Thursday, March 10, 2016

Meroktenos thabanensis: not Early Jurassic, just ahead of its time

It's been about a month since Meroktenos thabanensis made its (re)appearance, but I'm just now getting around to it. A bunch of other topics muscled ahead of it, which is fitting, I suppose. Prosauropods (by which the reader should understand "sauropodomorphs what ain't sauropods"; if you should want me to write "basal sauropodomorph" over and over again instead, I accept large cash bribes) are among those dinosaurs which never seem to get much respect. They have been fertile subjects for arm-waving, though, one result of which being that a survey of popular-audience dinosaur books will turn up wildly differing depictions over the years. Bipedal or quadrupedal? Herbivorous, omnivorous, predatory, or scavenging? A stem leading into sauropods, a bunch of rungs, or a distinct empire of prosauropods? Gleefully extreme lumping or splitting? What about herrerasaurids? What about Teratosaurus? (if that last one means anything to you, congratulations, you're in your mid-30s or older, or you study rauisuchids.) Prosauropods have also not been helped by a certain vague uniformity of body shape, nor have they been done any favors by geography. North America has an unimpressive record for these dinosaurs, so far including Anchisaurus (let's not kid ourselves about Ammosaurus being distinct), Sarahsaurus, Seitaad, the unnamed Nova Scotia form, and odds and ends. For a group that did not have horns, frills, spikes, bony armor, crests, ridge-backs, spectacular size, or big pointy teeth, and had the poor sense to go extinct before the appearance of tyrannosaurs or dromies to prey upon them, not being well-represented in North America has not helped their exposure. But I digress.

Genus and species: Meroktenos thabanensis; "Meroktenos" is derived from the Ancient Greek for "femur" and "animal", a reference to being originally based on a femur, and "thabanensis" is for the Thabana Morena area of the Mafeteng district of Lesotho. Free tips: if a species ends in "-ensis", it's a reference to a place. If it ends in "-i" (or rarely "-ii"), it's a reference to a person. If it ends in "-orum", it's a reference to multiple people.
Citation: Peyre de Fabrègues, C., and R. Allain. 2016. New material and revision of Melanorosaurus thabanensis, a basal sauropodomorph from the Upper Triassic of Lesotho. PeerJ 4:e1639.
Stratigraphy and geography: Lower Elliot Formation, Upper Triassic (probably within the middle Norian to Rhaetian stages), Mafeteng, Lesotho
Holotype: Muséum National d’Histoire Naturelle (MNHN).F.LES16, a right femur, partial right ilium, neural arch, left pubis, and right metatarsal II

Meroktenos thabanensis made its first appearance as a named dinosaur not in 2016, but in 1993 (Gauffre 1993), as Melanorosaurus thabanensis. The thing about Melanorosaurus thabanensis was that it could always fall back on "most recent melanorosaurid!" in Dinosaur Icebreakers, or at least it could back in the 1990s when being a melanorosaurid meant you were among the world's greatest sauropod mimics. Of course, you could never be a true sauropod because you had a short fifth metatarsal. That's all it took to get you barred "forever" in 1993. However, you may have noticed that the stratigraphy listed above puts this animal in the Late Triassic, which isn't all that impressive. The disconnect is because back when this was Melanorosaurus thabanensis, it was initially thought that it came from the younger Upper Elliot Formation, a Lower Jurassic unit. Further work burst that bubble, though. So, does Meroktenos no longer have anything to bring up at icebreakers?

Meroktenos actually has a fairly complicated history. Peyre de Fabrègues and Allain (2016) go over it in detail, but here's the dime tour. The type material was found back in 1959. It included not only the femur but the other handful of bones mentioned above. In addition, a cervical vertebra, a left ulna, and a radius were associated with the type material, but Peyre de Fabrègues and Allain were more hesitant in assigning these fossils to Meroktenos. The bones made a couple of cameo appearances between 1962 and 1970, but the dinosaur didn't break through until Gauffre (1993), where it gained its first name Melanorosaurus thabanensis. At this time, apparently the only specimen that was available was the femur. Following the initial description M. thabanensis faded into the background. The rediscovery of the other material, as well as the tangle that is Melanorosaurus readi, prompted Peyre de Fabrègues and Allain's revision. With the additional material, as well as more than 20 years of study on prosauropods, it turned out that while M. thabanensis is indeed a derived non-sauropod sauropodomorph, it doesn't quite plot with true Melanorosaurus, prompting the new genus.

Despite the (undeserved) reputation of "melanorosaurs" for great size, Meroktenos was not especially large. The type femur is only 480 mm (a shade less than 19 inches) long. Femora assigned to Melanorosaurus clock in over 620 mm (about 24.4 in) long, while the largest measured prosauropod femora of southern Africa hit 780 mm (close to 31 in). The relatively small stature of Meroktenos is combined with a couple of unusual features: the femur is quite robust and has a sauropod-like elliptical cross-section. Again like sauropods and unlike more basal prosauropods, the femur does not have a sigmoid (s-shaped) curve in side and anterior views (Peyre de Fabrègues and Allain 2016). These points suggest an animal that was "ahead of its time", so to speak, having acquired several features that are characteristic of sauropods while not being a sauropod.

The femur of Meroktenos thabanensis, from Peyre de Fabrègues and Allain (2016). The caption is as follows: "Right femur of Meroktenos, MNHN.F.LES16c. (A) Anterior, (B) medial, (C) posterior, (D) lateral, (E) proximal, and (F) distal views. ef, extensor fossa; fc, fibular condyle; fh, femoral head; lc, lateral condyle; lt, lesser trochanter; mc, medial condyle; pf, popliteal fossa; tfc, tibiofibular crest; 4t, fourth trochanter. (Photo credit: L Cazes.)"

References:

Gauffre, F.-X. 1993. The most recent Melanorosauridae (Saurischia, Prosauropoda), Lower Jurassic of Lesotho, with remarks on the prosauropod phylogeny. Neues Jahrbuch für Geologie und Paläontologie 11:648–654.

Peyre de Fabrègues, C., and R. Allain. 2016. New material and revision of Melanorosaurus thabanensis, a basal sauropodomorph from the Upper Triassic of Lesotho. PeerJ 4:e1639.

3 comments:

  1. I was looking at your 'Coming Attractions in Dinosauria?' post, and it got me thinking. Is there somewhere that compiles a larger list of unnamed dinosaurs? Your post did a pretty good job with the most important/well known ones, but is there anyway to see a larger amount of them?

    Also, given the amount of sauropodomorphs in the Lower Elliot Formation, is it possible that _Meroktenos thabanensis_ is the same as one of them? I wasn't sure because I don't know much about it. I'm not sure if you'll know the answer, but I thought I should give it at try.

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    1. I'm not sure if there is a full compilation across (non-avian) Dinosauria, but if you like theropods, Mickey Mortimer's Theropod Database (http://theropoddatabase.com/) has tons and tons of unnamed theropods, from isolated teeth on up. Tom Holtz's online supplement to his encyclopedia (linked at https://www.geol.umd.edu/~tholtz/dinoappendix/appendix.html) has a scattering of the most well-known unnnamed dinosaurs as well.

      With regard to _Meroktenos_ and its Lower Elliott friends, it appears to be distinct from all those where the femur is known, which eliminates all but _Blikanasaurus_ (type _Melanorosaurus_ is also something of a problem, and the type of _Euskelosaurus_ is supposed to include a partial femur, but it wasn't included in the publication and in any case may not have been complete enough for comparison). _Blikanasaurus_ comes out at about the same place as _Meroktenos_ in the phylogenetic analysis, and conceivably they could be the same thing. We just need more material (the only overlap is Metatarsal II).

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