Paleognathae - Evolution

Evolution

See also: List of paleognaths

No unambiguously paleognathous fossil birds are known until the Cenozoic, but there have been many reports of putative paleognathes, and it has long been inferred that they may have evolved in the Cretaceous.

One study of molecular and paleontological data found that modern bird orders, including the paleognathous ones, began diverging from one another in the Early Cretaceous. Benton (2005) summarized this and other molecular studies as implying that paleognaths should have arisen 110 to 120 million years ago in the Early Cretaceous. He points out, however, that there is no fossil record until 70 million years ago, leaving a 45 million year gap. He asks whether the paleognath fossils will be found one day, or whether the estimated rates of molecular evolution are too slow, and that bird evolution actually accelerated during an adaptive radiation after the Cretaceous–Paleogene boundary (K–T boundary).

Hope (2002) reviewed all known bird fossils from the Mesozoic looking for evidence of the origin of the evolutionary radiation of the Neornithes. That radiation would also signal that the paleognaths had already diverged. She notes five Early Cretaceous taxa that have been assigned to the Palaeognathae. She finds that none of them can be clearly assigned as such. However, she does find evidence that the Neognathae and, therefore, also the Palaeognathae had diverged no later than the Early Campanian age of the Cretaceous period.

Vegavis is a fossil bird from the Maastrichtian period of Late Cretaceous Antarctica. Vegavis is most closely related to true ducks. Because virtually all phylogenetic analyses predict that ducks diverged after paleognathes, this is evidence that paleognathes had already arisen well before that time.

An exceptionally preserved specimen of the extinct flying paleognathe Lithornis was published by Leonard et al. in 2005. It is an articulated and nearly complete fossil from the early Eocene of Denmark, and thought to have the best preserved lithornithiform skull ever found. The authors concluded that Lithornis was a close sister taxon to tinamous, rather than ostriches, and that the lithorniforms + tinamous were the most basal paleognaths. They concluded that all ratites, therefore, were monophyletic, descending from one common ancestor that became flightless. They also interpret the paleognath-like Limenavis, from Late Cretaceous Patagonia, as possible evidence of a Cretaceous and monophyletic origin for paleognathes.

An ambitious genomic analysis of the living birds was performed in 2007, and it contradicted Leonard et al. (2005). It found that tinamous are not primitive within the paleognathes, but among the most advanced. This requires multiple events of flightlessness within the paleognathes and partially refutes the Gondwana Vicariance Hypothesis (see below). The study looked at DNA sequences from 19 loci in 169 species. It recovered evidence that the paleognathes are one natural group (monophyletic), and that their divergence from other birds is the oldest divergence of any extant bird groups. It also placed the tinamous within the ratites, more derived than ostriches, or rheas and as a sister group to emus and kiwis, and this makes ratites paraphyletic.

A related study addressed the issue of paleognath phylogeny exclusively. It used molecular analysis and looked at twenty unlinked nuclear genes. It study concluded that there were at least three events of flightlessness that produced the different ratite orders, that the similarities between the ratite orders are partly due to convergent evolution, and that the Palaeognathae are monophyletic, but the ratites are not.

Other authors have questioned the monophyly of the Palaeognathae on various grounds, suggesting that they could be a hodgepodge of unrelated birds that have come to be grouped together because they are coincidentally flightless. One point is that unrelated birds have developed somewhat ratite-like anatomies multiple times around the world through convergent evolution. McDowell (1948) asserted that the similarities in the palate anatomy of paleognathes might actually be neoteny, or retained embryonic features. He noted that there were other feature of the skull, such as the retention of sutures into adulthood, that were like those of juvenile birds. Thus, perhaps the characteristic palate was actually a frozen stage that many carinate bird embryos passed through during development. The retention of early developmental stages, then, may have been the mechanism by which various birds became flightless and came to look similar to one another.

Read more about this topic:  Paleognathae

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