p. 189-196, [5] leaves of plates : ill. ; 24 cm

Case, E. C. (Ermine Cowles), b. 1871.

American Museum of Natural History Scientific Publications

56.81.1DArticle XIX.- DESCRIPTION OF A SKELETON OFDIMETRODON INCISIVUIS COPE.BY E. C. CASE.PLATES XV-XIX.This specimen, No. 4636, Am. Mus. Nat. Hist., was found by theauthor in 1906, in the red clay on the south side of Godlin Creek in ArcherCo., Texas. It consists of a skull and lower jaws, the vertebral columncomplete as far as the seventh caudal, the pelvis, femur, tibia and fibula ofthe right side and most of the ribs. The vertebral column was in position,while the pelvis, limb bones, ribs and skull were separated. The skull, asis common in the fossils of the genus, was broken in the median line andthe right and left halves were somewhat separated. The condition of thebones and the careful work of the preparator, Mr. Hermann, has made itpossible to restore the skull and vertebral column in proper position andpractically free from distortion. This is one of the rare occasions where afossil has been discovered in the Texas beds not distorted and with thebones in position. The happy chance which has preserved the skull soperfectly makes it especially valuable as it is possible to give the proportionsand shape very exactly and to add several new points in the morphology.As previously shown by the author,' the skull is very high in the facialregion, with narrow, flat frontal and parietal regions and the posteriorsurface falling off very steeply but not vertically. In general the bones ofthe skull are as already described and figured.2The bones of the brain case have been crushed somewhat to the leftand the paroccipital slightly displaced; flat facets on the lower part of thesquamosals (postparietals, suggested by Broom) indicate where they werenormally attached.A separate element homologous with that described by Cope and Caseas a stapes in Theropleura retroversa is considered as the stapes of Dimetro-don. Dr. Broom, from analogy with elements of similar position in thel Publication 55, Carnegie Institution, Washington, 1907.2 In working on this specimen I was so fortunate as to have the opportunity to go over thematerial with Dr. R. Broom, the able authority on the Permian reptiles of South Africa, and aperfect agreement was reached as to the position and relations of the bones; differences remainedin one or two instances as to the proper names and homologies of certain elements, as notedin the text.189Bulletin American Museum ofNatural History. [Val. XXCIV,-South African forms, would call this a tympanic, an element reaching fromthe exoccipital to the quadrate and leavingabove it an opening in which he would expectto find the distal end of a stapes. He would1<- regard the bones identified as stapes in Labido-saurus and Naosaurus (Edaphosaurus) as thetympanic also.The basisphenoid has, on each side, descend-ing from the outer edge of the lower surface, aflange-like process with an articular face onthe outer surface. It is to this that the headof the stapes was applied, with the outer endfitting against the inner side of the quadrate.In form the stapes resembles the expandedwing of a bird. The proximal end, correspond-Fig. 1. Stapes of Dimetrodon ...incUivus. X i nat. size. ing to the base of the wing, is divided by a deepgroove into a circular face, which is appliedto the face on the basisphenoid, and an elongate face on the portion of thestill thickened edge which lies below and behind the circular face. Justbelow the circular face the bone is pierced obliquely by a large foramen.The main portion of the bone is expanded and proportionately thinner, theanterior edge is convex forward and rather heavy; the tip is broken off sothat it is impossible to give the exact length, but it evidently reached nearlyto the posterior edge of the quadrate. It seems to have been applied to thequadrate for a considerable portion of its length.The quadrate extends far forward and is overlapped for a considerabledistance by the pterygoid which lies on its inner side.The position and relations of the pterygoid are shown to be different fromprevious interpretations in one or two important points. The strong externalprocess, with its single row of teeth in sockets on the lower surface is placedobliquely to the jaw instead of horizontally; the flat outer faces are not on aplane with the side of the face but are inclined inwards, correspondingly thelower, tooth bearing edge, which is at a right angle to the outer face, isinclined upward and inward. The lower edges of the processes are thusinclined toward each other forming a space like an inverted V instead ofbeing on a level.In most reptiles the basipterygoid processes of the basisphenoid areapplied to the pterygoid near the middle point and opposite the inner endof the external process of the pterygoid when such a process is present. InDimetrodon the basisphenoid is short and the middle point of the pterygoidsfar in advance, so there is a considerable interval between them. Well back190--1mbstq~~~~~~~~~~~~~-qFig. 2. Dimetrodon incisivus, inferior aspect of skull, slightly less than one half naturalsize. * facet on basisphenoid. for head of stapes, bpt; basipterygoid process; I.t. f., lateraltemporal fenestra.Iz.-I--PMX:F-WS.Isoli.7 \\,\VBulletin American Museum of Natural History. [Vol. XXVIII,of the external process, the slender epipterygoids are applied to the innerside of the pterygoid, not resting upon them as is usual; the lower end isheavier and broader and is fitted into a shallow groove on the pterygoid;in this thick lower end there is a flat facet which articulated with the basi-pterygoid process of the basisphenoid. This condition is verv peculiar, forin the Cotylosauria,t Rhyncocephalia (Sphenodon), most Lacertilia andDinosaurs, as Allosaurus and Tyrannosaurus, the basisphenoid articulatesdirectly with the pterygoid at or near a point opposite the external process.Above the pterygoid the epipterygoid is very slender and its upper end is incontact with the lower surface of the postorbital.Anterior to the external process the pterygoids are flat, horizontaleptIV~~~~~~~~~~~~~~~~~-P_ ~ PFig. 3. Dirnetrodon incisirus. Inner view of pterygoquadrate region with the bonesnearly in the natural vertical position. X i nat. size. t facet for basipterygoid process. Theportion of the pterygoid below the letters pt is horizontal; the part above rises vertically fromthe inner edge of the horizontal portion.plates; these approach in the median line but are separated by a considerableinterval. From the inner edges of the pterygoids rise vertical plates whichare well shown on the left side. These rise very gradually from the posteriorend but at about the middle of the anterior part of the pterygoid they risevery abruptly to the full height. The posterior edge of the vertical plate isconcave forward and the whole plate is of little anteroposterior extent.Previous description 2 shows that these plates united anteriorly or were veryclosely pressed together and that they were in contact with tht prevomers1 In Diadectes there is at bone seemingly overlying the pterygoid posteriorly, its Limits areuncertain and it may be a part of the pterygoid but if it is an epipterygoid appearing on theunder surface of the pterygoid it would bas-e the same relations as in Dimetrodon, for the basi-sphenoid articulates with it.2 Carnegie Publication 55, p. 104.192Case, Skeleton of Dimetrodon incisivus.below, anterior to the pterygoids. Two small fragments of bone whichcan not be placed in the skull are probably the anterior ends of these platesor of the prevomers and fitted close against the lower side of the premaxil-laries. The vertical plates of the pterygoids and the ethmoid formed themedian septum of the skull.The temporal region is as described previously, except that there is in thisspecimen no supratemporal opening. This seems to have been a variablecharacter in Dimetrodon. Some skulls show a definite elongate openingof good size (1002 Univ. of Chicago), others show an indefinite opening andthe bones with very thin edges (1001 Univ. of Chicago), in others the opening9'/1oilFig. 4. Dimetrodon incisivwPSgi~~~~~~~~~~pto3, posterior aspect of skull. Slightly less than one half naturalseems to be closed completelv. In this specimen there is an opening at theproper place but it has everv appearance of being due to the displacement ofthe bones due to crushing. The attachment of the bones was weak at best;an opening was either imminent or just disappearing.The axial skeleton offers little new. The exact number of presacralvertebrae is shown to be twenty-seven. The spines of the third to the seventhvertebrae are cupped at the distal end showing cartilaginous attachment.The anterior cervical ribs are double headed with large capitulum andtllberculum, the separation of these graduallv decreases until on the four-size.1910.] 193Bulletin American Museum of Natural History. [Vol. XXVIII,teenth, the separation between the two is no longer distinct and the wholehead of the rib is in contact with the vertebrae. The last free rib is attachedto the twenty-third or twenty-fourth vertebra, after this the ribs are fusedwith the transverse processes.Discusion of the skull.The condition of the temporal region in the skull suggests several impor-tant considerations.In such forms as Captorhinus and Labidosaurus the roof of the skull isformed posteriorlv by the parietals, frontals, postfrontals, postorbitals andtwo lateral elements, an upper, squamosal, and a lower, prosquamosal orquadratojugal. On the posterior surface there are three: a small pair, thetabulare ("epiotic," this name should be abandoned as it suggests a con-nection with the otic region which does not exist), a pair of median plates,the supraoccipitals, and an outer pair uniting with the supraoccipitals above,the squamosals, quadrates, prosquamosals in front, and the paroccipitalsbehind. This last pair has beenzP fi referred to by me in a previousdescription of Dimetrodon as thequadratojugals, because of its rela-/ i~iI~I11,,I~t~iI \ tion to the quadrate and becauseex.o0 there is a quadrate foramen sepa-pfg o Mrating it from the quadrate.Dr. Williston has objected toFig. 5. Captorhinus angusticep8. Posterior calling this element the quadrato-aspect of skull of No. 4457, A. M. N. H. jugal because of its relations to theparoccipital. In Dimetrodon thecondition is as shown in the figure and as previously described. The desig-nation prosquamosal and quadratojugal being retained provisionally for themoment, it will be seen that the bones have essentially the same position asin Captorhinus and Labidosauru with the exception of the lost tabulare.The supraoccipitals (not the dermal supraoccipital plates) have united withthe exoccipitals and paroccipitals to form a posterior plate. The squamosalslie behind the lower process of the parietals and unite with a lower pair ofelements exactly as in Captorhinus. Dr. Brooni suggests that these arepostparietals, homologizing them with the elements here called supraoe-cipital plates in Captorhinus, and would call the element here called pro-squamosal, the squamosal. It seems to me much more probable that theyare squamosals. The close relation of the primitive reptilia of the TexasPermian or Carboniferous to the higher amphibia warrants homologizing194Case, Skeleton of Dimetrodon incisivus.the posterior plates with the supraoccipital plates of the Stegocephalia ratherthan with postparietals, elements which occur in the African forms of un-doubtedly higher position and which have never been definitely determinedin the American forms. With the development of the supraoccipital bonedistinct from the supraoccipital dermal plates of the Stegocephalia and suchforms as Captorhinus, the need for such plates would disappear and theywould disappear with the need. This would leave the squamosals as theposterior lateral elements having the relations seen in Dimetrodon and I amdisposed to retain the name.With regard to the inferior pair of posterior elements there is perfectagreement among Drs. Broom, Williston and myself regarding their identityand relations. Now as to the proper name. It has been called quadrato-jugal by me because it lies outside of the quadrate, articulating with itsposterior edge and is in part separated by a foramen, the quadrate foramen.It is true that this foramen carries no blood vessels or nerves but it is aspersistent as the temporal fenestrse, which carry no vessels or nerves, and isentitled to as much weight in any natural classification. In the Dinosauirs,Allosaurus and Creosaurus there is a similar element which is only partlyseparated from the squamosal above, the suture runs part way between thebones and then disappears. In the Dinosaur Tyrannosaurus the suture andseparation are complete. The relation of the element to the quadrate isexactly as in Dirnetrodon. In all of these forms the paroccipital comes incontact with this element and in Tyrannosaurus with the parietal as well.So the condition in Captorhinus and Labidosauruus is not unique.There is no inherent improbability that the element cofnnectino thequadrate with the jugal is the prosquamosal (supratemporal), indeed ifthe argumenits offered above have weight it must be so by exclusion.Other facts, however, may be cited in favor of this idea. The temporalregion in the Stegocephalia is covered by the parietal, squamosal, prosqua-mosal, quadratojugal, and jugal. The prosquamosal is commonly thelargest element of the temporal region; it articulates with the postorbitalanteriorly, with the jugal below, and in many forms, as Branchiosaurus, etc.,posteriorly, with the quadratojuigal; it overlies the quadrate. The quadrato-jugal lies oii the outer side of the quadrate and frequently extends up behindit; the portion which extenids up behind the quadrate articulates with theposterior edge of the prosquamosal. This is exactly the relation of the outerpair of posterior elements in Captorhinus and the Pelycosauria.If we assume that this element covering the posterior and outer surfaceof the quadrate in Captorhinus, Diadectes and the Pelycosauria, and sepa-rated from the quadrate by a foramen in the last two, is the quadratojugal,then we have in the temporal region the full number of bones of the Stego-1951910.]196 Bulletin A*mercan Museum of Natural History. [Vol. XXVIII.cephalian skull, and this bone is, in many forms, where it articulates directlywith the jugal (Rhyncocephalia, Theropodous Dinosaurs and Phytosaurs)unhesitatingly called the quadratojugal. If, on the other hand, we assumethis bone to be a new element and the bone anterior to it in Pelycosaurs andCotylosaurs to be the quadratojugal, the skulls are radically different fromthe Stegocephalian skulls, having one more element on the posterior surfaceof the skull, and one less in the temporal region.The condition of the Pelycosaurian skull gives a hint as to the conditionof the skull of Sphenodon. Baur claimed that in Sphenodon the squamosaland prosquamosal had united in a single element, but Swinnerton andHowse found no trace of any division of this element in even the youngestembryos. Is it not just as probable that the prosquiamosal dropped downto the lower edge of the skull and formed the connecting link between thejugal and quadratojugal as that it rose and fused with the squamosal? InPalceohatteria the jugal joins the quadrate directly' but the squamosal (pr-squamosal?) projects down nearly between them. The quadratojugal. fpresent, was on the back of the skull. As the Cotylosauria can no longerconsidered as the primitive reptilian type, and, as the Rhyncocephaliano longer be considered as derived from them by the simple development ftemporal openings, the necessity of finding a prosquamosal bone in the skullof the Rhyncocephalia has passed.The more definitely the anatomy of the primitive reptiles is known themore apparent it becomes that the theory which places the Rhyncocephaliaas a central type directly derived from the Cotylosauria and giving rise totwo main branches of Reptilia is inadequate and must undergo a seriousreexamination.ICredner Zeitsch. d. Deutsch. Geol. Gesell., Jahrg. 1888, s. 546.¢F-P-.i¢picqc13cdcz4-60FQ¢VOL. XXVIII, PLATE XVI.Dimetrodorn incisivus. Superior aspect. Less than j nat. size.BULLETiN A. M. N. H.VOL. XXVIII, PLATE XVII.Dimetrodon incisivus. Inferior aspect of skull. Less than i nat. size.BULLETIN A. M. N. H.91-4IP..4m1-4-.4.4NP..Alx.40*4c)~04.;0c3~4(000¢z000"U 1jSrDE37utSErofA-0{DT4ffk-w l X-at ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~O vts^Z ^ .it;'f ~~~~~~~~~Xi F v '-~~~~~~~~~~~~~~~~~~1. . .0 - .,1 =~~~~~~~~~~~~~~~~~~''''=Sz-=w-4,;< .<_-. :-,Sad:- - aRD i o~~~~CCt

Skeleton of Dimetrodon incisivus

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