This post will be the latest as of 1st April, so it's that time again...
A modern thrush
About 40% of all living mammal species are rodents. Over a third of those that aren't are bats, leaving just 38% belonging to any other group - primates, cloven-hoofed mammals and all the rest. These are, at least in terms of speciation, clearly very successful body plans.
But the bias is even stronger with birds. A whopping 60% of living bird species belong to just one order: the Passeriformes. The chances are that, if you think of a "typical bird" the image that pops into your head is of a passerine. There are about 140 different families of passerine (the exact number being a matter of taste among ornithologists), of which 123 constitute the songbirds.
Songbirds are distinguished by advanced musculature in their syrinx, an organ analogous to the larynx (or "voicebox") in mammals. Unlike the larynx, which birds also have, it's located at the base of the trachea, near the lungs, rather than up at the top in the throat, but it's this that explains why songbirds are so good at... well, singing. So far as we can tell, this more complex syrinx only evolved once, so all songbirds belong to the same suborder within the passerines, and it's one that's obviously of huge importance.
Even so, that leaves 17 families - three of which have over 100 species each - that are passerines, but not songbirds. They do have the syrinx, because all birds do, but it's less complex, and any birdsong they produce is strictly limited. They're mostly tropical, so not necessarily familiar to those of us living further north, but we're talking about antbirds, tyrant flycatchers, and the like.
Passerines are typically small birds, with the raven being the largest by body weight, and most being much smaller than that. (The raven is, perhaps surprisingly, a songbird... just not a very musical one). The distinguishing feature of passerines as a whole, songbirds included, lies in the structure of their legs.
Like most birds, the feet of passerines are anisodactylous, which is to say, they have four toes in total, three pointing forward and one (what would be the big toe in a human) pointing backwards. Where they differ is that they have a mechanism for locking their tendons in place that allows them to sleep while perched. The conventional wisdom is that their grip on a branch is maintained by their own body weight pressing down on the lock. It's probably not quite that simple, but, nonetheless, the common English term for passerines is "perching birds".
Given their importance, it could be argued that the appearance of the passerines was the most significant event in the evolution of birds, other than the development of flight itself. However, we know relatively little about the early history of the passerines, or what the earliest ones might have looked like. That's largely because they are so small and fragile, being unlikely to leave even partially intact fossil remains. So, while molecular evidence and biogeography tell us that they most likely evolved somewhere in the Southern Hemisphere during the Early Eocene, with the songbirds probably first appearing in Australia shortly thereafter, the details remain obscure.
A recent discovery, however, sheds some light on one small corner of this puzzle... and it comes from a bird that wasn't originally thought to be a passerine at all.
In 2010, a fossil bird unearthed in Denmark was formally described for the first time, being given the name Morsoavis sedilis. The fossil was unusually complete, including the whole of one leg and part of another. It lived 54 million years ago, during the Early Eocene, and, unsurprisingly given that age, could not be placed in any living family of birds. Nonetheless, the evidence at the time suggested that it was a shorebird, a cousin to the bird families that contain, among others, the seagulls and plovers.
But there was an oddity; the hindleg had some features in common with those of passerines, suggesting that it may have been able to perch, rather than being adapted for wading as modern shorebirds are. Over the following years, more fossils of the same and closely related species were discovered near London and in Virginia in the US. Our best guess as to where these birds should fit in the larger family tree switched around a bit, but in 2023, Morsoavis and two of its presumed relatives were placed in a new family, the Morsoavidae.
Crucially, this family was identified, not as belonging to the shorebirds, but as a stem group of the passerines. Classifying it as a "stem passerine" means that it is not thought to have any living descendants, and is not a passerine itself, but is closer to them than it is to anything else alive today. In other words, this newly described family of extinct birds is thought to represent an early branch from the ancestors of the true passerines that ultimately failed.
Moreover, living as early as it did, it was probably a very close relative of those passerine ancestors, and so could give us, if not proof, at least hints as to what they may have been like.
Last year, a fourth genus of birds was added to the family, based on a fossil recovered from a lagerstätte in Wyoming. Lagerstätten are sites with exceptional preservation of not just bones, but also of the outlines of soft body parts, due to their burial in fine sediments without any oxygen, killing off most of the bacteria that would normally cause decomposition.
In this case, the fossil was almost complete, including the remains of the feathers. These latter are sufficient to indicate that it probably had a short tail and does not appear to have had a crest on its head. Its formal description gives it the name Consoravis turdirostris, which translates as "thrush-beaked consort bird", the first part having been chosen so as to sound similar to that of two of the other genera in the family - Morsoravis ("bird from Mors") and Soroavis ("sister bird"). It has been dated to 51.6 million years ago, during the Early Eocene.
A particularly key part of this new fossil is the preservation of the feet, which are notably incomplete in all the known fossils of Morsoravis. This enabled the researchers to conclusively prove that it isn't a shorebird, while various other aspects of the skeleton suggest that the "stem passerine" attribution is probably correct. However, it turns out that the foot was not anisodactyl, as any real passerine foot should be.
Instead, it's "semi-zygodactyl". This is a pattern seen in owls and ospreys, among other birds, and it means that the fourth toe can be rotated backwards on a bony flange so that it can face in either direction as the bird wishes. Most of the other stem passerines known have fully zygodactyl feet (that is, the fourth toe can only ever point backwards, so there are two toes to the front, and two to the back), so the morsoravids are closer to the modern pattern than some of their relatives from the same time period.
That they don't appear to be any closer in other respects suggests that this modification of the feet is a case of parallel evolution from an ancestor that, like parrots and woodpeckers, was fully zygodactyl. The authors suggest that, like ospreys, Consoravis and its relatives relied more on their feet for manipulating food than their beaks.
That beak, as the scientific name suggests, resembles that of a thrush. This isn't hugely helpful in itself, since thrushes are omnivores with diets that vary across the 194 living species. However, other aspects of the skull and legs suggest, according to the authors, that, while Morsoravis would have been suited to poking into the soil to pull out worms and other invertebrates, Consoravis used the grip of its feet to climb about in tree branches in search of fruit and insects, while also occasionally descending to the ground to forage for invertebrates among the leaf litter.
[Photo by Dion Art, from Wikimedia Commons.]
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