Besides the Angeac taxon, Thecocoelurus has been compared to several taxa with surprisingly similar cervical vertebrae- caenagnathids (Naish et al., 2001; Naish and Martill, 2002); Falcarius (Kirkland et al., 2004; Zanno, 2010a); and noasaurids (Naish, 2011). These all have elongate amphicoelous cervicals with low neural spines, anterior peduncular fossae, two pairs of pleurocoels and a transversely concave ventral surface defined by lateral ridges confluent with the parapophyses. Which is most similar to Thecocoelurus?
Naish et al. (2001) and Naish and Martill (2002) both argue it is closer to oviraptorosaurs than to therizinosauroids based on the rounded pleurocoel and thin neural spine, but this is also the case in basal therizinosaurs (Falcarius, Jianchangosaurus) (as noted by Zanno, 2010a) and noasaurids. Naish (2011) on the other hand, felt "the idea that large caenagnathids were present in Western Europe during the Barremian is difficult to reconcile with what we know of oviraptorosaur biogeography and distribution", thus favored an abelisauroid identity (I suppose based on Genusaurus). I'd argue basal oviraptorosaurs could and do have similar morphologies (e.g. the Early Cretaceous Similicaudipteryx) and that small theropod diversity in Cretaceous Europe is very poorly known.
Allain et al. make two new comparative arguments for Thecocoelurus being closest to the Angeac taxon.
1. They distinguished both from Noasaurus based on their concave anterior central surface, but this is true in Masiakasaurus as well. It is also true of all coelurosaurs being compared.
2. They also paired Thecocoelurus with the Angeac taxon based on pneumatic foramina above the prezygapophyses which invade the neural arch. Yet these are present in cervicals 6-10 of Masiakasaurus (Allain et al. state they are "in a modified form" but don't elaborate), at least cervical 9 of Heyuannia, and in Conchoraptor and "Ingenia" (Lu, 2004; contra Allain et al.'s claim they are unknown in oviraptorosaurs). They are also only present on the left side of Thecocoelurus (pf? in figure below, lower left), further posterior than in at least Masiakasaurus and the Angeac taxon, perhaps suggesting breakage of a naturally hollow area or pneumatic asymmetry. The only preserved posterior cervical of Falcarius doesn't preserve this area, nor does the illustrated and best preserved cervical of Chirostenotes.
So let's compare! Contra Naish and Martill, the specimen resembles posterior cervicals more than anterior ones, and indeed the supposedly identical Angeac vertebra matches the seventh or eighth of Harpymimus based on elongation, central articular surface orientation, prezygapophyseal length and orientation, etc. (contra Allain et al.).
The elongate parapophyses resemble Falcarius, Chirostenotes and Similicaudipteryx more than Masiakasaurus or the Angeac taxon.
The anterior pleurocoels are placed in an obvious fossa, like Falcarius and the Angeac taxon, but unlike Masiakasaurus, Chirostenotes or Similicaudipteryx.
The infradiapophyseal fossa is developed as an elongate groove, as in Falcarius but unlike Masiakasaurus, the Angeac taxon, Chirostenotes or Similicaudipteryx.
The centrum is taller than wide (midline height / width minus parapophyses 133%) as in Falcarius (118%), but unlike the Angeac taxon (95%) and especially Chirostenotes (74%) and Masiakasaurus (64%).
The anterior peduncular fossae are well defined as in Chirostenotes and at least anterior Falcarius cervicals, but unlike Masiakasaurus or the Angeac taxon. They are however also well defined in anterior Masiakasaurus cervicals, so the condition in Falcarius isn't necessarily better than that genus or the Angeac taxon.
They are also placed far below the diagonal prezygapophyseal surface as in at least anterior Falcarius cervicals, but unlike the Angeac taxon, Chirostenotes or Masiakasaurus. The same could be said re: anterior Masiakasaurus cervicals.
The prespinal fossa is broad like Chirostenotes and Masiakasaurus but unlike the Angeac taxon.
It has anteroposteriorly broad exposure dorsally as in the Angeac taxon and at least anterior Falcarius cervicals, but not Masiakasaurus (including anterior cervicals of the latter).
Overall, Thecocoelurus is most similar to Falcarius and least similar to Masiakasaurus. There are four good characters shared with Falcarius to the exclusion of the Angeac taxon, and three characters that are more similar to Chirostenotes than to the Angeac taxon, but two characters that are more similar to the Angeac taxon than to Chirostenotes.
|Thecocoelurus holotype completed with the posterior of Falcarius (modified from Naish and Martill, 2002 and Zanno, 2010). This results in a centrum length of 68 mm for Thecocoelurus, compared to Naish's (2011) estimate of 70-90 mm.|
Falcarius does differ from Thecocoelurus in having a ventral median ridge on its centra, but this is an autapomorphy not seen in other therizinosaurs. Besides this, no characters differ between the specimen except exact size and shape of pneumatic features, which themselves vary between right and left sides of Thecocoelurus. Both are Barremian, and Thecocoelurus is 58% the size of the Falcarius individual that preserved the posterior cervical (though a growth series is known, where that individual falls is unreported). Whether Thecocoelurus and Falcarius share derived characters to the exclusion of other therizinosaurs would require more study, but at the moment is seems most parsimonious to consider Thecocoelurus a basal therizinosaur and not closely related to the Angeac taxon.
References- Sues, 1997. On Chirostenotes, a Late Cretaceous oviraptorosaur (Dinosauria: Theropoda) from Western North America. Journal of Vertebrate Paleontology. 17(4), 698-716.
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Naish and Martill, 2002. A reappraisal of Thecocoelurus daviesi (Dinosauria: Theropoda) from the Early Cretaceous of the Isle of Wight. Proceedings of the Geologists’ Association. 113, 23-30.
Kirkland, Zanno, DeBlieux, Smith and Sampson, 2004. A new, basal-most therizinosauroid (Theropoda: Maniraptora) from Utah demonstrates a Pan-Laurasian distribution for Early Cretaceous therizinosauroids. Journal of Vertebrate Paleontology. 24(3), 25-26.
Lu, 2004. Oviraptorid dinosaurs from Southern China. PhD Thesis. Southern Methodist University. 249 pp.
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Allain, Vullo, Le Loeuff and Tournepiche, 2014. European ornithomimosaurs (Dinosauria, Theropoda): An undetected record. Geologica Acta. 12(2), in press. http://www.geologica-acta.com/pdf/vol1202a05.pdf