Saturday 1 April 2023

First of the Falcons?

Crested caracara
As I write this, it's 1st April, and that means, in lieu of trying to make a joke that somebody in the future will think is a real science item when they fail to notice the date, that it's time for: birds!

Trying to figure out the higher-level evolutionary relationships among animals can be tricky. Until the last few decades, we had to rely on physical comparisons and visible points of similarity, essentially a more sophisticated and well-informed version of what Linnaeus did back in the 18th century. With lower-level groups this can be reliable; nobody is surprised to discover that a moose is a type of deer or that rats are related to mice. But even here, parallel evolution can leave a misleading signal. It is not, for example, obvious that hyenas are more closely related to cats than to dogs (although, of course, they're neither). 

This problem gets bigger when we move to higher-level groups where physical similarity is no longer a reliable guide at all. Is a mouse more closely related to a dolphin than an elephant? That's not an easy one to answer on physical grounds alone. (Dolphin, probably, before anyone asks). It's much the same with birds.

During the 20th century, the diurnal birds of prey, as distinct from the owls, were placed together in the order Falconiformes, including the falcon, hawk, condor, and osprey families, along with the secretarybird. This is the same taxonomic rank as "primate" or "rodent" so it's reasonably high, but we'd expect a fair degree of similarity between its members, which is, indeed, what we get. (Of course, such ranks are arbitrary and defined for convenience; nature doesn't really divide up into neat levels in this way, so we can't say that the two things are "equivalent" in any biological way). 

However, it's been clear since at least 2006 that this is wrong. Genetic analysis, confirmed several times over the years, has shown that falcons in turn belong to a group called the "Australaves" or "southern birds". This is by far and away the largest group within the birds, in terms of number of species, because it also includes all the songbirds, as well as the parrots. All of the other diurnal birds of prey, while they do indeed, form a group, belong to quite a different branch, probably close to the owls, kingfishers, and woodpeckers, although the exact details have yet to be nailed down.

Either way, the similarity between falcons on the one hand, and hawks and ospreys on the other, is another case of parallel evolution; similar solutions to the same lifestyle. The end result of this is that, while the Falconiformes remains as an order, it now contains just one family: the falcons. Which is only about a quarter the size of the hawk family - although, for what it's worth, it does also include the kestrels and caracaras.

The fossil record of falcons is generally poor. Like other flying birds, their bones are light, fragile, and often don't preserve well, and they don't have teeth, which can be so helpful in classifying fossil mammals. The genetic studies reveal that the Falconiformes must have split from other birds quite early in the Age of Mammals, but the earliest known fossil falcon is Falco hezhengensis from northern China, which dates back only to the Late Miocene 5-10 million years ago. 

That has a reasonably complete skeleton, so we can be confident that it is what it appears to be. A much earlier fossil, dating back over 50 million years, comes from Antarctica (relatively hospitable at the time) but it consists of a not very-well preserved leg bone, making its identity less certain, and telling us little even if its correct. 

There is, however, another possible candidate for an early falcon. Massilaraptor was first described in 2006 from a fossil uncovered from 47 million year old deposits near Frankfurt in Germany. Unlike the Antarctic specimens, it includes a fairly complete skull, as well as some other, less well-preserved, bones from elsewhere in the skeleton. This is enough to show that it was definitely a bird of prey of some kind, but at the time, there wasn't enough to confirm whether it was a hawk, a falcon, or neither - and you'll also note that this was the same year that we first confirmed hawks and falcons aren't close relatives, so the taxonomic situation was still in flux.

What was clear was that, whatever the bird's affinities, it wasn't close enough to anything alive today to be placed in a living family. It was given its own family, to which further samples from Germany and Belgium have been added and there's apparently another possible contender from Wyoming that has yet to be formally described. It turns out, however, that better fossils may have been hiding elsewhere.

Back in the 1990s, a private fossil collector uncovered remains of some birds from a site in Essex, in southeast England. He described them in a letter as belonging to small relatives of the mighty terror-birds, giant flightless carnivores native to South America. This, it has to be said, seemed a bit unlikely. But, since no professional scientists were able to examine in detail what he kept in his private collection, there was no way to know what he actually had.

However, the collector passed away in September 2021, donating his entire collection of around 700 fossils to National Museums Scotland. Last year, some of the larger specimens of the supposed terror-birds were formally described and given a scientific name: Danielsraptor, after their discoverer. 

The fossil came from 55-million year old deposits, meaning that the bird was much older than the estimated origin of the falcon family 34 million years ago. Nonetheless, the new study does confirm that it, and the massilaraptorid family as a whole, were falconiformes, making them the closest known  fossil relatives of the modern falcon family. 

In particular, they had some features in common with the most primitive known falcons, such as long legs. These might suggest that they were largely ground-dwelling birds, like caracaras of Latin America and the southwestern US, which are basically scavenging, slow-flying falcons. On the other hand, the study also pointed to some features of the tail and wing bones that suggested long flight feathers were attached to them - something that would make ground-dwelling unlikely.

The beak is long, with a straight upper surface and a hooked tip. This, again, looks very similar to the beaks of caracaras but, in all fairness, also has a strong resemblance to the beaks of terror-birds. This could be another case of parallel evolution, but it's worth noting that the terror-birds are not exactly distant relatives of the falconiformes. Indeed, although it's not the general consensus, some ways of analysing the genetic data suggest that the cariamiformes (the order to which terror-birds probably belonged) are the closest living relatives of the falcons. 

There are, of course, multiple ways of interpreting that. It could be parallel evolution among two groups of related predatory birds. It could be that the terror-birds and falcons are more closely related than generally thought, and we have here a sort of "missing link" that connects the two. Or it could be that the common ancestor of terror-birds and the falcons had these features and the massilaraptorids just never lost them. 

Which, if true, would likely mean that the first "southern birds" - the ancestors not only of falcons and seriemas, but of parrots and the entire vast array of songbirds, too - was a long-legged and possibly fast-flying, predator.

[Photo by Dan Pancamo, from Wikimedia Commons. Cladogram adapted from Mayr & Kitchener 2022.]

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