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Another look at Microraptor

Thursday, December 31, 2015 12:29

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(Before It's News)

As reported earlier
the theropod community is not happy with the large reptile tree nesting of Yutyrannus with Allosaurus (see discussion yesterday). Today we’ll look at the heretical (non-traditional) nesting of Microraptor with Compsognathus (Fig. 1).

The traditional nesting of Microraptor
is with other ‘raptors’, like Velociraptor (Fig. 1). By contrast, the large reptile tree nested Microraptor between Compsognathus and Sinornithosaurus, and not far from Tianyuraptor at the base of the Tyrannosaurus clade.

Figure 3. Provisional sisters to Microraptor (middle) now include Compsognathus (above) and Velociraptor (below). Which ones appears to share more traits with Microraptor?

Figure 2. Skull of Microraptor (color, middle) compared to Compsognathus (above) and Velociraptor (below). The two skulls that resemble each other more are more closely related.

The following traits
are shared between Microraptor with Compsognathus to the exclusion of Velociraptor in the large reptile tree, a study that includes a wide gamut of reptiles, not just theropods.

lacrimal not deeper than maxilla
narial opening dorsolaterally
naris at snout tip, not elevated
frontal separated from upper temporal fenestra
posterior parietal 20-40º
jaw joint descends
caudal transverse processes present beyond eighth caudal
Mc2-3 align at or beyond m1.2
Mt2-3 align with p1.1

Of course
there is also a list of traits shared between Compsognathus and Velociraptor to the exclusion of Microraptor. And indeed there is also a list of traits linking Microraptor to Velociraptor to the exclusion of Compsognathus.

Look at that face!
If you had to lump and split Compsognathus, Microraptor and Velociraptor based just on the skull alone (Fig. 2), which two would you lump together?

Feathers
The presences of extensive feathers on all four limbs of Microraptor — not in the direct ancestry of extant birds — points to a possible convergent evolution in this clade… OR more extensive (bit not preserved) feathers in a last common ancestor, like Compsognathus.

The presence of only protofeathers
in the contemporaneous Sinosauropteryx indicates a likely reduction in plumage in that short-legged taxon, slightly off the main line of bird evolution represented by Archaeopteryx of the Late Jurassic. Previously (Ji and Ji 1996) Sinosauropteryx was considered the basalmost taxon with the basalmost protofeathers. Not so both chronologically and phylogenetically.

Figure 3. Sinosauropteryx fossil. Apparently the presence of simple feathers here is derived from or a reversal of more primitive taxa with more extensive, more derived feathers according to its place on the large reptile tree. This dinosaur appears to be the Early Cretaceous analog to our extant squirrels in its niche and appearance.

References
Ji Q and Ji S-A 1996. On the Discovery of the earliest fossil bird in China (Sinosauropteryxgen. nov.) and the origin of birds. Chinese Geology 233:30-33.
Ostrom JH 1978. The osteology of Compsognathus longipes. Zitteliana 4: 73–118.
Osborn HF 1905. Tyrannosaurus and other Cretaceous carnivorous dinosaurs. Bulletin of the AMNH (New York City: American Museum of Natural History) 21 (14): 259–265.
Osborn HF 1924. Three new Theropoda, Protoceratops zone, central Mongolia”. American Museum Novitates 144: 1–12.
Ostrom JH 1970. Stratigraphy and paleontology of the Cloverly Formation (Lower Cretaceous) of the Bighorn Basin area, Wyoming and Montana. Bulletin of the Peabody Museum of Natural History 35: 1–234.
Wagner JA 1859. Über einige im lithographischen Schiefer neu aufgefundene Schildkröten und Saurier. Gelehrte Anzeigen der Bayerischen Akademie der Wissenschaften 49: 553.
Xing L, Persons WS, Bell PR, Xu X, Zhang J-P, Miyashita T, Wang F-P and Currie P 2013. Piscivery iin the feathered dinosaur Microraptor. Evolution 67(8):2441-2445.
Xu X, Zhou Z, Wang X, Kuang X, Zhang F and Du X 2003. Four-winged dinosaurs from China. Nature, 421: 335–340.