Seems dinosaurs and humans do coexist
March 11, 2008 1:33 PM   Subscribe

Why aren't turtles dinosaurs?

Turtles are ancient reptiles that have existed since the Triassic, the time of the dinosaurs. Why aren't they considered dinosaurs?
posted by Listener to Science & Nature (18 answers total) 16 users marked this as a favorite
 
This book seems to say that they are.
posted by ND¢ at 1:39 PM on March 11, 2008


Dinosaurs are archosaurs, turtles are anapsids.
posted by zamboni at 1:43 PM on March 11, 2008


The important bit being that archosaurs are diapsid.
posted by zamboni at 1:46 PM on March 11, 2008


Hold on. Apparently, turtles are sort of up for grabs. That'll teach me to rely on my childhood dinosaur research!
posted by zamboni at 1:50 PM on March 11, 2008


The point of taxonomical classification (at least in its modern practice) is to understand evolutionary relationships between species. That turtles are not considered dinosaurs is the result of an understanding of physiological, paleontological, and genetic information in an evolutionary framework. Paleontologists and zoologists have looked at all the data and have come to a conclusion that class of animals we call turtles branched off from the set of all reptiles in an evolutionary event distinct from the event in which dinosaurs branched off from all reptiles. They're different lineages, with different evolutionary histories. There's no simple heuristic for understanding it: it's the result of a confluence of many observations and decades of theory.
posted by mr_roboto at 1:56 PM on March 11, 2008 [2 favorites]


Yeah, turtles are anapsids, but from what I remember consensus is that this is a derived rather than an ancestral trait - that is, they evolved from non-anapsid ancestors, and subsequently lost their skull perforations to become similar enough to the ancestral anapsid stem group that they were long considered to be one and the same.

Biggest issue is that turtles as a group are so old we don't have any good examples of turtle ancestors to compare them to.

[Just in case it helps anyone: anapsid, synapsid and diapsid refer to the number of openings in the skull behind the orbit; anapsid = 0, synapsid (mammal) = 1, diapsid (reptile) = 2]
posted by caution live frogs at 1:56 PM on March 11, 2008


And by "class", I mean "order".
posted by mr_roboto at 1:59 PM on March 11, 2008


Where turtles fit in the tree of life is an open question, as zamboni points out. Molecular and physical evidence makes is pretty clear that they are not archosaurs, so they are not dinosaurs. The question is how closely they are related to dinosaurs, lizards, and (for that matter) mammals.

All this brings up the question of what it means when we say that X is or is not a dinosaur. The scientifically accepted definition, which probably does not jib with your gut level understanding, is that X is a Y if any of X's ancestors were a Y. So dinosaurs can all trace their ancestry down to the first creature that we would recognize as a dinosaur. This includes birds, which are dinosaurs (so humans and dinosaurs do coexist!) but not archosaurs, which are a slightly larger group that includes crocodiles, nor diapsids, an even larger group that includes lizards and snakes (and maybe turtles).
posted by Maastrictian at 1:59 PM on March 11, 2008


nickerbocker - not necessarily. Theropod dinosaurs = stem group for birds, by all current evidence; if all dinosaurs were extinct, birds would also be extinct by strict definition. (All theropods are dinosaurs. Birds are theropods. Therefore birds are dinosaurs. It screws with out notions, sure, but cladistically that's where they fit, and as defined groups must be monophyletic, we can't just exclude Aves from Sauropodia because we want birds to be a separate class for purely historical reasons, can we?)
posted by caution live frogs at 2:05 PM on March 11, 2008


[few comments removed - dumb guesses can be written on pieces of paper and tossed out the window]
posted by jessamyn (staff) at 2:05 PM on March 11, 2008 [24 favorites]


Don't the legs have something to do with it, or is that just true with lizards?
posted by Sys Rq at 2:06 PM on March 11, 2008


Listener, the take-home lesson is that not all "reptiles" that were alive in the Triassic were dinosaurs.

Relevant Tree of Life link:
http://www.tolweb.org/Amniota/14990
posted by Eothele at 2:38 PM on March 11, 2008


Now with more happy linky goodness.
posted by Eothele at 2:39 PM on March 11, 2008


Piggyback: Actually, I always wondered where birds stop being classed as dinosaurs, too. The first birds WERE dinosaurs, right? So where does that line break? So I just went with turtles, which at least are reptiles.
posted by Listener at 2:40 PM on March 11, 2008


The first birds WERE dinosaurs, right? So where does that line break?

It doesn't. Birds are dinosaurs, at least according to cladistic taxonomy.
posted by mr_roboto at 2:44 PM on March 11, 2008


Listener, the take-home lesson is that not all "reptiles" that were alive in the Triassic were dinosaurs.

Exactly so. For instance, the Pterosaurs and Plesiosaurs are not considered to be Dinosaurs.
posted by Class Goat at 2:51 PM on March 11, 2008


There are natural groups, like Romans, The French New Wave, and the Dinosauria including birds.

There are artificial groups, like barbarians, foreign film, and the Dinosauria excluding birds.

Again with the Tree of Life. Think of the node immediately above and to the left of the S in Saurischia, where all of the lines come together, as a hypothetical anscestor of all dinosaurs. In other words, "Dinosauria" is our name for the genealogical process that began with this hypothetical ancestor. That process includes the birds. Excluding the birds is arbitrary, and makes the Dinosauria artificial (in this case, paraphyletic).

What's not immediately apparent on the Tree of Life page is all of the rigorous empirical work that went into hypothesizing that ancestor. Check the ref's & links for that. For birds & dinos in particular see also here. For the turtle thing see here, or the Tree of Life page that I linked above.
posted by Eothele at 5:22 PM on March 11, 2008 [5 favorites]


>There are natural groups, like Romans, The French New Wave, and the Dinosauria including birds.

>There are artificial groups, like barbarians, foreign film, and the Dinosauria excluding birds.

This answer is double plus good. Thank you.
posted by Listener at 8:03 PM on March 11, 2008 [1 favorite]


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