Arctometatarsals should have been a great clade trait…

But it’s not.
Arctometatarsals describe a type of theropod metatarsals in which metatarsal 3 is pinched off proximally by flanking metatarsals 2 and 4. In addition, distally metatarsal 3 is typically just slightly anterior to its flanking metatarsals, which slightly back it up. You would think such a trait would only develop once and identify or diagnose a clade – but that is not the case, as earlier workers (like Snively et al. 2004) discovered and described.

Figure 1. Arctometatarsals on T-rex vs. normal metatarsals on Allosaurus.

Figure 1. Arctometatarsals on T-rex vs. normal metatarsals on Allosaurus.

Similarly
the large reptile tree (LRT, subset Fig. 1) does not find an arctometatarsal clade. The LRT finds at least six convergent instances where the third metatarsal is pinched between the flanking second and fourth metatarsals. None of these instances are related to each other. Rather, all are separated from one another by normal metatarsals.

Figure 1. Theropod dinosaurs and the arctometatarsal taxa (in blue).

Figure 1. Theropod dinosaurs and the arctometatarsal taxa (in blue).

Snively et al report, 
“A Bayesian phylogenetic analysis indicates that an arctometatarsus evolved in the common ancestor of the Tyrannosauridae + (Ornithomimosauria + Troodontidae) clade, but other optimizations are plausible.” That hypothesis was not supported by the LRT. Moreover, only one tested troodontid has an arctometatarsal trait. Troodon also has this trait, but it is not yet a tested taxon.

Snively et al report, 
“The most likely selective benefit of the structure was increased agility; if so, homoplasy indicates multiple exaptive and adaptive pathways towards predation and escape roles.”
Perhaps so! But these taxa have little in common distinct from other theropods, other than the pinched third metatarsal.

As we’ve seen before,
and this fact just hammers it home: convergence in RAMPANT throughout the LRT. That’s why we leave it to the software to recover the tree. No one wants to pull a Larry Martin here by creating or disputing relationships based on a single trait.

References
Snively E, Russell AP and Powell GL 2004. Evolutionary morphology of the coelurosaurian arctometatarsus: descriptive, morphometric and phylogenetic approaches. Zoological Journal of the Linnean Society 142:525–553.

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