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Saturday, 8 December 2012

Possible Further Support for Continued Plesiosaurian Survival

Scott Mardis sent me a notice about some fossils from the Miocene ("Beginning of the End" for the Age of Mammals) which he noticed had some features of Plesiosaurian anatomy and which famous 19th Century Paleontologist Edward D Cope had thought actually was a surviving Plesiosaur. The anatomical features that Scott had noticed were actually present in the fossil but this was an unusual sort ot actually be looked upon as a surviving Plesiosaur ANCESTOR, a Choristodere. The genus which Cope had named was called either Ischyrotherium or Ischyrosaurus,but the latter name was pre-empted by a Sauropod dinosaur. The familiar Choristoderan Champsosaurus is shownb at left in its Wikipedia reference photo. So while the fosdsil material in question might not be evidence of a Plesiosaurian survival, we  something even more surprising: a line of Plesiosaurian ANCESTORS had vanished for most of the length of the Age of Dinosaurs, most of the Jurassic and Cretaceous, and then reappeared on the fossil record  , and continued on through most of the Age of Mammals, but with pretty poor fossil representation all along. If this could be true of Plesiosaur ancestors then it could certainly also have been true of Plesiosaurs themselves at the same timeby way of the same unknown mechanisms.

Two Triassic Nothosaurs (Plesiosaur ancestors) pictured  below, both of which have the unusual rib formation which Scott Mardis pointed out in the Oligocene- Miocene Choristodere Ischyrotherium

Lazarussuchus, Miocene Choristoderan presumably close to Cope's Ischyrotherium, has a skull like a little Plesiosaur with what look very like the standard Eyryapsid skull openings.


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Temporal range: Late Permian–Present
Reconstruction of Trilophosaurus, a primitive archosauromorph
Scientific classification e
von Huene, 1946
Archosauromorpha (Greek for "ruling lizard forms") is an infraclass of diapsid reptiles that first appeared during the late Permian and became more common during the Triassic. It was defined by Michel Laurin (1991) as the clade containing the most recent common ancestor of Prolacerta, Trilophosaurus, Hyperodapedon and archosaurs and all its descendants;[1] David Dilkes (1998) formulated a more inclusive definition of Archosauromorpha, defining it as the clade containg Protorosaurus and all other saurians that are more closely related to Protorosaurus than to Lepidosauria.[2]
Included in this infraclass are the groups Rhynchosauria, Trilophosauridae, Prolacertiformes and Archosauriformes. While superficially these reptiles vary in appearance (at one time they were even included in different subclasses – the trilophosaurs were considered euryapsids, and the rhynchosaurs were considered lepidosaurs and were included in the same order as the tuatara), they are actually united by a number of small skeletal and skull-related details that suggest they form a clade that descended from a single common ancestor. Additional groups with uncertain phylogenetic position that are included in Archosauromorpha by some authors (and excluded from it by others) are Choristodera, drepanosaurs[2], thalattosaurs, ichthyopterygians, sauropterygians[3][4][5] and turtles.[6]
Of the taxa mentioned above rhychosaurs, trilophosaurs and prolacertiforms died out at or before the end-Triassic extinction. The choristoderans continued as a minor group until the Miocene, and the Archosauriformes were important factors in early Triassic environments before giving rise to the even more successful Archosauria.




Genus: Ischyrotherium LEIDY, 1856 (nomen dubium)
= Ischyrosaurus COPE, 1870 (non) HULKE, 1874 Species: antiquum LEIDY, 1856
= Ischyrosaurus antiquus (LEIDY, 1856) COPE, 1870
Syntypes: 11559
Locality: Between Moreau River and Grand River, Nebraska.
Horizon: Lignite.
Age: Cretaceous. [this turned out to be incorrect and the material is MIOCENE in date]
Material: Dorsal vertebrae and 7 rib fragments.
NOTE: possibly a Plesiosaur. [Later called a Choristodere]

Wikipedia discussion page on status of Ischyrosaurus/Ischyrotherium

Ischyrosaurus Cope, 1876
The correct date of the publication of Ischyrosaurus Cope is 1876, not 1869 [1], making Ischyrosaurus Hulke, 1874 in Lydekker, 1888 unpre-occupied. Cope [2] tried to create Ischyrosaurus for Ischyrotherium on the grounds that Ischyrotherium was a reptile and not a mammal as originally classified by Leidy [3]. However, the name Ischyrosaurus Cope, 1876 is a nomen nudum, on the condition of Cope's attempt, and a junior synonym of Ischyrotherium Leidy, 1856.
[2] Cope, E. D., 1876, On some extinct reptiles and Batrachia from the Judith River and Fox Hills beds of Montana: Proceedings of the Academy of Natural Sciences, Philadelphia, 1876, p. 340-359.
[3] Leidy, J., 1856, Notices of remains of extinct vertebrated animals of New Jersey, collected by Prof. Cook, of the State Geological Survey, under the Direction of Dr. W. Kitchell: Proceedings of the Academy of Natural Sciences, Philadelphia, v. 8, p. 220-221. —The preceding unsigned comment was added by (talk) 17:13, 31 December 2006 (UTC).
The link (once I found it; for some reason, I had to go through Google to get to it) claims it was Cope 1870 (which would be "Synopsis of the extinct Batrachia, Reptilia and Aves of North America". Transactions of the American Philosophical Society 14:1-252.), which is not 1869, but still predates it. A JSTOR link I found on Google to an 1871 Cope paper implied he'd coined it before 1871:
JSTOR: On the Families of Fishes of the Cretaceous Formation of Kansas
The case appears to me to be different with the name Ischyrosaurus, which I proposed to replace with Ischyrotherium (Leidy). The latter was given to a genus ...
So, it was in use before 1871. That it was a nomen nudum, or a superfluous name, does not matter. However, since it may have been abandoned by 1871 (if I'm reading that sentence fragment correctly), Cope's version could be declared a nomen oblitum if for some reason Ischyrosaurus Hulke became important to paleontology.[Subsequent discussion on the same Wikitalk page showed the name was NOT a nominum nudem]
Why some have it as Cope (1869) and some have it as Cope (1870) (it looks like it was a typo in this message to the DML, which looks like it was referring to the 1870 work) I don't know, but it was definitely coined before 1871. J. Spencer 20:30, 1 January 2007 (UTC)
Correction: Cope had several 1869 or 1870 works dealing with Batrachia (see for example the ref list here for a few, one of which is repeated in 1870 because of the famous Elasmosaurus-head-on-tail thing), so I can't firmly say which one without having read them. The important thing is still the pre-1874 date. J. Spencer 20:38, 1 January 2007 (UTC)
The reasoning for the different dates is explained by Spamer et al. (1995- pg. 333). The portion up to page 105 was issued in August, 1869, up to page 235 in April, 1870 and the rest was published in December, 1870. This is on page ii of the index in the volume itself. Since Cope named Ischyrosaurus on pages 38 and 39, the proper year is 1869. -Mickey Mortimer

The Longevity and Variety Within the Choristodera
Choristoderes appeared in the Late Triassic, but probably originated in the Late Permian, along with their sister taxa. Some survived into the Miocene. Despite the longevity of this clade, relatively few modifications to the basic body plan appeared. Oh, sure, the lateral temporal fenestra disappeared in Doswellia and Lazarussuchus. The rostrum elongated in Champsosaurus. The neck elongated in HyphalosaurusDoswellia was the giant of the clade, reaching 1.6 m in length, or slightly larger than Champsosaurus at 1.5 m, so their largest size was about the size of a human being. Traditional enigmas, choristoderes were a monophyletic clade that nested between Youngoides and Parasuchia, close to the base of the Archosauriformes. Relatively conservative in morphology, choristoderes were a relatively minor presence throughout the Mesozoic and into the Cenozoic.

ReferencesBrown B 1905. The osteology of Champsosaurus Cope. Memoirs of the AMNH 9 (1):1-26.
Cope ED 1876.
On some extinct reptiles and Batrachia from the Judith River and Fox Hills beds of Montana: Proceedings of the Academy of Natural Sciences, Philadelphia. 28, p. 340-359.
Dilkes D and Sues H-D 2009.
Redescription and phylogenetic relationships of Doswellia kaltenbachi (Diapsida: Archosauriformes) from the Upper Triassic of Virginia. Journal of Vertebrate Paleontology 29(1):58-79
Evans SE and Hecht MK 1993.A history of an extinct reptilian clade, the Choristodera: longevity, Lazarus-Taxa, and the fossil record. Evolutionary Biology 27:323–338.
Foster JR and Trujillo KC 2000.
New occurrences of Cteniogenys (Reptilia, Choristodera) in the Late Jurassic of Wyoming and South Dakota. Brigham Young University Geology Studies 45:11-18.
Gao K-Q, Tang Z-L and Wang X-L 1999.
A long-necked reptile from the Upper Jurassic/Lower Cretaceous of Liaoning Province, northeastern China. Vertebrata PalAsiatica 37:1–8.
Gilmore CW 1928.
Fossil lizards of North America. Memoirs of the National Academy of Sciences 22(3):1-201.
Hecht MK 1992. A new choristodere (Reptilia, Diapsida) from the Oligocene of France: an example of the Lazarus effect. Geobios 25:115–131. doi:10.1016/S0016-6995(09)90041-9.
Matsumoto R and Evans SE 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberain Geology 36(2):253-274. online pdf
Weems RE 1980.
An unusual newly discovered archosaur from the Upper Triassic of Virginia, U.S.A. Transactions of the American Philosophical Society, New Series 70(7):1-53

Compare Triliphosauria, Prolacertaforms ("Protosauria")

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