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Ameghiniana

versão On-line ISSN 1851-8044

Ameghiniana v.45 n.4 Buenos Aires dez. 2008

 

NOTA PALEONTOLÓGICA

"Chelys patagonica" from the Pliocene of Argentina: Not lost, not valid, not a chelid turtle

Walter G. Joyce1 and Cynthia Faux1,2

1Yale Peabody Museum of Natural History, 170 Whitney Avenue, New Haven, Connecticut 06511, USA. walter.joyce@yale.edu
2Department of Geological Science, University of Idaho, Moscow, ID 83844, USA.

Introduction

In 1923, Wieland briefly described in an addendum to the formal description of a new species of fossil turtle a fragmentary fossil from "well south in Patagonia" that had been presented to him as a gift by the Museo Nacional in Buenos Aires, Argentina. No definitive locality or age was available to Wieland (1923) beyond that it came from south of Bahía Blanca, Buenos Aires Province, and he assumed that the specimen came from the Tertiary of Patagonia. When describing this fossil, Wieland (1923) made comparisons with representatives of the land tortoise clade Testudinidae and with the extant matamata Chelus (also Chelys) fimbriatus of the clade Chelidae (all clade names used herein are sensu Joyce et al ., 2004). Although he explicitly noted several testudinid characters, Wieland (1923) nevertheless concluded that the fragmentary fossil might be a close relative of Chelus fimbriatus and provisionally suggested the name Chelys (?) patagonica. Neither a collection number, nor a figure, accompanied this description.
A few years later, Simpson (1942) noted that the fossil was not available for examination, thus hinting at the possibility that it had been lost. Furthermore, given that Wieland (1923) did not list any explicitly diagnostic characters and perhaps never intended Chelys patagonica to be an available designation, Simpson (1942) concluded that the name should be removed from all nomenclatural considerations. He furthermore noted that the few listed characters are more consistent with the specimen being a testudinid or would otherwise signify a highly deviant relative of the matamata. In his review of the fossil record of Chelus, Wood (1976) also noted the unavailability of Wieland's specimen, highlighted its vague provenance, and suggested that the referral to Chelus was indeed wrong, as Weiland (1923) himself was "uncertain as to the creature's proper generic allocation." Instead, he suggested that the fossil likely represents a testudinid, given that these are commonly found in the Miocene of Patagonia. Finally, Wood (1976) declared the name Chelys patagonica a nomen nudum, because Wieland (1923) only suggested the name for convenience.
Despite these doubts regarding its validity or affiliation with Chelus fimbriatus, Chelys patagonica has been featured in some faunal lists as a representative of Chelidae, most notably the authoritative summaries of fossil turtles provided by Kuhn (1964) and Mlynarski (1976). Although chances are slim, the possibility remains that Wieland's (1923) specimen indeed is a representative of the Chelus fimbriatus stem lineage and the only known find outside its usual Neogene range within the Amazonian sedimentary basin (Wood, 1976; Sánchez-Villagra et al., 1995). Similarly, possibilities remain that the suggested taxon name indeed designates a valid taxon, either of the Testudinidae or Chelidae. Fortunately, although this fossil apparently was not available for some time, it still safely resides at the Yale Peabody Museum of Natural History (YPM) and is available for study under the catalogue number YPM 351. The purpose of this short note is to figure and describe this specimen and to discuss its implications.

Description

The specimen is a partial carapace that is relatively uncrushed, although significant portions of the shell's surface have weathered, particularly in the anterior region. The edges of the specimen exhibit bone that is between 8 and 12 mm thick. In comparison to specimens of Chelonoidis carbonaria (YPM R 10640, R 10795, R 12237, R 13955, R 14343) and Chelonoidis denticulate (YPM R 11493, R 14341), we estimate this specimen to have had a carapace length of approximately 45 cm. The specimen consists of a fragment of the first neural, most of the right costals I-V, and the dorsal portions of the right peripherals IVII (figure 1). Although the outline of neural II generally appears octagonal, it is apparent that this element did not contact costal I, but rather costals II along a well-developed contact and III along a short contact and thus was hexagonal with short posterior sides. Similarly, given how strong the third costal expands medially, it is reasonable to speculate that the third neural was square. As such, the neural formula can reasonably be speculated to have been 6-6-4-?-?- ?-?. The sutures of the costals are not parallel, thus giving some, especially the third, a wedge shape. Laterally, costal I is in contact with peripherals I-III, costal II with peripheral IV and V, costal III with peripheral V only, costal IV with peripheral V and VI, and what is left of costal V with the remainders of peripheral VII. The peripherals are notably high. The only portion of the plastron that is preserved are the thin axillary and inguinal buttresses of the hyoplastron and hypoplastron, which articulate along the inner side of peripheral III and VII, respectively.


Figure 1. Photographs and line drawings of YPM 351, "Chelys patagonica WJ1." 1, 2, Dorsal view. 3, 4, Right lateral view. Abbreviations: ne = neural bone, co = costal bone, per = peripheral bone, Ve = vertebral scute, Pl = pleural scute, Ma = marginal scute. / Fotografías y dibujos de YPM 351. "Chelys patagonica WJ2." 1, 2, Vista dorsal. 3, 4, Vista lateral derecha. Abreviaturas: ne = hueso neural, co = hueso costal, per = hueso perisférico, Ve = escudo vertebral, Pl = escudo pleural, Ma = escudo marginal.

The scutes are represented by portions of the second vertebral, most of pleurals I-III, and most of marginals III-VIII (figure 1). All sulci are rather deep and straight. As in most turtles, vertebral I overlapped neurals I-III. This scute was hexagonal and had rather straight margins; however, given that much of the medial portions are missing, the proportions of this element remain unclear. Although this may be due to crushing, a slight grove coincides with the vertebral pleural sulcus thus giving the carapace a slight tricarinate appearance. The pleural I-II and pleural II-III sulci lie over costal peripheral II and IV, again as in most turtles. Finally, the pleural-marginal sulcus coincidences almost completely with the costal-peripheral suture.

Nomenclatural status of Chelys patagonica

The rules of the ICZN (1999) demand little from names published before 1931 to be considered available for nomenclatural considerations. In particular (Article 12, ICZN 1999), a name must be accomplished by a description, definition, or an indication. The first is clearly fulfilled by Wieland (1923) in his original description of Chelys patagonica. However, in addition, names must be used as valid when proposed (Article 11.5, ICZN 1999). When proposing the name, Wieland (1923: p. 14) explicitly stated Chelys patagonica was "a purely arbitrary name of convenience." Simpson (1942) believed Wieland did not consider Chelys (?) patagonica a formal designation and thus should be ignored. Unfortunately, it is impossible to know what Wieland intended, however, the question mark inserted within the name suggests a certain informality. We presume that Wieland indeed did not intend this to be a formal name and thus did not consider it valid. As such, we agree with Wood's (1976) assessment that this name should be considered neither available, nor valid. However, if our interpretation is incorrect and Wieland (1923) did intend the name to be valid, Chelys patagonica would have to be considered available. Nevertheless, given the highly fragmentary nature of the specimen and the lack of diagnostic characters, the name Chelys patagonica must be considered a nomen dubium. The name can therefore safely be disregarded from all future nomenclatural considerations.

Systematic identity of YPM 351

A series of characters clearly diagnose YPM 351 as being a representative of Testudinidae. In particular, YPM 351 exhibits alternating wedge shaped costals (already noted by Wieland, 1923), high and almost vertically standing bridge peripherals, strap-like buttresses that run along the visceral side of the peripherals, overlap of the pleural-marginal sulcus over the costal-peripheral suture, and deep and rather straight sulci with slightly raised edges. The radiating scutes Wieland (1923) noted as indicating a relationship with Chelus fimbriatus are clearly not developed. Instead, the specimen is smooth, as are most representatives of Testudinidae.
All extant South American representatives of the Testudinidae have traditionally been hypothesized to form a clade (e.g., Williams, 1950; Auffenberg, 1974), commonly referred to as Chelonoidis. Molecular phylogenies have recently corroborated this conclusion (Le et al ., 2006; Fritz and Bininda- Emonds, 2007). Conversely, all South American fossil tortoises found to date have been attributed to this clade as well (e.g., Auffenberg, 1971, 1974). Given the large variability that is apparent within Testudinidae in general, and Chelonoidis in particular, we find it difficult to find any characters that would unambiguously affiliate YPM 351 with Chelonoidis. However, given that no characters are present that contradict this assignment, we think it reasonable to assign YPM 351 to Chelonoidis based on geographic considerations.

Conclusions

The specimen described by Wieland (1923) under the name Chelys (?) patagonica is not lost, but rather still resides at the Yale Peabody Museum of Natural History. No significant observations can be added that would help clarify the provenience of the specimen beyond what was noted by Wieland (1923). However, a morphological analysis of the specimen clearly reveals that is does not represent a fossil of the stem lineage of Chelus fimbriatus, but rather a land tortoise of the Testudinidae, possibly Chelonoidis. It is unclear whether Wieland (1923) intended his suggested taxon name to be valid when he originally proposed it. Given that the taxon would be a nomen dubium even if the name were to be considered available, it is safe to disregard Wieland's Chelys patagonica from all future nomenclatural considerations.

Acknowledgements

We would acknowledge A. Logan for helping with photography and for producing the line art used herein. J. Sterli and M. de la Fuente provided helpful comments.

References

1. Auffenberg, W. 1971. A new fossil tortoise, with remarks on the origin of South American Testudines. Copeia 1971: 106-117.        [ Links ]

2. Auffenberg, W. 1974. Checklist of fossil land tortoises (Testudinidae). Bulletin of the Florida State Museum Biological Sciences 18: 121-251.        [ Links ]

3. Fritz, U. and Bininda-Emonds, O.R.P. 2007. When genes meet nomenclature: Tortoise phylogeny and the shifting generic concepts of Testudo and Geochelone. Zoology 110: 298-307.        [ Links ]

4. ICZN (International Commission on Zoological Nomenclature) 1999. International Code of Zoological Nomenclature, Fourth Edition. International Trust for Zoological Nomenclature. London, 306 pp.        [ Links ]

5. Joyce, W.G., Parham, J.F. and Gauthier, J.A. 2004. Developing a protocol for the conversion of rank-based taxon names to phylogenetically defined clade names, as exemplified by turtles. Journal of Paleontology 78: 989-1013.        [ Links ]

6. Kuhn, O. 1964. Fossilium Catalogus, Pars 107, Testudines. Ysel Press. Deventer, 299 pp.        [ Links ]

7. Le, M., Raxworthy, C.J., McCord, W.P. and Mertz, L. 2006. A molecular phylogeny of tortoises (Testudines: Testudinidae) based on mitochondrial and nuclear genes. Molecular Phylogenetics and Evolution 40: 517-531.        [ Links ]

8. Mlynarski, M. 1976. Handbuch der Paläoherpetologie, Teil 7, Testudines. Gustav Fischer Verlag. Stuttgart, 130 pp.        [ Links ]

9. Sanchez-Villagra, M.R., Linares, O.J. and Paolillo, A. 1995. Consideraciones sobre la sistemática de las Tortugas del género Chelus (Pleurodira: Chelidae) y nuevas evidencias fósiles del Mioceno de Colombia y Venezuela. Ameghiniana 32: 159-167.        [ Links ]

10. Simpson, G.G. 1942. A Miocene tortoise from Patagonia. American Museum Novitates 1209: 1-6.        [ Links ]

11. Wieland, G.R. 1923. A new Paraná pleurodiran. American Journal of Science 25: 1-14.        [ Links ]

12. Williams, E. 1950. Testudo cubensis and the evolution of western hemisphere tortoises. Bulletin of the American Museum of Natural History 95: 1-36.        [ Links ]

13. Wood, R.C. 1976. Two new species of Chelus (Testudines: Pleurodira) from the Late Tertiary of northern South America. Breviora 435: 1-26.        [ Links ]

Recibido: 4 de febrero de 2008.
Aceptado: 14 de julio de 2008.

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