Reevaluación del holotipo de Plesiosaurus (Polyptychodon) mexicanus Wieland, 1910 del ? Jurásico superior de México: un thalalattosuchio y no un sauropterygio
Marie–Céline Buchy
Museo del Desierto, Prolongación Pérez Treviño 3745, Centro Metropolitano Parque Las Maravillas, 25015 Saltillo, Coahuila, México. mcbuchy@gmail.com
Manuscript received: February 14, 2008
Corrected manuscript received: April 15, 2008 ]]>
Manuscript accepted: August 25, 2008
ABSTRACT
The holotype of the first sauropterygian described from Mexico, Plesiosaurus mexicanus Wieland, 1910, was recently rediscovered and is here described and discussed for the first time in almost a century. The holotype specimen belongs to an indeterminate metriorhynchine crocodyliform. Doubts remain as to the stratigraphic origin of the specimen, whether Upper Jurassic or Lower Cretaceous. It adds a specimen to the recently recognised wealth of the Mexican Gulf in thalattosuchians.
Key words: Thalattosuchia, Metriorhynchinae, Late Jurassic, Early Cretaceous, Mexico.
RESUMEN
Se realiza la redescripción y discusión del primer sauropterigio descrito para México, Plesiosaurus mexicanus Wieland, 1910, después de casi un siglo de su descubrimiento. El holotipo corresponde a un cocodrilo metriorinquido indeterminado. Existen dudas acerca del origen de los datos estratigráficos referentes al espécimen y puede asignarse al Jurásico Tardío o Cretácico Temprano. Este espécimen incrementa la riqueza del grupo Thalattosuchia reportada recientemente para el Golfo mexicano.
Palabras clave: Thalattosuchia, Metriorhynchinae, Jurásico Tardío, Cretácico Temprano, México.
]]> INTRODUCTION
On March 3rd, 1909, the great Yale palaeobotanist G. R. Wieland (1865–1963) embarked in Mexico City for a field trip to the region of Tlaxiaco, State of Oaxaca, Mexico (Figures 1,2). According to his field book (Figure 2), he was accompanied by Ygnacio S. Bonillas (1886/7–?), recently graduated in geology from the Massachussets Institute of Technology in Boston, the son of Ygnacio Bonillas (1858–1944), a geologist himself, future ambassador from Mexico in Washington and future presidential candidate for Mexico (CallahanHedden, personal communication). Wieland and Bonillas reached the train station at Parian, Oaxaca, on the evening of March 4th; then continued overland for three days, reaching Tlaxiaco in the afternoon of March 7th. The expedition appears to have been uneventful as dryly related by Wieland, until the discovery of cycads on March 13th, except for the discovery on March 8th of a partial rostrum with teeth, which Wieland immediately recognised as the first ichthyosaur or plesiosaur described from Mexico (Figure 2).
The following year, Wieland formally named the very first sauropterygian from Mexico (actually the first new fossil reptile taxon according to Reynoso, 2006) upon that specimen, as Plesiosaurus (Polyptychodon) mexicanus Wieland, 1910. Wieland had revisited the site on March 16th and determined the probable origin of the specimen within the briefly described section as it appears in the publication (Wieland, 1910: 361–362). The field book mentions 'Ammonites' and 'trigonal Oysters', without field determination nor the mention that samples were taken for further identification, in which case this material appears now lost. In his publication, Wieland (1910: 362) refers to the ammonites described by 'Felix y Lenk' (1891) to attribute a Neocomian or slightly older age to the rostrum (in fact, the reference is Felix [1891]). No ammonite is illustrated together with the specimen, no taxon name is given, and the finding area is geologically complex (e.g., Ferrusquia–Villafranca, 1993). Hay (1930: 116) referred the specimen to the Late Jurassic without further justification and, indeed, regional geology leaves that possibility open. Only further local geological studies (with the caveat that the 'way to Putla' [Wieland, 1910: 361] likely was renamed since 1909; however, Wieland [1910] gives an account ofthe section at the locality, and mentions quarries, possibly in relation to his find) and possibly examination ofthe remaining matrix may contradict Wieland's age determination ofthe specimen.
Welles (1952: 106) was the last expert to discuss the specimen (not having seen it though) who, mislead by the erroneous interpretation of the remains given by Wieland (1910: PI. LII), regarded it as anelasmosaur, whose remains, although 'unsatisfactory' are not 'indeterminate and topotypic material could establish the species'.
The long–lost holotype (Buchy etal, 2006a; Reynoso, 2006) was recently rediscovered at the Instituto de Geología of the Universidad Nacional Autónoma de México (UN AM) and a cast sent for study to the Museo del Desierto. Saltillo, Coahuila, Mexico. The holotype is here re–described and referred to an indeterminate metriorhynchine crocodyliform.
SYSTEMATIC PALAEONTOLOGY
]]> Metriorhynchinae indet.Infraorder Thalattosuchia Fraas, 1901
Family Metriorhynchidae Fitzinger, 1843
Subfamily Metriorhynchinae Fitzinger, 1843
1910 Plesiosaurus {Polyptychodon) mexicanus Wieland, p. 361, pl. LII
1930 Plesiosaurus (Polyptychodon) mexicanus Burckhardt, p. 99
1930 Plesiosaurus mexicanus Hay, p. 116
1935 Plesiosaurus mexicanus Kuhn, p. 67
1952 "Plesiosaurus" mexicanus Wieland, 1910, nomenvanum; Welles, pp. 106–107
1963 Plesiosaurus mexicanus Persson, pp. 38, 43
2003 Plesiosaurus (Polyptychodon?) mexicanus Buchy et al, p. 276
2006a Plesiosaurus (Polyptychodon?) mexicanus Buchy et al., p. 242
]]> 2006 Plesiosaurus mexicanus Reynoso, pp. 211, 216, 218, 222, nomen dubium p. 221Material. The specimen is housed at the Instituto de Geología of the UN AM (Colección Nacional de Paleontología) under accession number IGM 9026. It represents a portion of occluding mandibular and maxillary rostra with teeth, better visible in right lateral view (note that in pl. LII of Wieland, 1910, the specimen is upside down). The left side of the specimen requires preparation; it seems that the rostrum was split longitudinally at some point, as no tooth (nor tooth root) attributable to the left series is visible.
Origin. According to Wieland (1910: 361–362), the specimen was collected by himself on March, 8th, 1909, atapoint north of the way to Putla, about 6 km south–southwest of Tlaxiaco, Oaxaca, Mexico (Figures 1, 2).
Preservation. The specimen probably represents a fragment caudal to the mandibular symphysis, although preparation of the left side is necessary to confirm it. A poorly preserved bone fragment ventral to the right mandible might represent a portion of the left mandibular ramus. Both rostral and caudal extremities of the specimen are subvertical 'fresh' breaks, as was recognized by Wieland, who could not find more pieces (1910: 362). The rostrocaudal length of the specimen is 90 mm; its rostral height is 75 mm, its caudal height, 80 mm.
The maxilla preserves five teeth, regularly alternating with six dentary teeth (of which the rostral– and caudal–most ones are better visible along the breaks). The tooth outlined and labeled 'b' by Wieland (1910: pl. LII) is not a tooth but a triangular area of bone bounded by two breaks.
As mentioned by Wieland (1910), the bone surface and teeth are poorly preserved. However, tooth ornamentation is clearly visible where enamel is preserved.
Description (Figure 3).
Maxilla. The dorsal margin of the maxilla parallels its ventral margin. Little of the original surface of the bone is preserved, and no ornamentation pattern can be deduced.
Mandible. As far as can be judged, the dorsal margin of the dentary parallels the ventral margin of the maxilla. A seam of sediment raised against the weathered surrounding bone surface runs sub–horizontally at about mid–height of the dentary. This likely represents the infill of the groove described on the lateral surface of the dentary and surangular e.g., by Frey et al. (2002) in Geosaurus vignaudi and by Gasparini et al. (2006) in Dakosaurus andiniensis. According to the data provided by Gasparini et al. (2006, online supporting material), this surangulodentary groove appears to be present in Pelagosaurus typus (see also Pierce and Benton, 2006), but absent in Steneosaurus bollensis, Metriorhynchus superciliosus, M. casamiquelai and Geosaurus araucanensis. However, Young (2006; pers. com.) confirms the presence of the groove in all examined specimens adequately preserved attributed to Steneosaurus, Teleosaurus, Metriorhynchus and Geosaurus. The groove is present in Dakosaurus where it terminates rostrally as a foramen within the dentary (Gasparini et al., 2006; Young, 2006, pers. com.). A similarly situated groove, although shorter both rostrally and caudally and lacking a rostral foramen, is present in Recent crocodyliformes, and houses a cutaneous branch of the mandibular nerve (Salisbury, pers. com.; see Holliday and Witmer, 2007).
If present, the splenial cannot be distinguished from the dentary due to poor surface preservation.
]]> Dentition. Tooth implantation is thecodont, as is visible along the breaks. The right dentary tooth visible along the rostral–most break preserves a replacement tooth within the pulp cavity (as is also visible in pl. LII of Wieland, 1910, tooth 'a')– Replacement is therefore vertical. As far as can be judged, the teeth are subcircular in basal cross–section. The tooth crowns are almost straight (the dentary ones more so than the maxillary ones), while the roots are curved. Possibly, the distal convexity of especially the maxillary teeth as is now visible results from mediolateral compression, and the maxillary teeth were originally convex more labially than distally. The preserved replacement tooth as well as the fourth preserved maxillary tooth exhibit a discreet distal carina, which, at least in the case of the replacement tooth (which preserves the only intact apex), runs until the apex. Ornamentation wherever enamel is present is made up of fine ridges, irregular in extent and spacing. On the replacement tooth, few irregular ridges reach the apex.
DISCUSSION
Identification
The tooth replacement pattern proves the specimen belongs to a crocodyliform, and not a sauropterygian. The architecture of the rostrum and constituting bones, as well as the tooth morphology clearly designate a metriorhynchine thalattosuchian. The tooth ornamentation is reminiscent of longirostrine members of the genus Metriorhynchus(e.g., Vignaud, 1995, 1997; pers. obs.)– However, dentition of those species referred to Geosaurus known from the Late Jurassic Mexican Gulf is poorly known (see a review in Buchy, 2007); pending further discoveries of more complete specimens, IGM 9026 is therefore considered an indeterminate metriorhynchine.
Historical note
Wieland (1910) claims IGM 9026 to be the first determinable reptile remains from Mexico (most likely meaning of large Mesozoic reptiles; see also Reynoso, 2006). Wieland (1910: 361) notes that Mr. Aguilera, director of the Instituto Geológico Nacional, had previously found in Chihuahua a weathered vertebral centrum attributable to a' reptile', from layers equivalent to the North American Pierre Formation; this reference could not be found and is ignored in recent listing of reports of historical finds of Mexican Mesozoic reptiles(e.g. see Reynoso, 2006; it may refer to a personal or oral communication). Plesiosaurus mexicanus was long considered the first (and only) sauropterygian described from the country(e.g., Buchy et al, 2003). IGM 9026 actually represents the first Mexican thalattosuchian (see Gasparini, 1992; Frey et al, 2002). The partial dentary mentioned by Ferrusquia–Villafranca and Comas (1980) attributed to an indeterminate pliosaur represents the first mention of a sauropterygian from Mexico, while the giant pliosaur first described by Hahnel (1988) as a dinosaur and reassessed by Buchy et al (2003) is the second of the currently known seven Mexican pliosaurs and few poorly diagnostic elasmosaurs (Buchy, 2007).
Thalattosuchians of the Late Jurassic Mexican Gulf
The wealth of thalattosuchians of the Late Jurassic (and possibly Early Cretaceous) Mexican Gulf has only been recently revealed (Frey et al, 2002; Buchy et al, 2006b, 2006b, 2006c, 2006d; Buchy, 2007, 2008). Additional specimens awaiting preparation and study most likely will add at least one taxon to the assemblage (Buchy, 2007), while Upper Jurassic and Lower Cretaceous outcrops of Mexico still consist of a wide, unexplored territory for marine reptiles in general.
As emphasised by the indeterminate status of the holotype of P. mexicanus, taxonomy and affinities of Mexican thalattosuchians can only be resolved when material is prepared and proper studies are completed. In turn, then, their bearing upon the elucidation of origin and affinities of the Mexican marine reptiles assemblage and palaeobiogeography could be evaluated.
]]>ACKNOWLEDGEMENTS
M.C. Perrilliat (UNAM) found the lost holotype and provided an excellent cast, photos and data. For assistance during the quest, for supplying literature and data, access to specimens, casting, discussion, comments, general support, etc. thanks are due to the Museo del Desierto team, G. Alvarez Reyes and A.B. Villaseñor Martinez (UNAM), B. Andres, W Joyce, L. Klise and M.A. Turner (all Yale), K. Callahan Hedden (Annapolis), M. Everhart (Hays), R. Guzman (Aguascalientes), C. Ifrim (Karlsruhe), J.G. López Oliva (Linares), S.W. Salisbury (Brisbane), A.S. Smith (Dublin), K.T. Smith (Austin), P. Vignaud (Poitiers), M.T. Young (Bristol). Thanks to the reviewers J.I. Kirkland (Salt Lake City) and F.J. Vega (UNAM). The final version owes much to the efforts of the Scientific Editor T. Lawton (NMSU). For financial support via the DFG (grants number FR1314/6–1, 1314/7–1, 1314/9–1 and 9–2) and Museo del Desierto, thanks are due to E. Frey (Karlsruhe), A.H. González González (Museo del Desierto) and W. Stinnesbeck (Heidelberg).
REFERENCES
Buchy, M.–C., 2007, Mesozoic marine reptiles from north–east Mexico: description, systematics, assemblages and palaeo–biogeography: University of Karlsruhe, Germany, PhD thesis, 98 p. Available online at <http://digbib.ubka.uni-karlsruhe.de/volltexte/1000007307>. [ Links ]
Buchy, M.–C., 2008, New occurrence of the genus Dakosaurus (Reptilia, Thalattosuchia) in the Upper Jurassic of north–eastern Mexico with comments upon skull architecture of Dakosaurus and Geosaurus: Neues Jahrbuch für Geologie und Palaontologie, 249(1), 1–8. [ Links ]
Buchy, M.–C., Frey, E., Stinnesbeck, W., López–Oliva, J.G., 2003, First occurrence of a gigantic pliosaurid plesiosaur in the Late Jurassic (Kimmeridgian) of Mexico: Bulletin de la Société géologique de France, 174(3), 271–278. [ Links ]
Buchy, M.–C., Frey, E., Salisbury, S.W., Stinnesbeck, W., López–Oliva, J.G., Gótte, M., 2006a, An unusual pliosaur (Reptilia, Sauropterygia) from the Kimmeridgian (Upper Jurassic) of northeastern Mexico: Neues Jahrbuch für Geologie und Palaontologie, 240(2), 241–270. [ Links ]
Buchy, M.–C., Frey, E., Stinnesbeck, W., González–González, A.H., 2006b, A new Tithonian (Upper Jurassic) marine vertebrate concentration Lagerstatte in north–eastern Mexico: Hantkeniana, 5, 17–19. [ Links ]
Buchy, M.–C., Frey, E., Stinnesbeck, W., López–Oliva, J.G., 2006c, An annotated catalogue of the Upper Jurassic (Kimmeridgian and Tithonian) marine reptiles in the collections of the Universidad Autónoma de Nuevo León, Facultad de Ciencias de la Tierra, Linares, Mexico: Oryctos, 6, 1–18. [ Links ]
Buchy, M.–C., Vignaud, P., Frey, E., Stinnesbeck, W., González–González, A.H., 2006d, A new thalattosuchian crocodyliform from the Tithonian (Upper Jurassic) of north–eastern Mexico: Comptes Rendus Palevol, 5(6), 785–794. [ Links ]
Burckhardt, C., 1930, Etude synthétique sur le Mésozoíque mexicain: Société Paléontologique Suisse, 49/50, 1–280. [ Links ]
Felix, J., 1891, Versteinerungen aus der mexicanischen Jura– und Kreide– Formation: Palaeontographica, 37, 140–194. [ Links ]
Felix, J., Lenk, H., 1891, Uebersicht über die geologischen Verhaltnisse des mexicanischen Staates Puebla: Palaeontographica, 37, 117– 139. [ Links ]
Ferrusquía–Villafranca, J., 1993, Geology of Mexico: a synopsis, in Ramamoorthy, T.P., Bye, R., Lot, A., Fa, J. (eds.), Biological Diversity of Mexico: origins and distributions: Oxford University Press, 3–107. [ Links ]
Ferrusquía–Villafranca, I., Comas, O., 1980, Reptiles marinos mesozoicos en el Sureste de México y su significación geológica–paleontológica, in V Convención Geológica Nacional, Sociedad Geológica Mexicana, 104–105. [ Links ]
Fitzinger, L.J.F.J, 1843, Systema Reptilium, Fasciculus primus, Amblygossae: Wien, Braumüller et Seidel, 106 p. [ Links ]
Fraas, E., 1901, Die Meerkrokodile (Thalattosuchian.g.) eine Sauriergruppe der Juraformation: Jahreshefte des Vereins für vaterlandische Naturkunde Württemberg, 57, 409–418. [ Links ]
Frey, E., Buchy, M.–C., Stinnesbeck, W., López–Oliva, J.G., 2002, Geosaurus vignaudi n. sp. (Crocodyliformes: Thalattosuchia), first evidence of metriorhynchid crocodilians in the Late Jurassic (Tithonian) of central–east Mexico (Puebla): Canadian Journal of Earth Sciences, 39(10), 1467–1483. [ Links ]
Gasparini, Z., 1992, Marine reptiles of the Circum–Pacific region, in Westermann, G.E.G. (ed.), The Jurassic of the Circum–Pacific: Cambridge, U.K., Cambridge University Press, Series World and Regional Geology 3, 361–364. [ Links ]
Gasparini, Z., Pol, D., Spalletti, L.A., 2006, An unusual marine crocodyliform from the Jurassic–Cretaceous boundary of Patagonia: Science, 311, 70–73. [ Links ]
Hahnel, W., 1988, Hallazgo de restos de dinosaurio en Aramberri, N.L., México: Actas de la Facultad de Ciencias de la Tierra, U. A.N.L., 3, 245–250. [ Links ]
Hay, O.P., 1930, Second bibliography and catalogue of the fossil Vertebrata of North America: Carnegie Institution of Washington, Publication 390, 1–1074. [ Links ]
Holliday, C.M., Witmer, L.M., 2007, Archosaur adductor chamber evolution: integration of musculoskeletal and topological criteria in jaw muscle homology: Journal of Morphology, 268, 457–484. [ Links ]
Instituto Nacional de Estadística Geografía e Informática, INEGI, 1992, Carta Topográfica Oaxaca E14–9, escala 1:250,000: Aguascalientes, Ags., México, Secretaría de Programación y Presupuesto, Instituto Nacional de Estadística, Geografía e Informática, 1 mapa. [ Links ]
Kuhn, O., 1935, Fossilium catalogus, 1. Animalia, Part 69. Sauropterygia: Den Haag, Junk / Ascher, 127 p. [ Links ]
Persson, P.O., 1963, A revision of the classification of the Plesiosauria with a synopsis of the stratigraphical and geographical distribution of the group: Lunds Unveristets Arsskrift, 59, 1–59. [ Links ]
Pierce, S.E., Benton, M.J., 2006, Pelagosaurus typus Bronn, 1841 (Mesoeucrocodylia: Thalattosuchia) from the Upper Lias (Toarcian, Lower Lurassic) of Somerset, England: Journal of Vertebrate Paleontology, 26(3), 621–635. [ Links ]
Reynoso, V.H., 2006, Research on fossil amphibians and reptiles in Mexico, from 1869 to early 2004 (including marine forms but excluding pterosaurs, dinosaurs and obviously birds), in Vega, F. J., Nyborg, T.G., Perrilliat, M.C., Montellano–Ballesteros M., Cevallos–Ferriz S.R.S., Quiroz–Barroso S.A. (eds.), Studies on Mexican paleontology: Dordrecht, The Netherlands, Springer, 209–231. [ Links ]
Vignaud, P., 1995, Les Thalattosuchia, crocodiles marins du Mésozoique: systématique phylogénétique, paléoécologie, biochronologie et implications paléogéographiques: University of Poitiers, France, These de Doctorat, 271 p. [ Links ]
Vignaud, P., 1997, La morphologie dentaire des Thalattosuchia (Crocodylia, Mesosuchia): Palaeovertebrata, 26, 35–59. [ Links ]
Welles, S.P., 1952, A review of the North American Cretaceous elasmosaurs: University of California Publications in Geological Sciences, 29(3), 47–144. [ Links ]
Wieland, G.R., 1910, Plesiosaurus (Polyptychodon) mexicanus Wieland: Parergones del Instituto Geológico de México, 3(6), 359–365. [ Links ]
Young, M., 2006, Evolution and taxonomic revision of the Mesozoic marine crocodyliformes Metriorhynchidae, a phylogenetic and morphometric approach: University of London, UK, Masters Thesis, 76 p. [ Links ] ]]>