I know this is only a minor component of this study, but the authors' approach to troodontid taxonomy is intriguing. They refer the troodontid material from the Two Medicine Formation to _Troodon formosus_, the holotype of which is an isolated tooth from the Judith River Formation. _Stenonychosaurus inequalis_ (from Lower Dinosaur Park Formation) is considered a junior synonym of _T. formosus_. Nonetheless, Varricchio &c retain
_Latenivenatrix mcmasterae_ (from Upper Dinosaur Park Formation) as a separate species from
_T. formosus_ (including _S. inequalis_). I don't quite follow their logic.
There could be an argument for regarding Two Medicine troodontid and _S. inequalis_ as being a single species. According to van der Reest and Currie (2017), similarities between frontals and metatarsals suggest that they *might* be conspecific (although this needs further study). But I can't see why all this material (including from Two Medicine & Lower Dinosaur Park Formations) should be called _T. formosus_.
The teeth of _T. formosus_,
_S. inequalis_, and _L. mcmasterae_ cannot be
distinguished from each other - this was the conclusion of a slew of recent studies. Although _S. inequalis_ has been referred to _T. formosus_ in the past
(back in 1987, by Currie), this was reversed by van der Reest and Currie (2017). There is currently no evidence that the Two Medicine and Judith River
material belong to a single troodontid species to the exclusion of other
species (such as _L. mcmasterae_).
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_T. formosus_ can possibly be maintained as a valid species *if* diagnostic
material from the Judith River Formation is assigned to it *and* a
neotype is designated. But until then, the best option is to
restrict the name _T. formosus_ to the holotype. That means that _S. inequalis_ is the correct name for the diagnosable
troodontid known from Lower Dinosaur Park Formation. The Two Medicine troodontid might or might not be referable to _S. inequalis_ - that remains to be determined. But referring it to to _T. formosus_ will only make troodontid taxonomy more complicated than it already is.Â
Ben Creisler
A new open access paper:
David J. Varricchio, Martin KundrÃt & Jason Hogan (2018)
An Intermediate Incubation Period and Primitive Brooding in a Theropod Dinosaur.
Scientific Reports 8, Article number: 12454Â
Non-avian dinosaurs such as oviraptorosaurs and troodontids share several important reproductive characters with modern birds, including eggshell microstructure and iterative egg production. Nevertheless, debate exists concerning their incubation strategies. Here we estimate incubation period for the troodontid, Troodon formosus, by examining a near-term embryonic tooth. Synchrotron scanning and histologic thin sections allowed counting of daily (von Ebner) growth lines. The tooth preserves 31 intact lines with an average spacing of 3.3âÂâ0.96âÎm. Adding 8 more for the missing crown tip gives a total age of 39 days. Modern crocodilians begin to establish their functional dentition at approximately 47% through incubation. Thus, this tooth age suggests a Troodon incubation period of 74 days, falling midway between avian (44.4 days) and reptilian (107.3 days) values predicted by the Troodon egg mass (314âg). An accelerated incubation relative to modern reptiles supports brooding and concurs with a suite of features in oviraptorosaurs and troodontids (sequential laying, large complex clutches, and precocial young) that appear dependent upon both adult body and incubation temperatures elevated over ambient conditions. However, the largely buried condition of Troodon clutches may have prohibited efficient brooding, necessitating longer incubation than that of modern birds with fully exposed eggs.