Mike Keesey  

Just what is a turtle?

January 30th, 2007 by Mike Keesey :: 6 Comments

Turtles serve to demonstrate one of the quandaries of biological nomenclature (naming of groups of life forms). Many of us were taught in school (and may have forgetten) that Class Reptilia has four living orders, one of them being Order Testudines (also called Chelonia, although that’s the name of a sea turtle genus as well), which includes all turtles. What is a turtle? Well, it’s self-evident: those reptile things with shells and beaks and shoulder girdles positioned inside their ribs, etc. Everyone knows what a turtle is. (Well, except for some Brits, who seem to think that terrapins and tortoises are completely separate categories … but I digress.)

Well, it was not always so simple. Modern turtles have many traits in common, but they could not all have arisen at once. Perhaps the beak was first, then parts of the shell, then the migration of the shoulder girdle, then the rest of the shell, etc. Which animals were turtles: the ones with just a beak, the ones with part of a shell, or the ones with all of the shell? (What does this mean for the Teenage Mutant Ninja Turtles, with their half-shells?) Or was the order of traits different? (As noted in my last essay, the origins of turtles are not well-understood.) This kind of problem crops up in a lot of places: where do you draw the line if a group is well-delineated today, but was blurrier in the past? (Birds are another excellent and contentious example.)

It’s important to understand several types of groups in order to delve further into the issue. In phylogenetic nomenclature, all named groups are clades or species. (Arbitrary designations like “Order” and “Class” are eschewed—and actually, it’s debated as to whether “Species” might be arbitrary.) A clade is an ancestor plus all descendants. Examples of clades include Mammalia, Angiospermae (flowering plants), and Dinosauria (but only if birds are included). Groups that are not clades include outdated taxa like “Pisces” (which includes the last common ancestor of fish but excludes some of its descendants—namely, us tetrapods) and “Pachydermata” (which includes several distantly related lineages of large, thick-skinned mammal but does not include any of their common ancestors).

There are three relevant types of clade. A crown group includes the last common ancestor of living organisms, plus all descendants of that ancestor. A total group includes all organisms sharing more recent ancestry with a crown group than with any living organisms not in the crown group. As an example, Mammalia is often used as a crown group (the last common ancestor of monotremes, marsupials, and placentals, plus all descendants of that ancestor), while Synapsida is sometimes used as a total group, including everything closer to mammals than to living reptiles (this includes mammals as well as extinct relatives such as Dimetrodon, Moschops, Cynognathus, etc.) Every crown group is included by a corresponding total group. The total group consists of the crown group plus the stem group, the extinct relatives of the crown group. The non-mammalian synapsids may be referred to as “stem-mammals” (This is preferable to the older, less accurate nickname, “mammal-like reptiles”.)

--Synapsida (=Pan-Mammalia)
        `–Mammalia (crown group)
           |–Monotremata (platypuses, echidnas)
           `–Theria (marsupials, placentals)
* extinct stem-mammals

Finally, an apomorphy-based clade is defined based on a derived character (apomorphy). For example, Avifilopluma is defined as the first pan-avian ancestor of Vultur gryphus (the Andean condor) to possess feathers homologous with those of V. gryphus, plus all of that ancestor’s descendants. Apomorphy-based definitions have some special problems. In the case of Avifilopluma, it is not always possible to tell whether a given fossil organism possessed feathers. We can say pretty confidently that Archaeopteryx, Caudipteryx, Dilong, Sinosauropteryx, etc. must belong, since they preserve feathers. We can also say confidently that Deinonychus, Ichthyornis, Oviraptor, Tyrannosaurus, etc. also belong, since, even though their fossils don’t preserve feathers, other evidence shows that they descend from an ancestor that did (and probably had feathers themselves). But for some fossil taxa, such as Proceratosaurus, it’s not known whether the last common ancestor with V. gryphus had feathers or not, so the group lies in a sort of “avifilopluman limbo”.

--Pan-Aves (=Avemetatarsalia)
        |–Allosaurus (not feathered)
           |–Proceratosaurus (feathered? who knows?)
              |–Sinosauropteryx (feathered)
              |–+–Dilong (feathered)
              |  `–Tyrannosaurus (presumed feathered)
              `–+–+–Caudipteryx (feathered)
                 |  `–Oviraptor (presumed feathered)
                 `–+–Archaeopteryx (feathered)
                    |–Deinonychus (presumed feathered)
                    `–+–Ichthyornis (presumed feathered)
                       `–Vultur (feathered)

(I seem to have gotten around to dinosaurs; back to turtles.)

One solution to the question of what to include in Testudines was proposed by Joyce et al. (2004). They advocate limiting the term to the crown group, the last common ancestor of living turtles, plus all of that ancestor’s descendants. This general approach is preferred by some systematists, who argue that it prevents researchers from making “unjustified inferences” (de Queiroz and Gauthier, 1992) about extinct members of the stem group. For example, someone talking about Aves (birds) might talk about soft-tissue or behavioral characters such as warm-bloodedness, powered flight, patagia (skin flaps on the wing), etc. But it is not known exactly when these traits arose, and many organisms commonly referred to as “avian” may not have had them. (E.g., Archaeopteryx seems not to have had patagia, may not have flown, and a few researchers don’t think it was warm-blooded, either.) For this reason, these systematists feel that commonly-used clade names are best attached to crown groups.

(Damn, dinosaurs, again! Must … get … back … to … turtles!)

This idea is a bit contentious for Aves (although I lean toward it, myself) but works very well for Testudines, especially if (again following Joyce et al.) we use a similar name, Testudinata, for an apomorphy-based clade based on the possession of a full turtle shell (detailed in Joyce et al., 2004), and Pantestudines (or, as a new PhyloCode rule may have it, Pan-Testudines) for the total group, including everything closer to modern turtles than to lizards, tuataras, crocodylians, or birds. Thus, some fossil forms, such as Proganochelys (which the turtle in the strip is based on) fall outside Testudines, but are still within Testudinata. There must also be pan-testudines which are not testudinates, but, as previously discussed, we are not too certain what they are. (Pareiasaurs? Procolophonids? Euryapsids? Something as-yet-undiscovered?)

--Pan-Testudines (total group)
  |==shell-less stem-testudines
  `–Testudinata (apomorphy-based group)
     |==stem-testudines with shells, such as Proganochelys
     `–Testudines (crown group)

Crown and total groups are increasingly important types of organism group in biological systematics. Knowing the distinction is important whether studying stem-testudines like Proganochelys or stem-avians like Carnotaurus.


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