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I know that birds evolved from dinosaurs (or a related clade). However, some dinosaurs are large and others are small. Did birds evolve only from a line of dinosaurs that stayed small throughout evolutionary history, or did the ancestors of bids get large at some point and then shrink?

Concretely, what’s the largest known animal among the ancestors of birds?

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    $\begingroup$ I think you need to clarify this a bit. First, how do you measure "large" for birds, body mass or wingspan? And are you asking only about ancestral species, or include lineages that have died out? $\endgroup$
    – jamesqf
    Aug 8, 2021 at 17:18
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    $\begingroup$ @jamesqf: Good point, I meant mass, since the goal was to include non-bird ancestors. And by "ancestors of birds", I meant ancestral species. $\endgroup$
    – Geoffrey Irving
    Aug 8, 2021 at 19:07
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    $\begingroup$ It is basically impossible to identify direct ancestor, the best we get in paleontology is something closely related to the ancestral lineage. $\endgroup$
    – John
    Aug 9, 2021 at 3:20
  • $\begingroup$ consider Herrerasaurus the most primitive therapod, which is bigger than a human en.wikipedia.org/wiki/Herrerasaurus $\endgroup$
    – John
    Aug 9, 2021 at 3:27

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This is actually a very difficult question to answer - because how far back do you go. The clade Avialae which includes the true birds is fairly recent and contains a lot of new dinosaurs(! -debate over feathers), but the clade Avialae is included in the theropod (Theropoda) group of dinosaurs, which includes such things as Tyrannosaurus rex, Spinosaurus, and Giganotosaurus, all of which are fairly large. However, these are almost certainly sister clades to that of the birds rather than actual ancestors.

However, Avialae is a made up clade with no actual fossil representing it; it is purely hypothetical as an ancestral marker for birds.

It seems that there is quite a bit of debate among paleo ornithologists as to which groups of dinosaurs are most closely related to birds. It is thought that things like Archaeopteryx are part of the bird ancestral clade, but even this much is uncertain, as there are a number of other feathered dinosaurs within the larger clade, that definitely did not evolve into birds. You might consider Neotheropoda a suitable cut-off, as this contains the true birds, but is still quite diverse. You could also cut off at Averostra, which includes Aves, and Ceratosauria, a group of quite large feathered dinosaurs.

Having said all that, we are very unlikely to even know which species are directly ancestors of birds, it seems that there are a lot of species that are very similar and all closely related, but making the choice of which is down to experts and a paucity of specimens. See this bit on Birds for some of the difficulty in defining them. If I had to make a choice, I would probably go with Ichthyornis as a definite ancestor, but I am no expert (I'm a virologist...).

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  • $\begingroup$ Interesting, thank you! I suppose in the absence of known common ancestor species, answering this kind of question might require some entirely difference source of information to separate sister clades off, such as the presence or absence of genes related to large size. $\endgroup$
    – Geoffrey Irving
    Aug 8, 2021 at 14:31
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    $\begingroup$ @Geoffrey Irving: I doubt that genes for large size would work, considering that the elephants are closely related to the hyrax: en.wikipedia.org/wiki/Hyrax Or that the dodo is descended from pigeons, or the considerable size variation among domestic dog breeds. $\endgroup$
    – jamesqf
    Aug 8, 2021 at 17:24
  • $\begingroup$ @jamesqf Not sure what your mean by that, since we know that, in humans, size is mostly genetic. So there are definitely genes for size. $\endgroup$
    – Konrad Rudolph
    Aug 8, 2021 at 19:48
  • $\begingroup$ @Konrad Rudolph: But those genes are obviously not... is stable the word I want, or maybe conserved? Anyway, my point is that size can change drastically in relatively few generations, either through natural evolution (see e.g. "island dwarfism" and "island gigantism"), or by selective breeding. $\endgroup$
    – jamesqf
    Aug 9, 2021 at 5:38
  • $\begingroup$ @jamesqf I think it’s mostly that height is a complex, polygenic trait: hundreds of pleiotropic genes all have a tiny influence, and these genes are mostly not in linkage disequilibrium, so they are inherited independently and are subject to different selective pressures. $\endgroup$
    – Konrad Rudolph
    Aug 9, 2021 at 9:57
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The largest bird was Vorombe titan, 3m tall and weighing 650kg. It is likely that this was larger than any of its ancestors.

Working back through the lineage of birds in Theropoda, the animals that are on the avian linage are small. One finds some early therapods that may be close to the common ancestor that were "largish" but smaller than V. titan. Going back to basal Archosauriformes one finds a side-genus like Proterosuchus, a crocodile-like animal that could be 3m long, but not as massive as V. titan. It is possible that there is a large semi-aquatic creature in the archosaur lineage, but it is more likely that the common ancestors of crocodiles and birds were smaller. The megafauna of the Permian were all synapsids and more closely related to modern mammals than to birds.

Prior to that, there are lizard shaped creatures, salamander-like creatures, back to lobe-finned fish and so on. No megafauna.

None of these potential ancestors is as large as V. titan. I'd conclude that the largest animal on the "bird" lineage is this type of Elephant Bird.

Perhaps this is not so surprising. Large creatures are vulnerable to extinction. The common ancestors of the extant clades tend to small adaptable generalists.

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  • $\begingroup$ I was going to answer that, but V. titan isn't an ancestral bird, it is one of the extinct clades within Aves, just like any number of more recent species - passenger pigeon, dodo etc. For it to be ancestral it must have direct ancestors now. $\endgroup$
    – bob1
    Aug 8, 2021 at 21:17
  • $\begingroup$ @bob1 I assume you meant it must have direct descendants, right? $\endgroup$
    – terdon
    Aug 9, 2021 at 12:04
  • $\begingroup$ @bob1, that is a very reasonable interpretation of the question, in which case it is likely that the African ostrich is the answer to the question: extant birds are descendants of generally smaller ancestors, though there may be some theropod ancestors that were larger. As your answer implies, we probably don't have actual fossils of theropods on the line to modern birds, we only have various side branches, but if we do, it is very hard to establish which, if any are ancestors and which are cousins. My interpretation is "clade Aves and ancestors", and V. titan is certainly in Aves. $\endgroup$
    – James K
    Aug 9, 2021 at 16:10
  • $\begingroup$ I should say that “the largest ancestor is a large current or recent bird” is a good answer to the underlying motivation of whether the ancestral line went large and then shrunk or not, which is why I accepted the answer. I should probably have phrased my question this way explicitly rather than trying to concretize it. $\endgroup$
    – Geoffrey Irving
    Aug 9, 2021 at 19:06
  • $\begingroup$ @GeoffreyIrving in that case the question should have been "what was the largest bird ever?", not a specific ancestral bird. BTW, Dinornis robustus was taller but probably not heavier than V. titan, having said that, there were Dinornis species that were about the size of a turkey too... $\endgroup$
    – bob1
    Aug 9, 2021 at 20:02

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