essentially those of the Branchiosauria but they differ from these in the specialization of the aygosphene and zygantrum, which have not been detected in the branchiosaurs, and in the structure of the ribs. The fact that the ribs are borne on the middle of the centrum on an elongate transverse process would be sufficient to indicate its complete separation from the Micro- sauria in which the ribs are universally intercentral. The pres- ence of an epicondylar foramen in the humerus is another distinc- tive character of Diplocaulus and entirely lacking in both Bran- chiosauria and Microsauria. In short the characters presented by Diplocaulus are so confusing and contradictory that they compel us to perceive that after all a final classification is impossible. There will always be new classifications so long as there are new forms and new intellects at work upon the material. But if we

DIPLOCAULUS MAGNICORNIS 37

must have a classification for convenience why not have it consist- ent at least? If we use a character to distinguish two orders of Amphibia like the Branchiosauria and Microsauria, and the char- acter has been accepted for nearly half a century, then why not, apply the same rule to another group of amphibians, and on the structure of the vertebrae, ribs and limb bones establish the new order Diplocaulia? It seems consistent at least if nothing more.

In pursuance of this I give here the ordinal characters of this new group of Amphibia-Diplocaulia : Skull proportionately very large with epiotic angles drawn out into fan-shaped horns, the expansion being due to the supratemporal, epiotic, parietal, squamosal and supraoccipital. Frontals fused into a single plate. Lachrymal probably absent. Pineal foramen absent. Orbits small, circular and anteriorly placed. Sclerotic plates unknown. Nostrils on or near the anterior edge of cranium. Teeth borne on mandible, premaxillae, maxillae, palatines, vomers and ectopterygoids. Teeth rounded, acrodont, denticles, abun- dantly present and apparently fitted for crushing hard substances. Palatal region restricted to the anterior portion of the skull. It does not take part in the epiotic prolongation. The palatal aspect of the cranium interrupted by five paired openings which are : the internal nares, the palatine foramina, the infratenporal foramina, the quadrate foramina for the attachment of the masse- ter and temporalis muscles and the auditory slits or external auditory meatus. The occipital condyles occur under the pro- jecting shelf of the supraoccipital plates Basioccipital partly cartilaginous and condyles borne by exoccipitals. Lateral line grooves present on skull and mandible.

Atlas ribless and essentially urodelous in structure. The ribs bicipital and borne on large transverse processes springing from the arch and centrum. The zygosphenal articulation not so well developed as the zygopophysial one. Vertebral formula unknown. Vertebrae elongate with low spine. Notochord but partly persistent and absent in the middle portion of the centrum, persisting as a double cone in the intervertebral regions. Clavic- ular girdle composed of interclavicle, clavicles and coracoids (?)

38 ROY L. MOODIE

the first two of which are sculptured. These are quite large and indicate the presence of limbs in species where actual limbs have not been found. Humerus with endochondral ossifications resem- bling the Branchiosauria. Epicondylar foramen and muscular expansions present. Carpus and tarsus unossified. Femur elon- gate and somewhat twisted.

There are three species of this order known. Diplocaulus salamandroides Cope, described from fragmentary

material collected in the upper carboniferous beds of Salt Creek, Vermilion County, Illinois, in 1877.

Diplocaulus limbatus Cope, described from fragmentary material from the Permian of Texas. Further descriptions and figures of the skull, girdles and limb bones given by Williston in 1909.

Diplocaulus magnicornis Cope, described from a nearly com- plete cranium from the Permian of Texas.

Closely related forms of this group are possibly to be found in the Crossotelidae from the Permian of Oklahoma, but the group is as yet very imperfectly known.

They are:

BIBLIOGRAPHY

BROOM, R.

COPE, E. D.

1910 Comparison of the Permian reptiles of North America with those of South Africa. Bull. Amer. Mus. Nat. Hist., vol. 28, p. 214.

1882 Third contribution t o the history of the vertebrata of the Permian formation of Texas. Proc. Amer. Phil. SOC., vol. 20, p. 453.

1881 Catalogue of vertebrata of the Permian formation of the United States.

1882 Permian vertebrata, Amer. Nat., vol. 16, p. 925.

1888 Systematic catalogue of vertebrata from the Permian. Trans. Amer. Phil. SOC., vol. 16, p. 286.

1896 The reptilian order Cotylosauria. Proc. Amer. Phil. SOC., vol.

Amer. Nat., vol. 15, p. 162.

34, p. 455. PI. 9.

MILLER, S. A. 1889 N. A. Geology and paleontology. p. 621.

CASE, E. C. 1900 Vertebrates from Permian bone bed, Illinois. Journ. Geol.,

Pop. Sci. Monthly,

vol. 8, p. 710.

1908 A great Permian delta and its vertebrate life. vol. 73, p. 567, figs. 12, 13.

DIPLOCAULUS MAGNICORNIS 39

BROILI, F. 1902 Beitriige zur Kenntniss von Diplocaulus Cope. Centralblatt far Mineralogie, p. 536,

1904 Permische Stegocephalen und Reptilien. Paleontographica, Bd. 51, p. 8, pls. I, IV, V.

JAEKEL, O n 0 1903 n e r Ceraterpeton, Diceratosaurus und Diplocaulus. Neues Jahrb. Mineral., p. 126.

MOODIE, ROY L. 1908 The dawn of quadrupeds in North America. Pop. Sci. Monthly, vol. 72, p. 565, fig. 5. 1908 The lateral line system in extinct amphibia. Jour. Morph., vol. 19, p. 522, figs. 9, 9a, 10.

1908 Carboniferous quadrupeds. Trans. Kans. Acad. Science, vol.

1909 The Microsauria. Geol. Mag., Dec., vol. 6, p. 220.

WILLISTON, 5. W. 1908 The skull and extremities of Diplocaulus. Trans. Kans. Acad. Science, vol. 22, p. 122, pls. 1-5. Describes more fully Diplocaulus limbatus Cope, locates Diplocaulus in Microsauria. 1910 Dissorophus. Journ. Geol., vol. 18, p. 534. Giveslist of Permian amphibia of North America.

22, p. 243.

PLATE 1

EXPLANATION OF FIQURES

1 Dorsum of skull of Diplocaulus magnicornis Cope. Infraorbital canal a t

2 Ventral surface of the mandible. The operculo-mandibular canal a t the

3 Oblique view of one ramus of the mandible t o show the entire course of the

point of arrow. X +. point of the arrow.

operculo-mandibular canal. Nearly natural size.

X +.

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DIPLOCAULUS MAGN ICORNIS ROY L. MOODIE

JOURNAL OF YORPAOLOOY, VOL. 23. NO. 1

41

PLATE 1

PLATE 2

EXPLANATION OF FIGURES

4 Outline of the cranial elements of Diplocaulus magnicornis Cope. E, epiotic; F, frontal; J , jugal; Mx, maxilla; N, nasal; P, parietal; Pf, postfrontal; Po, post- orbital; Pr, prefrontal; Px, premaxilla; Qj, quadratojugal; So, supraoccipital; Sq, squamosal; St, supratemporal. 5 Mandible from the side to show arrangement of the operculo-mandibular

canal. 6 Outline of the openings and elements of the palate of the skull. Af, internal

nares; Co, condyle; Ep, epiotic; Es, auditory fossa or ear slit; Exo, exoccipital; Mx, maxilla; Pa, parasphenoid; Paf, palatine foramen; Pal, palatine; P1, ptery- goid; Px, premaxilla; &, quadrate; Qj, quadratojugal; T, ectopterygoid or trans- verse bone.

7 Side view of the skull. J , jugal; Mz, maxilla; Pr, prcfrontal; Px, premaxilln; Qj, quadratojugal; S?. squamosal.

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DIPLOCAULUS MAGNICORN I S 'ROY L. MOODIE

PLATE 2

5