The troodontid Sinornithoides: finally a taxon nesting that almost matches tradition

Lately
we’ve been having trouble nesting taxa where they have traditionally nested. Earlier we looked at some reasons why that might be so.

Figure 1. Sinornithoides youngi figure modified from Russell and Dong 1993.

Figure 1. Sinornithoides youngi figure modified from Russell and Dong 1993. The skull is tucked under the tail and matrix supporting the gastralia, so it is probably complete, just not exposed. The ‘killer’ toe claw is not so large n this specimen. 

Today
I’m pleased to announce the nesting of Sauronithoides youngi (Russell and Dong 1993, Early Cretaceous, Aptian/Albian, IVPP V9612), about where it nests in other cladograms, between the dromaeosaurids and the pre-birds + birds.

One little problem
In the large reptile tree, however, the order has been shifted around compared to the cladogram of Turner, Makovicky and Norell (2012) in which Anchiornis, Xiaotingia, Jinfengopteryx and Mei nest as basal troodontids, rather that pre-birds and birds, while Sauronithoides nests in a derived node with Troodon. Their cladogram, like the large reptile tree, nests the above named pre-birds closer to birds. The major difference is the separation of dromaeosauridae from the quite similar overall Sauronithoides by a long list of intervening transitional taxa. That can happen in cladistic analysis. Parsimony rules, of course.

Note the further separation
of Velociraptor from Archaeopteryx. If valid, the retroverted pubis developed by convergence in both clades. Intervening and basal taxa have a ventral pubis without a pubic foot. We looked at bird and pre-bird convergence earlier here.

Figure 2. Sinornithoides cladogram. This taxon nests at the base of the pre-birds and birds, derived from the same ancestors as Velociraptor and Balaur.

Figure 2. Sinornithoides cladogram. This taxon nests at the base of the pre-birds and birds, derived from the same ancestors as Velociraptor and Balaur. We’re not getting a clade of troodontids, but a grade of pre-birds here. 

Sinornithoides youngi
 
is one of the most complete troodontid theropod dinosaurs. It was preserved in a bird-like resting posture. In the large reptile tree (632 taxa, Fig. 2) it nests derived from Tanycolagreus and the Velociraptor clade and basal to the pre-birds and birds. Distinct from the predecessor taxa, the rostrum and nares were low, the forelimbs were shorter, the pubis lacked a foot, the pelvis was smaller. Sinornithoides was one of the earlier bird-like dinos to come out of China. It, too, was fossilized in a resting posture with both tail and neck wrapped around its presumably warm-blooded body.

Everything here, of course, is provisional
as are all scientific hypotheses. That the present cladogram (Fig. 2) makes sense in terms of sister taxa appearing similar and of roughly the same size and that predecessors to derived taxa demonstrate a gradually accumulating set of traits I think bodes well for it.

References
Russell D and Dong Z 1993. A nearly complete skeleton of a new troodontid dinosaur from the Early Cretaceous of the Ordos Basin, Inner Mongolia, People’s Republic of China. Canadian Journal of Earth Sciences, 30: 2163-2173.

wiki/Sinornithoides

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s