Monday, June 14, 2010

Rachitrema is an ichthyosaur of course

Along with Actiosaurus, Sauvage described another supposed dinosaur now thought to be an ichthyosaur.

Rachitrema Sauvage, 1883
R. pellati Sauvage, 1883
Rhaetian, Late Triassic
limestone at Conches-les-Mines, Autun, Saône-et-Loire, France
Lectotype (proposed)- (Pellat coll.) (~6.5 m) posterior dorsal neural arch (140 mm tall)
Paratypes- ?(Pellat coll.) scapula (220 mm)
(Pellat coll.) (Amniota incertae sedis) skull fragment, three dorsal rib fragments, proximal ?humerus, distal ?ilium or ?fibula, partial ?pelvic element

Rachitrema pellati type neural arch in posterior (4b), right lateral (4) and anterior (4a) views.  After Sauvage (1883).

Comments- Sauvage (1883) described this material as a new taxon of dinosaur, though an oddly primitive one which he does not favorably compare to any other species. The specimen Sauvage based the name on is a neural arch he assigned to the caudal series, while he referred additional bones ("undoubtedly to the same animal") identified as a braincase fragment, at least three dorsal rib fragments, a scapula, proximal humerus, proximal radius and distal pubis. He notes the neural arch was unfused to the centrum, citing this as a primitive character. The structure of the prezygapophyses and ligament pits are compared to Actinodon (now recognized as a junior synonym of the temnospondyl Onchiodon), while the cranial fragment, scapula and humerus are compared to crocodilians and the latter two at least are said to differ from Megalosaurus and Cetiosaurus. Boulenger (1883) attempted to respell the name Racheotrema without comment. It has also sometimes been misspelled Rhachitrema, starting with Boettger (1884). Zittel (1890) stated it was a possible zanclodontid theropod (though insufficiently characterized), though he later (1895) stated it was a megalosaurid close to Zanclodon instead. Pocta (1905) also placed it in Megalosauridae, while Simroth (1907) placed it in Zanclodontidae. Nopsca (1901) assigned it to Anchisauridae, which was thought to be a theropod family at the time. Huene (1902) identified Rachitrema's type neural arch as that of an ichthyosaur and synonymized the genus with Shastasaurus without comment. Sauvage (1903) agreed Rachitrema was an ichthyosaur and synonymized it with the contemporaneous Ichthyosaurus? rheticus, since he believed that species to be more similar to Shastasaurus than the also contemporaneous I? carinatus. He regarded the scapula to be more similar to Toretocnemus (as Leptocheirus) than Ichthyosaurus, and the reidentified distal ilium (previously thought to be a distal radius) to be like Toretocnemus and Shastasaurus. The supposed proximal humerus and distal pubis could not be compared well. Merriam (1908) kept Rachitrema as a synonym of Ichthyosaurus? rheticus and believed the type neural arch was more primitive than Jurassic plesiosaurs. Huene (1908) synonymized it with Ichthyosaurus, crediting his 1902 paper despite the fact that work had it synonymized with Shastasaurus. Huene (1922) now listed Rachitrema as a junior synonym of Leptopterygius? rheticus (Leptopterygius has since been replaced by Leptonectes), though referring to not only the type neural arch but also a "supposed ischium". He later (1951) listed Rachitrema as a possible synonym of Merriamia (now a synonym of Toretocnemus), as did Romer (1976). Bardet and Cuny (1993) accepted some of the material as possibly ichthyosaurian, but assigned the rest to Reptilia indet.. McGowan and Motani (2003) consider Rachitrema to be probably dinosaurian without justification.

When evaluating Rachitrema, it should first be noted that no neural arches have been described for rheticus, so the two taxa cannot be shown to be synonyms. Thus rheticus can be ignored in the following discussion. In addition, there is no evidence the material described as Rachitrema belongs to one individual or taxon. It was all found isolated and by two different collectors. The neural arch is here proposed to be the lectotype, as the genus name refers to it and Sauvage states "The remarkable characters present in this neural arch make us think they indicate a dinosaurian of unknown type which we will indicate under the name Rachitrema." The neural arch is not dinosaurian, as it differs from dinosaur presacrals and proximal caudals in lacking transverse processes and having reduced zygapophyses which are placed near the midline. Nor is it from a distal caudal, which have reduced neural spines. The neural arch does strongly resemble ichthyosaurs in these characters however, so Huene's identification was correct. Within Ichthyopterygia, Rachitrema is outside Neoichthyosauria based on its divided postzygapophyses (Maisch and Matzke, 2000), as expected for a Triassic taxon. This indicates it is not synonymous with Ichthyosaurus or Leptonectes, though it still may be Shastosaurus or Toretocnemus. It is probably a member of Longipinnati, as no more basal ichthyosaurs are known from the Late Jurassic. Pending further study, Rachitrema is considered Ichthyopterygia incertae sedis. The scapula has a short contact surface for the coracoid (~23% of scapular length), as in Cymbospondylus and non-longipinnatin taxa. The moderately sized anterior blade expansion is similar to Cymbospondylus, Shastasaurus and Shonisaurus, unlike the large flange of non-longipinnatins or the parvipelvians, Besanosaurus, Californosaurus, Mikadocephalus and Callawayia. Similarly, the low posterior blade expansion is unlike non-longipinnatins and neoichthyosaurs. The referred scapula is thus from a longipinnatin ichthyosaur that seems most similar to Cymbospondylus. The supposed distal ilium could indeed belong to an ichthyosaur like Cymbospondylus, Toretocnemus or Californosaurus, or it could be a partial long bone shaft of a non-ichthyosaur (e.g. choristodere or dinosaur distal fibula) as originally described. The other paratype fragments deserve detailed comparison to a wide variety of Triassic taxa, though the supposed humerus is certainly not an ichthyosaur humerus based on its elongation and the supposed distal pubis (which could also be a choristodere ventral ilium for instance) resembles parvipelvian pubes in rough shape but not Triassic ichthyosaurs.

References- Boulenger, 1883. Reptilia and Batrachia. in Rye (ed.). The Zoological Record for 1883. Record of Zoological Literature. 20, 24 pp.

Sauvage, 1883. Recherches sur les reptiles trouves dans l'etage Rhetien des environs d'Autun. Annales des Sciences Geologiques. 14(6, Article 3), 1-44.

Boettger, 1884. Bericht über die Leistungen in der Herpetologie während des Jahres 1883. Archiv Fur Naturgeschichte. 50(2), 379-434.

Zittel, 1890. Handbuch der Palaeontologie. Volume III. Vertebrata (Pisces, Amphibia, Reptilia, Aves).

Zittel, 1895. Grundzüge der Palaeontologie (Palaeozoologie). 971 pp.

Nopcsa, 1901. Synopsis und Abstammung der Dinosaurier. Földtani Közlöny. 31, 247-288.

Huene, 1902. Übersicht über die Reptilien der Trias [Review of the Reptilia of the Triassic]. Geologische und Paläontologische Abhandlungen (Neue Serie). Gustav Fischer Verlag, Jena. 6, 1-84.

Sauvage, 1903. Note sur les reptiles de letage Rhetien des environs d'Autun. Bulletin Societe d'Histoire Naturelle d'Autun (France). 16, 309-318.

Pocta, 1905. Rukovet Palaeozoologie. II. Cast: Vertebrata. 310 pp.

Simroth, 1907. Die Pendulationstheorie. Leipzig Konrad Grethlein's Verlag. 564 pp.

Huene, 1908. Die Dinosaurier der europäischen Triasformation mit Berücksichtiging der aussereuropäischen Vorkommnisse [The dinosaurs of the European Triassic Formation, with consideration of non-European occurrences]. Geologische und Paläontologische Abhandlungen Supplement-Band. 1, 419 pp.

Merriam, 1908. Triassic Ichthyosauria: With special reference to the American forms. 196 pp.

Huene, 1922. Die Ichthyosaurier des Lias und ihre Zusammenhänge. Jahresversammlung der palaeontologischen Gesellschaft. Verlag Bornträger, Berlin. 114 pp.

Huene, 1951. Eine neue Ichthyosaurier-Gattung der mitteleren Trias. Neues Jahrbuch fur Geologie und Palaontologie, Abhandlungen. 94, 80-92.

Romer, 1976. Osteology of the Reptiles. University of Chicago Press. 772 pp.

Bardet and Cuny, 1993. Triassic reptile faunas from France. Paleontologia Lombarda. 2, 9-17.

McGowan and Motani, 2003. Handbook of Paleoherpetology: Ichthyopterygia, Part 8. 175 pp.

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