The Cervical and Caudal Vertebrae of the Cryptodiran Turtle, Melolania Platyceps, from the Pleistocene of Lord Howe Island, Australia
Total Page:16
File Type:pdf, Size:1020Kb
AMERICAN MUSEUM Nornltates PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 2805, pp. 1-29, figs. 1-22, tables 1-3 January 30, 1985 The Cervical and Caudal Vertebrae of the Cryptodiran Turtle, Melolania platyceps, from the Pleistocene of Lord Howe Island, Australia EUGENE S. GAFFNEY' ABSTRACT Meiolania platyceps, a cryptodiran turtle from platyceps, but a conservative reconstruction based the Pleistocene of Lord Howe Island, Australia, on degree of serial variation and comparison with has cervical and caudal vertebrae exhibiting a Recent turtles, yields a total ofat least ten caudals number of phylogenetically interesting features. plus tail club. The caudals are all opisthocoelus The cervicals have fully formed central articula- and have well-developed haemal spines; charac- tions with a formula of (2( (3( (4) )5) )6) )7) )8), ters that are primitive for cryptodires. Meiolania interpreted as a synapomorphy of eucryptodires. has an ossified tail club consisting ofa conical layer Free ribs are present on cervicals two through six, of bone surrounding and fused to the terminal a retained primitive character in Meiolania, lost caudals. Proganochelys has a similar, but not iden- independently in other eucryptodires, baenids, and tical, terminal ossification, and the presence of a pleurodires. The presence in Meiolania ofcervical tail club is interpreted as a retained primitive fea- ribs articulating with paired intercentra shows that ture in Meiolania, being lost independently in the supposed vestigial ribs identified by previous pleurodires and other cryptodires. The tail of authors in the neck of Recent turtles are probably Meiolania platyceps has a series of ventrally in- intercentra and not rib remnants. The neural spines complete rings, contrasting with the complete rings of cervicals seven and eight articulate with the found in "Meiolania" oweni and Niolamia argen- nuchal bone; this is interpreted as an autapomor- tina. The vertebral features ofMeiolaniaplatyceps phy, occurring independently in chelonioids. are consistent with its hypothesized systematic po- Complete tails are unknown for Meiolania sition as a primitive eucryptodire. INTRODUCTION The cervical and caudal vertebrae ofMeio- and phylogenetically interesting features. The lania platyceps show a number of unusual well-developed cervical ribs and the tail club I Curator, Department of Vertebrate Paleontology, American Museum of Natural History. Copyright © American Museum of Natural History 1985 ISSN 0003-0082 / Price $2.90 2 AMERICAN MUSEUM NOVITATES NO. 2805 are found in the Triassic turtle, Proganoche- lys, and suggest the presence ofvery primitive chelonian features in this extinct turtle. The purpose of the present paper is to describe and systematically assess these and other fea- tures in the vertebral column of Meiolania platyceps. Further information on Meiolania platy- ceps, including geographic and geologic oc- currence, previous work, and cranial mor- phology can be found in Gaffihey (1983). Lists of specimens examined are also available in FIG. 1. Meiolaniaplatyceps, AM F:61105. Left, anterior view of atlantal centrum; right, posterior that paper. view of fused atlantal intercentrum and right at- lantal neural arch. See figure 5 for views of the ACKNOWLEDGMENTS articulated atlas. Much of this work was undertaken while I was a Visiting Curator at the Australian Mu- seum in 1980. I am indebted to that insti- F:5536 (centrum) and AM F: 18315 (centrum tution for its support and for the cooperation fragment; figured in Anderson, 1925, pl. 37, ofits staff, particularly Dr. Alex Ritchie. Dr. fig. 1). John Pickett, Mining and Geological Mu- As in other turtles (and amniotes generally) seum, Sydney, and Dr. Angela Milner, British the atlas of Meiolania (figs. 1, 5-8) consists Museum (Natural History) both aided me by of four elements: paired neural arches an- permitting me to study their collections. More terodorsally, an intercentrum anteroventral- extensive acknowledgments relating to the ly, and a centrum posteriorly. The Meiolania Meiolania project can be found in Gaffney atlas is generally similar to the generalized (1983). chelonian atlas, as in Proganochelys, but it This paper was supported in part by NSF differs in being somewhat shortened antero- grant DEB 8002885. Ms. Mary Jane Spring posteriorly and broadened laterally. did figures 5, 11, 14, 15, and 21. Most of the The chelonian neural arch (see Kasper, photographs were taken by me. Mr. Frank 1903), can be roughly divided into a dorsal Ippolito greatly enhanced the value of the portion covering the neural cord and having paper with his work on all the figures and photographs. ABBREVIATIONS AM, Australian Museum, Sydney BM(NH), British Museum (Natural History), Lon- don MM, Mining Museum, Sydney SMNS, Staatliches Museum fir Naturkunde, Stuttgart ATLAS The specimens available for the atlas are as follows: AM F:49141 (complete, articu- lated), MM F: 13825a (complete, articulated; figured in Owen, 1888, pls. 31, 32; Gaffney, 1983, figs. 42, 43), AM F:57984 (left arch, FIG. 2. Meiolania platyceps. Medial views of centrum fragment), AM F:61105 (arch and right atlantal neural arches, anterior to the left. intercentrum), AM F:18835 (arch), AM Left, AM F:57984; right, AM F:18835. Cu 00 o 4 0 0 z3 0 .C C) Cu cn Cu 0' o 03 w GMC Cu ,OU (A o 0 C) I - 0 0 C) C) 10'0. '0 -uCu C) '0 Cur 0 Cu Cu C) $- C) 0 - N. 0 C.) ni '0 o gr '0 04 0 ._ oC); m Co *1. -u 0~ '0 C O - Ed C) ~0 .eC) 10 '0 2 Cu (A 3 hIIt C) 83 _ 0 Q o Cu-_ '0 Cu3 0 Cu4 C) 0 0 en .-C.._ nu z C-4 o a toCu'34b (10 CA Cu 6I 0 0 r Cu Cu~~ ~ ~ ~ ~ ~ ~ (A 0 N~ ~ ~ ~ ~ ~ 0 04 0 t Cd N 0 CAC) C) C)~-. 0 I-4 Z(U 01 4 AMERICAN MUSEUM NOVITATES NO. 2805 a posteriorly directed postzygapophyseal Neural Spine flange articulating with the axis, and a ventral Postzygapophysis portion bearing articular facets for the occip- ital condyle, the atlantal centrum and the at- lantal intercentrum. In Meiolania platyceps Prezygapophysis (figs. 1, 2) the neural arch has a particularly broad dorsal portion that inclines medially to lie almost horizontally above the space for the neural cord. The neural arches do not Diapophysis meet in the midline in known specimens. The postzygapophysis has a well-developed artic- ular facet for the prezygapophysis ofthe axis. The very large proportion ofthe ventral part Parapophysis ofthe neural arch is occupied by the articular facet for the centrum of the atlas. This facet Centrum faces posteromedially and lies approximately at right angles to a smaller facet facing an- FIG. 3. Diagram showing morphologic fea- teromedially that articulates with the occip- tures of cervical vertebrae. Seventh cervical of ital condyle of the skull. Ventral to the facet Meiolania platyceps. with the centrum, the neural arch has a small contact area with the atlantal intercentrum. The lateral surface of the neural arch has a variation in other areas and I interpret this transverse process with a broad base but a as another example. relatively shallow lateral extension. The An unassociated neural arch, AM F: 18835 transverse process extends more than in Pro- (fig. 2) is not so easily interpreted as individ- ganochelys and most cryptodires but less than ual variation, but I see no other alternative. in pleurodires. This is a right neural arch, roughly similar to The transverse process of the neural arch the others but about a third narrower in lat- extends ventrally to a variable extent in the eral view. The dorsal process that extends direction of a short dorsal extension of the over the neural cord is a cylindrical process intercentrum. In MM F: 13825a (see Gaffney, rather than a broad plate as in the other spec- 1983, fig. 42, middle, for a stereophotograph) imens. The ventral portion of the arch has these processes join and enclose a small fo- the central and occipital facets but much of ramen, whereas in AM F:57984 (figs. 2, 5) the surrounding bone is reduced in compar- they do not meet but end in surfaces that ison to other neural arches. The arch is clearly appear to have been finished in cartilage. It Meiolania; it has no particular similarities to is possible that a cervical rib participates in sea turtles or pleurodires. It could be from a the ossification of this area but no evidence juvenile but the dorsoventral dimension is of a discrete element has been seen. the same as in AM F:57984. The contact between the neural arch and The intercentrum ofthe atlas in Meiolania intercentrum is fused on both sides of MM platyceps (figs. 1, 5) is a crescentic element, F:13825a and on the right side of AM connecting the bases ofthe neural arches, and F:61105 (fig. 1). The other specimens (in- forming the ventral third ofthe atlantal ring. cluding AM F:57984 and AM F:49141, which It articulates with the occipital condyle an- are both well preserved) show no indication teriorly and with the atlantal centrum pos- of fusion. Fusion ofthese elements occurs in teriorly. The intercentrum of Meiolania is pleurodires and trionychids and it may be narrow transversely, being distinctly wider interpreted as characteristic of certain taxa than long, in contrast with most turtles which within these groups, but I do not think its have a more equidimensional intercentrum. presence in Meiolania platyceps has system- The intercentrum of Proganochelys, how- atic significance. Meiolania platyceps is char- ever, is similar to Meiolania, suggesting that acterized by a high degree of morphologic this is the primitive chelonian condition. The 1985 GAFFNEY: MEIOLANIA PLATYCEPS 5 intercentrum is fused to the neural arch in Meiolania platyceps has well-developed MM F: 13825a and on the right side of AM cervical ribs (figs.