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Post by Exalt on Sept 11, 2023 6:07:25 GMT 5
Why are Homotherium reconstructions so diverse?
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Post by Exalt on Sept 12, 2023 6:29:06 GMT 5
Why did small non-avin dinosaurs and pterasaurs die out after the KPG event? For the latter, why did they fail while birds succeeded?
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Post by theropod on Sept 12, 2023 14:39:28 GMT 5
Why did small non-avin dinosaurs and pterasaurs die out after the KPG event? For the latter, why did they fail while birds succeeded? As for pterosaurs, that’s easier to answer. Pterosaur diversity (while not as low as formerly thought) was clearly already declining significantly. Right now only two families of pterosaurs are confirmed to have survived up until the K/Pg boundary, azhdarchids and nyctosaurid pteranodontians (though admittedly the latter were only recognized to have lasted that long quite late). Azhdarchids were large, terrestrial predators of small to mid-sized prey, Nyctosaurids were large marine piscivores. So considering that they were this ecologically specialized, already had low diversity, and occupied high trophic positions, it is not surprising that pterosaurs went extinct. Azhdarchids were among the largest predators in many of the environments they occupied, and large predators are naturally among the most at-risk of extinction of any trophic position. Nyctosaurids, as large fish-eaters, were likely particularly vulnerable to a disruption of notoriously vulnerable ocean food chains. The small, non-avian theropods are the more complex question. I think for that it’s important to put things in perspective. You shouldn’t imagine that birds just generally survived, while other theropods went extinct. Rather, the extinction among birds was essentially almost as severe as for other dinosaurs. Avian diversity, just like non-avian theropod diversity, was very high immediately preceding the mass-extinction, and the vast majority of that diversity went extinct (see for example Longrich et al 2011). Latest Cretaceous avian biodiversity was very high, and highly dominated by stem-ornithurines and enantiornithines, but none of these lineages survive into the Cenozoic, only the Neornithes that appear to have just started radiating at the very end of the cretaceous did. There’s a hypothesis that stem-birds were primarily ecologically tied to forests, and the end-cretaceous collapse of forest ecosystems favoured the survival of basal Neornithines, that occupied more diverse, non-forest habitats (Field et al. 2018). So in rounded terms, almost as large a share of bird species went extinct as of other dinosaurs, the thing is just that that tiny difference, for which there does not necessarily have to be a hugely obvious reason, can be the difference between going extinct (extinction share of 1) and not going extinct (extinction share of 0.99≈1). In fact birds were probably simply one of the most diverse dinosaur groups at that point, which, added to being generally small in size, predisposed them to having the highest chance that at least some of those species would survive. Maybe it was pure dumb luck that it happened to be small Neornithes that survived, rather than some other lineage of small theropods. Or maybe there were slight ecological differences that slightly favoured neornithine birds over other small theropods. --- Field, D.J., Bercovici, A., Berv, J.S., Dunn, R., Fastovsky, D.E., Lyson, T.R., Vajda, V. and Gauthier, J.A. 2018. Early Evolution of Modern Birds Structured by Global Forest Collapse at the End-Cretaceous Mass Extinction. Current Biology 28 (11): 1825-1831.e2. Longrich, N.R., Tokaryk, T. and Field, D.J. 2011. Mass extinction of birds at the Cretaceous–Paleogene (K–Pg) boundary. Proceedings of the National Academy of Sciences 108 (37): 15253–15257. |
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Post by Exalt on Sept 12, 2023 15:44:02 GMT 5
I did learn a couple of weeks ago that birds and mammals both were hit hard, but I wasn't thinking about this at the time, and dd not consider luck, nor was I aware of the diversity at the time. Thank you.
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Post by Infinity Blade on Sept 12, 2023 16:10:24 GMT 5
Why are Homotherium reconstructions so diverse? There are certain aspects of its anatomy that have been debated, aside from different paleoartists having different levels of understanding on its proportions and whatnot. There's been debate on whether it was digitigrade or plantigrade, and whether or not the upper canines were exposed when the mouth was shut. Correct answer for these, btw, is digitigrade feet and covered canines (unlike Smilodon, they're short enough to be covered by lips). This recent (2020) publication I found uses Amphibia (Linnaeus, 1758)-> to list the class the Albanerpetontidae belongs to. Albanerpetontids were lissamphibians, and so they do indeed belong to the one monophyletic clade all modern amphibians belong to. Wouldn't it have been more appropriate to use Lissamphibia? I didn't think Amphibia was still referred to as a valid clade in modern circles, unless their idea of Amphibia isn't 100% the same as Linnaeus'. |
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Post by Exalt on Sept 12, 2023 18:49:02 GMT 5
Two more things:
1. What is the difference between basal and derived? I can't find anything. Looking at context suggests that basal members of a group would be early, and derived members are later and have more defined characteristics.
2. This is a pet peeve of mine, and I promise that I'm not trying to bring up Smilodon all the time, but how is it that on one hand, people still use the term, "saber-toothed tiger", and then on the other, some insist that Machairodonts aren't even cats? Maybe I'm being unfair but do we have any thoughts there?
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Post by Exalt on Sept 12, 2023 20:36:35 GMT 5
One more: how did pinnipeds end up at both poles, but little in-between?
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Post by theropod on Sept 12, 2023 23:33:41 GMT 5
Oh boy, this is a more complex question than you think. I recently got into an argument with another paleontologist about this, because even we can’t really agree on the proper, scientific definition of "basal". But I’ll give you my two cents.
Firstly, it’s important to keep in mind that "basal" is and will always be a pretty informal term, and that there are many paleontologists and evolutionary biologists who frown at it’s use and insist on using other, more precise terms. So don’t be surprised should your professor ever tell you to not use the term basal–I have had more than one who did exactly that.
Generally I think the the best objective, unarbitrary definition of these terms is that "basal" is stem-ward (i.e. closer towards the root of a phylogenetic tree) while "derived" is crown-ward, (i.e. more closely related to the smallest group to include all the extant representatives). An equivalent term would be "early-branching", or simply "stem-lineage"/"stem-ward".
The implication is that it’s always relative to the group you are talking about (there can be no inherently "basal" organisms, it depends on your frame of reference), and that you can only have "basal members" of a total-group that also has at least one extant member. In this scenario, the less recent your most recent common ancestor with the extant members is, the more basal you are.
"Derived" is basically the opposite of basal, meaning closer to the crown group, or within it. Again, one member of the crown group cannot really be more "derived" than another, objectively speaking. However, of course you can usually further subdivide a crown-group into smaller groups that each have their own stem-lineage and crown-group, and within those it then again becomes possible to be more basal/stem-ward or derived/crown-ward. It simply all depends on the frame of reference.
This principle is also sometimes taken a bit more liberally, not only to clades with an extant crown-group, but generally to lineages within a clade that branched off and went extinct earlier than others. Under this less strict definition, a crown-group can have "basal" members, but still only extinct ones. There’s no way under either of these definitions for one extant animal to be more "basal" than another extant animal.
For example there are "basal Tetanurans" (usually used to refer to Allosauroids and Megalosauroids, whose exact relationship to each other and to Coelurosaurs is not totally clear, but which are clearly both less closely related to the crown group of theropods (birds) than any coelurosaur, and are thus more "basal". So this is a case where basal vs derived is used properly, because the crown group is objectively defined, and those groups that branched off from it earlier are more basal than those that branched off later.
A slightly more informal example would be to talk of something like "basal Ornithischians" or "basal ornithopods", or "basal Ceratopsians". Strictly speaking such a thing doesn’t exist, but it will generally be understood what you mean; basal ornithischians are ornithischians that branched off early, before the diversification onto the major clades that survived until the end of the mesozoic, basal Ornithopods are ornithopods that branched off early relative to, for example, hadrosaurs, and basal Ceratopsians are Ceratopsians that branched off before the split into Leptoceratopsids and Coronosaurs (the group including ceratopsidae), the two clades that survived until the end of the Cretaceous.
Now for an example of clearly improper usage of the term, calling Monotremes or Marsupials "basal mammals" is not correct. Mammals are defined as the crown-group of mammaliaformes, which is the clade consisting of Monotremes, Marsupials and Placentals, so they are all within this crown-group, and they are all extant. It is true that Monotremes do exhibit some "primitive" characteristics, such as laying eggs, lacking pinnae and teats etc., but they also exhibit derived characteristics that do not reflect the condition of their common ancestor with Placentals and Marsupials, for example a lack of teeth and electrocereption. So there’s no objective grounds on which to call these animals "more basal" than therians, such a labelling would entirely depend on your vantage point; from the perspective of a monotreme, we would be the basal ones, the offshoot from the evolutionary lineage that led to themselves.
In addition, "derived" can also refer to a trait characterizing a group (it doesn’t matter if one considers the group itself "basal" or "derived" here, as long as it is monophyletic) by being a newly evolved characteristic of that group, also known as an apomorphy.
However, these terms, esp. "basal" do get tossed around a whole lot, often with a slightly different meaning whenever it’s convenient, and are also sometimes misused to make a point, even if they have no significance as to said point.
Often people will use "basal" in a relative fashion that is basically completely arbitrary and depends on the vantage point (such as the above example with the Monotremes). In such a case it may be entirely obvious to them what they mean, but anyone else not looking at it from the same perspective will be completely confused. Another most common tendency is for people to call things basal that they believe (whether based on evidence or not) to be more "similar" (usually in some arbitrary way) to the ancestral condition in a group. But again, as with the monotreme example above, animals tend to exhibit a mixture of basal and derived characteristics, meaning this is basically impossible to determine objectively (an animal could be basal in some ways and derived in others, according to this logic…so who’s to say what animal is basal overall?). Generally using such labels can still be ok, depending on the context and whether it manages to communicate something meaningful, which has to be evaluated on a case by case basis. However it’s always important to keep in mind that there’s no inherent biological significance or factual nature to describing something as "basal" or "derived" in this manner, it’s just a convenient terminology to label things.
"Sabre-toothed tiger" is a term that is basically exclusively used by laypeople who have no regard at all for phylogenetic accuracy.
As for "insisting" that machairodonts aren’t even cats, who does that? I’ve never seen this claimed, but then, I’m not that into Machairodonts. It is my understanding that Machairodonts are members of the stem-lineage of cats, so if one were to make the case for a node-based, crown-group definition of cats, then Machairodontines would fall outside it.
However I’ve never actually seen anyone do that, I always got the impression that the definition of Felidae as including Machairodontinae is pretty much consensus. That being said, it is also general consensus to not include certain other sabre-toothed feliforms, such as Barbourofelids, despite them also being (more basal) members of the cat stem-lineage, so in the end I’d say this is basically an arbitrary decision on how to name things; if the definition of cats is "the last common ancestor of Smilodon fatalis, Felis catus and Panthera leo, and all its descendants", then Machairodonts will always be in it even if it turned out that they were more closely related to insects than to Felis or Panthera.
And then of course there are even more distantly related sabre-toothed feliforms, the Nimravids, which are less closely related to cats than other extant animals, such as Viverrids and Hyaenids, so they cannot be within Felidae unless we want to include those other (distinctly un-feline) groups as well.
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Post by Exalt on Sept 13, 2023 3:11:28 GMT 5
Thank you once again.
Honestly, I think that for the first answer, that's enough of a bear that I may need to come back here a time or two before I really get it. I guess taxonomy isn't complicated enough, so we had to bring semantics into the mix...
As for the second one, maybe the latter idea isn't very widespread, but I heard about it from someone else. My perspective is not the most informed, but until I see something contradictory, I'm going with, "If it's a felid, it's a cat.'
What do you mean by the stem lineage of cats? As far as I can tell, Felinae, Pantherinae, and Machairodontinae are all believed to descend from Pseudaelurus, so I'm at a bit of a loss here. Does it have to do with the latter splitting off comparatively early?
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Post by theropod on Sept 13, 2023 10:32:27 GMT 5
Yes. Felinae and Pantherinae are the only extant cats. Together, they, their most recent common ancestor, and all its descendants form the crown group of cats. Everything outside this group, but more closely related to them than to any other extant, is part of the stem-lineage of cats. If I saw this right, that includes both barbourofelids and machairodontids, though the latter is more derived (=closer to extant cats), and seems to be included in Felidae by convention. But if someone was of the opinion that Felidae/cats should he defined to include only the crown group, then Machairodonts (being less related to either felines or pantherines than those groups are to each other) would fall outside it.
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Post by Exalt on Sept 13, 2023 20:56:09 GMT 5
Can you explain the purpose of the crown group concept? It just seems needlessly exclusionary to my uninformed self.
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Post by theropod on Sept 13, 2023 22:02:00 GMT 5
The purpose of this concept is to have a term to refer to the least inclusive group including all the extant members of a group.
For example "crown-group birds" refers to Neornithes, the group of birds to which all extant birds (and in fact all known birds after the K/Pg-extinction) belong, as opposed to the many lineages of stem-lineage birds that existed during the Mesozoic, but are less closely related to any extant bird than all of those extant birds are related to each other.
That’s important because it succinctly communicates something about how these animals are related to their extant relatives, and what kind of extant taxa we’d need to look at for phylogenetic bracketing. We cannot use a phylogenetic bracket only of extant birds on a stem-lineage bird, because all those extant birds share a more recent common ancestor with each other than with stem-lineage birds (so the stem-lineage bird is bracketed only from one side by extant birds), which tells us we’d need to look at the next-closest extant group, crocodilians, in order to get a complete bracket.
It’s also potentially important for definitions of groups to be aware what is and isn’t in the crown-group, because historically, many groups were first defined on the basis of extant animals only, so the crown-groups are sort of the minimum/least inclusive definitions for these taxa, whereas the total-groups (i.e. including the entire stem-lineage) are the maximum/most inclusive definitions. For many groups there are conventions of how to define them that are pretty much consensus, but for some others these definitions are in a permanent state of flux.
For example there’s no real agreement on what exactly "birds" even are, what they include or don’t include, We just know that it’s at least the crown-group of Neornithes, and at most the total-group, which includes all bird-line archosaurs (a terminology that would have made a lot of sense actually, but never really caught on in favour of somewhat more arbitrary definitions of what a bird is).
Is Archaeopteryx a bird? What about Deinonychus? Or Rahonavis? Or Balaur? Or Epidexipteryx? If you just say "birds", then it’s unclear what exact definition of birds you are using, and whether or not you are thinking of any of these taxa. But if you say "crown-group birds", then it’s clear that you are excluding these stem-lineage birds, and if you say "stem-lineage birds, then it is clear you are referring to something outside Neornithes, but at least closer to Neornithes than to crocodiles.
So that’s why we need the terms "crown-group" and "stem-lineage". Of course, keep in mind that in paleontology we are always looking at the stem-lineage of something, because every extinct taxon is on some extant taxon’s stem-lineage if you look closely enough (e.g. an extinct member of Panthera that’s closer to lions than to other members will be on the stem-lineage of Panthera leo).
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Post by Exalt on Sept 13, 2023 23:22:37 GMT 5
Thank you. I will perhaps address the bulk of your answer later, I think I may have looked at the concept from the wrong angle.
But I see your, "What is a bird?" and I raise you, since birds are dinosaurs, what is a reptile?
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Post by theropod on Sept 14, 2023 2:09:09 GMT 5
A reptile is any animal more closely related to birds, lizards and crocodiles than to mammals.
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Post by Exalt on Sept 14, 2023 2:17:43 GMT 5
I feel dumb for asking that now.
Are fish and amphibians considered equally close or distant from the two? Or would you just slot "amniote" in there?
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