Yesterday we looked at several early birds (Fig. 1). Earlier we considered the phylogenetic nesting of Balaur (Fig. 2; Csiki Z et al. 2010), which some workers (Cau et al. 2015) considered an early flightless bird.
Figure 7. Bird cladogram with the latest additions. Here the referred specimen of Yanornis nests with enantiornithes while Archaeovolans nests within the Scansoriopterygidae, not with Yanornis.
the first flightless birds, one must first decide which taxon represents the first or basal bird. In the large reptile tree (subset Fig. 1) the last common ancestor of Enantiornithes and Euornithes is Archaeopteryx siemensi, represented by the Berlin and the Thermopolis specimens. Thus they represent, in this cladogram, the first or basal birds. Both the Enantiornithes and Euornithes produced specimens with a locked down coracoid and expanded sternum, anchors for powerful flight muscles attached to long feathered forelimbs.
Thus the purported first flightless bird,
Balaur, nests outside the bird clade (Fig. 1) established by the large reptile tree.
Figure 1. Balaur compared to various dromaeosaurids and to Sapeornis, both to scale and enlarged for detail. Cau, Brougham and Naish wondered if Balaur was the first neoflightless bird, a sort of dodo of the Cretaceous.
the Scansoriopterygidae produced the first taxa in the Eurornithes with more of a dinosaur/theropod look, with Mei (Early Cretaceous) having the smallest forelimbs relative to the rest of the body in that clade. No doubt it was flightless — and with shorter coracoids and a tiny sternum, reduced its flapping. By contrast, its current sister, Archaeovolans (Fig. 3), retained a robust pectoral girdle and long forelimbs.
Figure 2. Sister taxa at the base of the scansoriopterygidae include Jeholornis, Mei and Archaeovolans, here shown to scale.
As everyone knows,
flightless birds have arisen several times since the Early Cretaceous with Hesperornis and Struthio as examples in the large reptile tree. In evolution everything is gradual and often enough, reversible. And behavior is best determined at the extremes of morphology. More generalized taxa probably had more generalized behavior.
In Dinosaurs of the Air
author Greg Paul Paul “argues provocatively for the idea that the ancestor-descendant relationship between the dinosaurs and birds can on occasion be reversed, and that many dinosaurs were secondarily flightless descendants of creatures we would regard as birds.” According to the large reptile tree, dromaeosaurids and basal troodontids were not birds. But birds are derived troodontids. And troodontids arise from basal dromaeosaurids.
Along these same lines Kavanau 2010 reported
“Varricchio et al. propose that troodontids and oviraptorids were pre-avian and that paternal egg care preceded the origin of birds. On the contrary, unmentioned by them is that abundant paleontological evidence has led several workers to conclude that troodontids and oviraptorids were secondary flightless birds. This evidence ranges from bird-like bodies and bone designs, adapted for climbing, perching, gliding, and ultimately flight, to relatively large, highly developed brains, poor sense of smell, and their feeding habits.” Not so, according to the large reptile tree. But, to their point, bird-like theropods have arisen about 8 times by convergence, as we looked at earlier here.
Cau A, Brougham T and Naish D. 2015. The Phylogenetic Affinities of the Bizarre Late Cretaceous Romanian Theropod Balaur bondoc (Dinosauria, Maniraptora): Dromaeosaurid or Flightless Bird? PeerJ. 3: E1032. DOI: dx.doi.org/10.7717/peerj.1032
Csiki Z, Vremir M, Brusatte SL, Norell MA 2010. An aberrant island-dwelling theropod dinosaur from the Late Cretaceous of Romania. Proceedings of the National Academy of Sciences of the United States of America 107 (35): 15357–15361.
Kavanau JL 2010. Secondarily flightless birds or Cretaceous non-avian theropods? Med Hypotheses 74(2):275-6.
Paul G 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Johns Hopkins University Press, Baltimore, 472 pp.
newslink on secondarily flightless bird Epidexoptryx.