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Posts Tagged ‘palaeontology

some stuff about dinosaurs and their relationship to birds

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Archaeopteryx lithographica with its long bony tail – I took this pic myself at London’s Natural History Museum

Jacinta: Let’s talk about dinosaurs. Are they a thing?

Canto: Of course they are, what are you talking about?

Jacinta: Well I read recently in a New Scientist article that for quite some time in the recent past dinosaur experts didn’t really think ‘dinosaur’ existed as a scientific classification. A new way of classifying was needed because some dinosaurs were bird-hipped and some were lizard-hipped, though they were neither birds nor lizards. So, new names were required.

Canto: Right, so some had hips like lizards, but were clearly not lizards because they had anatomical features that set them apart, and the same went for those that had hips like birds?

Jacinta: Yes I think that’s right. Let’s talk as we learn. Bird-hipped dinos are ornithischians – think ornithology – and the lizardy ones are called saurischians. It was Harry Seeley who shook up the dinosaur-loving world back in 1887 when he argued, before the Royal Society, that what they’d thought were dinosaurs (a term coined by Richard Owen) were really two separate groups, based on those hip bones. Seeley was right about the two groups, but the term ‘dinosaur’, which of course has never disappeared in popular writing, has been rescued over time for science by agreement on other features which bespeak ‘dinosaur’. This has much to do with cladistics, which we may or may not discuss later.

Canto: So the first dinos appeared some 235 mya in the late triassic period, but interestingly they flourished between two major extinction events, the Triassic-Jurassic extinction event about 201 mya, a very sudden event that allowed dinosaurs to fill vacated ecological niches on land, and the Cretaceous-Paleogene (or Cretaceous-Teriary, or K-T) extinction event of 66 mya, which wiped out all the non-avian dinos.

Jacinta: And it should be mentioned that birds are now considered feathered avian dinosaurs, descended from earlier therapods, which strangely are saurischians (lizard-hipped), though a very recent and still controversial paper has reclassified them as ornithischians. I should also mention that dinosaur researchers are a notoriously feisty and bickering tribe, from what I’ve heard.

Canto: I’ve started ploughing through a course on dinosaurs via youtube – The Natural History of Dinosaurs – and I’ve already learned some words, just as background: lithify, diagenesis and coprolite. I’ll let you know if anything exciting crops up, but tell me more about birds being the only remaining dinosaurs and how we know that.

Jacinta: Well, it’s been known since at least the discovery of Archaeopteryx, the type specimen of which was found just two years after Darwin published The Origin of Species, that there are clear anatomical similarities between birds and non-avian dinosaurs. Feathers and hollow bones, for example. There’s also evidence that they share nesting and brooding behaviour. There are also relations with non-avian dinosaurs, some species of which also had feathers, and these discoveries are raising fascinating questions about the origin of flight in these creatures. Of course it’s all very controversial and some researchers are still holding out on the dinosaur-bird link, suggesting other archosaurs were the ancestors.

Canto: What’s an archosaur?

Jacinta: It means ‘ruling reptile’ and these are creatures which first emerged some 300 mya, and they’re the ancestors of living reptiles today. They’re also the ancestors of birds, and dinosaurs. So they’re a larger and older group. Presumably the hold-outs have reason to think birds emerged out of some reptilian line that was distinct from theropod dinosaurs. But that’s nothing to the arguments about the evolutionary steps that led from maniraptoran theropods (perhaps) to modern birds, or the arguments about the origin of flight. Now let me point out that theropods are a suborder of dinosaurs with hollow bones and three-toed limbs, which have long been classed as saurischians until this very recent paper discussed in the New Scientist article, which reclassifies them as ornithiscians. And this seems to be another step – if it holds – towards our understanding of the relationship between birds and their ancestral dinosaurs. An earlier but still pretty recent step were the discoveries, particularly out of China, of a number of fossilised dinosaurs with evidence of feathers, or proto-feathers, and all this, together with advances in analysing and categorising existing specimens using cladistics described in Wikipedia as ‘a method of arranging species based strictly on their evolutionary relationships, using a statistical analysis of their anatomical characteristics’.

Canto: I get very confused about all this. Weren’t there flying dinosaurs – we used to call them pterodactyls – and did they have feathers, or were their means of flight completely different? I seem to remember them depicted like gliders – I mean of the animal kind, with great flaps of skin to catch the wind… Of course that was long before any talk of feathered dinos.

Jacinta: Well hopefully I’ll get to that. Let me talk first about Archaeopteryx, which they reckon dates back to about 150 million years ago. It was probably about the size of a magpie, though there may have been different species of different size (only 11 fossil specimens have been discovered so far). They had feathers, but it’s not known whether they flew like modern birds (flapping flight) or merely glided. A recent study (which I’ve not read) has argued that their flight capabilities were quite limited. They had long, bony tails, which I’m assuming would’ve hampered long-term flight. Interestingly, complex and, for me, impossible-to-verify coloration analyses have presented evidence that the feathers of these critters were a matte black, at least predominantly. Of course it’s hard to prove all this conclusively with 150 million-year-old animals, but speculation and analyses continue, for example on the brain-case of one Archaeopteryx specimen, to determine whether it had a brain for flight (e.g. adequate eyesight, hearing and muscle manipulation). Most of this converges on a limited flight ability, but just how limited will be endlessly argued. And concerning the evolution of birds and flight, there’s a ‘trees-down’ theory (think of sugar gliders etc) and a ‘ground up’ theory. Where does Archaeopteryx fit with those alternatives? That’s still up for grabs.

Canto: Okay, so what about pterodactyls, are they still a thing? Dactyl means digit or finger, doesn’t it?

Jacinta: Winged finger. Yes, they’re a species of pterosaur, with thirty known specimens. They presumably achieved fame among the children of the world as the first-known flying dinosaurs – but they’re not dinosaurs. It’s confusing because ‘saur’ means ‘lizard’, and ‘dinosaur’ means ‘terrible lizard’ and ‘pterosaur’ means ‘winged  lizard’ and they all seem to be connected…

Canto: So what about their relation to birds? Any sign of feathers?

Jacinta: They may have had downy feathers here and there, but not for flight. Their wings were more like those of bats, and they were originally classified as an archaic type of bat. In fact, in the early days of taxonomy, many fossils that had vague similarities to the first pterodactyl fossils discovered in the late 18th century were wrongly designated as pterodactyls, which probably explains their general popularity. It has taken years and many improvements in analysis and dating to sort out the mess, and apparently it still hasn’t been sorted. Anyway, they’re not seen as ancestral to birds. But I may be wrong.

Canto: Wow. Disappointing.

Jacinta: So getting back to the origin of birds, the question of clavicles (collar bones) is important. Birds have wishbones (furculae), which are fused clavicles. The question of bird ancestry has hung on these clavicle bones to a large degree. They’re delicate bones, not easily preserved, and it was long thought that they didn’t exist in dinosaurs. This view has been completely overturned, and in fact most of our understanding about the relationship between birds and earlier dinosaurs come from skeletal studies, or re-examinations, as well as studies of musculature and internal organs, though of course it’s feathers that capture the public’s imagination. But of course there’s a lot of controversy about the how and when of bird evolution, and the evolution of flight, which you’d expect from such scant solid evidence together with intense scientific and public interest.

Canto: Well, I’ve learned something more than the little I knew before about dinosaurs. And their hips. I’ll watch the rest of The Natural History of Dinosaurs’, and we’ll speculate some more in a later post.

Written by stewart henderson

September 4, 2018 at 1:03 pm