The Compsognathus-Tyrannosaurus clade to scale

There has been a recent rise in interest
in the new nestings of the theropods recovered here, here, here and here. The present post provides a summary of images and traits from the large reptile tree (Fig. 1).

Figure 1. Latest cladogram of the Theropoda, including the Compsognathus-Tyrannosaurus clade.

Figure 1. Latest cladogram of the Theropoda, including the Compsognathus-Tyrannosaurus clade.

The large reptile tree 
nests Compsognathus at the base of a clade that includes Microraptor, Sinornithosaurus, Tianyuraptor, Zhenyuanlong and Tyrannosaurus (Fig. 2). The middle taxa (above) are traditionally considered dromaeosaurs and indeed nest with dromaeosaurs in very large cladograms focused on theropods (Cau et al. 2015).

Figure 2. Taxa in the Compsognathus/Tyrannosaurus clade, a subset of the large reptile tree to scale. Also included are Microraptor, Sinornithosaurus, Dilong and Zhenyuanlong.

Figure 2. Taxa in the Compsognathus/Tyrannosaurus clade, a subset of the large reptile tree to scale. Also included are Microraptor, Sinornithosaurus, Tianyuraptor and Zhenyuanlong.

Unfortunately
the large reptile tree does not confirm many of the nestings recovered by the Cau et al. study, which employed many more taxa and characters specific to theropods (to their credit), not generalized to all reptiles. They also did not create reconstructions of included taxa, so the researcher has to look at their character scores to confirm validity. Sometimes a picture streamlines difficult tasks like this. Cau eta al updated the matrix of Lee et al. 2014 (which included two of the former authors). So they didn’t have to examine each taxon, nor photographs or drawings of each and every taxon. Likely they accepted most of the data as correct — which it indeed may be. But in Science, remember, EVERYTHING is provisional, even what you read and see here.

The Cau et al. study
nested Archaeopteryx (as only one taxon) basal to Xiaotingia, Rahonavis and Balaur. By contrast the large reptile tree nests these taxa basal to six specimens of Archaeopteryx, each basal to distinct derived avian clades

Distinct from other theropods
(in the large reptile tree), this clade (Fig. 2) shares the following traits:

  1. skull not shorter than cervicals
  2. lateral rostral shape convex, smooth curve (reversed in T)
  3. frontals separated from upper temporal fenestrae (reversed in Z and T)
  4. procumbent premaxillary teeth (reversed in S and T)
  5. ventral mandible convex anteriorly, concave posteriorly (reversed in T)

The Tianyuraptor/Zhenyuanlong/Tyrannosaurus clade
shares the following traits:

  1. snout occiput length not less than half the presacral length
  2. dorsal nasal shape triangular, wider anteriorly
  3. ventral premaxilla tilted up
  4. orbit not larger than antorbital fenestra
  5. parietal skull table strongly constricted
  6. quadratojugal hourglass shaped
  7. posterior mandible deeper anteriorly
  8. midcervical centrum taller than long
  9. cervical size decreases cranially
  10. cervicals parallelogram shaped in lateral view
  11. anterior chevrons parallel to centra (reversed in T)
  12. pedal 4 length sub equal to mt4

Of course
these and many other traits are shared with other taxa by convergence. As always, it is the suite of traits that nests taxa.

Figure 3. Bird origins. Apparently the four-winged micro raptors were no in the direct line of living birds.

Figure 3. Bird origins. Apparently the four-winged microraptors were no in the direct line of living birds.

 

Figure 4. Sinornithosaurus (holotype) pelvis. Note the pubis is oriented ventrally, then curves posteriorly.

Figure 4. Sinornithosaurus (holotype) pelvis. Note the pubis is oriented ventrally, then curves or angles posteriorly.

The Sinornithosaurus pelvis
is indeed similar to that of Velociraptor pelvis (Fig. 3). Similar, yes, but also distinct in morphology.

I’m just reporting results
and they appear (Fig. 2) that make a certain amount of sense. There’s no reason to criticize a worker for methodology. If specific mistakes are found, please alert the author.

Let’s find common ground and figure this out.

References
Cau et al. 2015. The phylogenetic affinities of the bizarre Late Cretaceous Romanian theropod Balaur bondoc (Dinosauria, Maniraptora): dromaeosaurid or flightless bird? PeerJ 3:e1032; DOI 10.7717/peerj.1032

Lee MSY, Cau A, Naish D, Dyke GJ. 2014. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds. Science 345(6196):562–566. DOI 10.1126/science.1252243.

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