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When I was a child, I thought I knew what a dinosaur was. But, as I grew older, I was told that many of the extinct species that I thought were dinosaurs (dimetrodons, pterosaurs, plesiosaurs, etc.) were not in fact dinosaurs. Is there a reasonably quick way for a lay person to tell apart true dinosaurs from non-dinosaurs without taking up an in-depth, species-by-species study of paleontology?

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There are several defining morphological characteristics of dinosaurs. Including their ankle structure and perforated acetabulum, Aka the hole in their hip goes all the way through, as well as a few other minor features. These combined makes it literally impossibly for a dinosaur to take on a sprawling pose without breaking hind leg in many places, this is why you will often see an erect posture given as the reason, but that is a product of these feature not the defining feature itself, there are many ways to achieve upright posture.

Now of course we are moving away from purely morphological definitions, cladistically dinosaurs are defined as The last (most recent) common ancestor of Triceratops horridus and Passer domesticus (common pigeon), and sometimes Diplodocus carnegii as a third, and all of its(the ancestor's) descendants.

A more superficial quick and dirty method is if it is a pterosaur or lives in the ocean it is not a dinosaur, If its limbs sprawl it is also not a dinosaur. I should also include this guys work, since he attempted to create a complete cladogram of every dinosaur, so you can visualize the group.

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  • $\begingroup$ Could you explain how dinosaurs could possibly be defined as "the last common ancestor...", since birds diverged from the dinosaurs tens of millions of years before the K-T extinction? $\endgroup$ – jamesqf Jul 22 at 16:08
  • $\begingroup$ Sorry that should say the "most recent" common ancestor, which is probably closely related to Nyasasaurus. It is worded that way because it is assumed we will never find the exact ancestral species this way any new discoveries can be assessed. If the common ancestor thig is giving you trouble try this, evolution.berkeley.edu/evolibrary/article/0_0_0/… $\endgroup$ – John Jul 22 at 16:23
  • $\begingroup$ I understand the common ancestor bit, what I don't understand is the choice of species. IIRC Triceratops was a late Cretaceous (group of) species, while birds diverged from theropods sometime in the Jurassic. But there are dinosaurs going back to the Triassic (e.g. Nyasasaurus). So how can "dinosaurs" be defined as the descendants of something that existed tens of millions of later? Wouldn't there be many dinosaur lineages that coexisted with that ancestor, but diverged earlier and followed different evolutionary paths? $\endgroup$ – jamesqf Jul 23 at 1:16
  • $\begingroup$ @jamesqf You misunderstand, all the descendants of the common ancestor, not those two (although their descendants would be included). Since those two are from each of the major dinosaur lineages respectively that means the descendants of their common ancestor will include all dinosaurs. Triceratops and birds are about as distantly related as you can get within dinosaurs. $\endgroup$ – John Jul 23 at 3:19
  • $\begingroup$ But aren't the ancestors (or at least some of them) of that last common ancestor also dinosaurs? At least back to the point where they could be confused with reptiles? Also lineages that branched from the ancestors of that last common ancestor? $\endgroup$ – jamesqf Jul 24 at 4:13
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Yes, the legs of dinosaurs are directly beneath their bodies. This is more efficient that the sprawling stance of lizards, crocodilians, pelycosaurs, and other mammal-like reptiles. It is likely no coincidence that another successful group, mammals, also have their legs directly beneath them. Also, as far as I know there were no fully marine dinosaurs.

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    $\begingroup$ Please provide support for the various claims you make in your answer. Thanks, Karl. $\endgroup$ – theforestecologist Apr 1 at 15:07
  • $\begingroup$ This would hardly seem to apply to pterosaurs and plesiosaurs. $\endgroup$ – jamesqf Apr 1 at 17:12
  • $\begingroup$ Points of fact do not need to be supported by citation. Also, it does apply to both pterosaurs and plesiosaurs, whose limbs come out from the sides. $\endgroup$ – Karl Kjer Apr 2 at 16:26
  • $\begingroup$ There are many examples of "upright" postures among animals you list as having sprawling posture. $\endgroup$ – kmm May 1 at 17:11

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