Now that we’ve looked at a few theropod dinosaurs,
it might be interesting to see how many clades in the large reptile tee produced bird-like and VERY bird-like taxa (Fig. 1). Of course, only one clade (#9) actually produced extant birds. Every one off the eight others came (more or less) close, though.
Feathers no longer make the bird
We looked at the origin of feathers on dinosaurs here, here and here. If you’ll remember, the key to feather genesis is their origin on naked, not scaly, skin. Based on phylogenetic bracketing feathers were present on basalmost dinosaurs because both theropods and basal phytodinosaurs like Kulindadromeus had primitive feathers and quills that first appeared on the lower back both in phylogeny and embryology.
- Microraptor and kin – This is the most primitive theropod clade that produced elongate wing feathers, both on the forelimbs and hind limbs. At the base of the clade are the ‘ostrich-mimic’ dinosaurs. Somewhere near the top of the small taxa are the four-winged dinosaurs. None of these taxa had elongated coracoids. So they didn’t flap.
- Rahonavis and kin – Wikipedia reports, “Rahonavis has historically been the subject of some uncertainty as to its proper taxonomic position – whether it is a member of the clade Avialae (birds) or a closely related dromaeosaurid. The presence of quill knobs on its ulna (forearm bone) led initially to its inclusion as an avialan; however, the rest of the skeleton is rather typically dromaeosaurid in its attributes. The large reptile tree nests Rahonavis with Falcarius and the feathery therizinosaurs, admittedly on very few Rahonavis bones, some of which include the raised ‘killer claw’ typically found in dromaeosaurs.
- Khaan and kin – the oviraptorids lost teeth and reduced tail bones convergent with birds.
- Velociraptor and kin – the dromaeosaurs elongated the coracoid and rotated the pubis posteriorly independent of birds. Yes, Ostrom 1970 noted similarities between Deinonychus and Archaeopteryx, but with intervening taxa that include Anchiornis, Aurornis, Xiaotingia and Eosinopteryx, we now know the retroverted pubis was not homologous, but developed independently. Surprised me, too. But in Science you have to be ready to accept the surprises and paradigm busters.
- Shuvuuia and kin – the alvarezsaurs were briefly sensationalized as short-armed bird-like dinosaurs. Choinierre et al. 2014 reported, “Alvarezsauroidea is a clade of theropod dinosaurs whose derived members possess remarkably birdlike features, including a lightly built, kinetic skull, several vertebral modifications, a keeled sternum, a fused carpometacarpus, a fully retroverted pubis and ischium that do not contact at the body midline, and a gracile hind limb.”
- Archaeopteryx and kin – Most theropod cladograms use only one taxonomic unit and therefore one specimen of Archaeopteryx, despite the fact that over a dozen diverse specimens are known. Earlier we saw that several were basal to other bird-like clades, one of which produced extant birds. Based on their longer coracoids, they flapped. This clade arose from sisters to Anchiornis, Aurornis, Xiaotingia and Eosinopteryx, all of which had short coracoids.
- Cathayornis and kin – The enantiornithes or “opposite birds” arose from a sister to the Berlin specimen of Archaeopteryx. All were flappers and flyers. None survive today.
- Scansoriopteryx and kin – The scansoriopterygids arose from a sister to Archaeopteryx bavarica. The longer manual digit 3 sets them apart. None survive today.
- Confuciusornis and kin – This clade arose from Archaeopteryx (Jurapteryx) recurva and ultimately produced extant birds, starting with sparrow-sized Archaeornithura. Confuciusornis and Wellnhoferia (Archaeopteryx) grandis could represent yet another divergent clade.
One could argue
that convergence stopped at stage 6 when divergence took over. The first or most basal Archaeopteryx, the one that finally started flapping with elongated coracoids, might just be the first bird.
Phylogenetic miniaturization? Maybe not.
Usually I’m a big proponent of phylogenetic miniaturization as it typically is found at the base of major clades. I supported Lee, Cau, Naish and Dyke 2014, who produced a paper that showed how birds are the product of a long series of smaller and smaller and smaller theropods (Fig. 2). Now that I’ve tested this with the addition of several bird-like taxa (above), I now see the large reptile tree does not support sustained miniaturization producing birds at the end of a long line of shrinking theropods. Rather, small bird-like and giant carnosaur-like taxa appear in nearly every theropod clade. And that pattern has continued until recently with secretary birds and related terror birds.
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