And now, a problem....



Testudo, actualy Malaclemys terrapin
We have failed to address one sauropsid group that is dear to our institutional heart. We have saved Testudinata - turtles - for last because they represent the greatest challenge of any enigmatic vertebrate to phylogenetic systematics, despite their being very easy to recognize.


The Shell:


The Skull:



Tu'i Malila and companions in the 1950s from tongaturismo.info

Longevity:

Confirmation of evolution: Biologically turtles illustrate a fundamental fact about evolution - Life span is correlated with one's ability to avoid accidental death.

Suppose that humans have a variety of genes, some of which confer great advantage for individuals above age 150 and some that are lethal for people that age, but that don't effect younger people. Can natural selection select for the advantageous gene? No, because it never gets the chance. People don't live that long. Indeed, for all critters, natural selection for mechanisms of self-repair of the body can only work if at least a few individuals are still alive and mating at the age when this would be an issue.

Animals like marsupials are usually dead from predation, disease, or accident before they reach this point. But what happens with animals that posess some adaptation that keeps them relatively safe? These may reach advanced ages where natural selection can work on mechanisms for body repair - i.e. longevity. For this reason, when you compare mammals and birds of similar size, the birds almost invariably have the longer life span. But they can't compare with turtles. These creatures, in natural settings, at least, are so immune to predation and accident that they have evolved extreme longeivity. Indeed, it's not clear at what point they do start aging. E.G. Tu'i Malila, a Malagasy radiated tortoise given to the King of Tonga by Capt. Cook in 1777 passed away in 1965 at a minimum age of 188.


Diversity:

Turtles, despite their differences from all other amniotes, encompass a great deal of ecological variety. Although many are fresh-water aquatic, some are adapted to dry environments, and some are fully marine. Some are adapted for rapid swimming, some for a more leisurely mode. In coming to grips with their phylogenetic diversity, we have to keep a few names straight.

Pantestudines: The total group containing turtles and everything more closely related to them than to other living amniotes.

Testudinata: The apomorphy-based group containing turtles and their fossil relatives with an unambiguous turtle shell.

Testudines: The crown-group containing the last common ancestor of all living turtles and all of its descendants. Its diversity falls into two major groups:



Terrapene carolina bauri the Florida box turtle of Gloria Lennon.
Cryptodira: (hidden neck - Early Jurassic - recent)

The monophyletic group containing all turtles that pull their heads straight into their shells. (This includes most of the turtles you know.)

Their synapomorphies include:

Noteworthy plesiomorphy: Otic capsule serves as trochlea for adductor muscles (below). Among living turtles, this distinguishes cryptodires, but it is also present among stem Testudinata.

Cryptodiran superlatives:



Pelusious castaneus the African side-necked turtle by Jenni Bodey from Allturtles.com.
  • Pleurodira: (side-neck - Early Jurassic - recent)

    The so-called side necked turtles, that fold their heads sideways against their bodies. These are found, today, only in the southern continents. Side-necked turtles retract their heads flexing their necks at the well developed articulations between cervical centra. Restricted to the southern (gondwanan) continents, they are the dominant turtles of Australia.


    Comparison of stem testudate (left), cryptodiran (center) and pleurodiran trochlear pattern from Joyce, 2007
    Their synapomorphies include:

    Pleurodiran superlatives:


    Proganochelys: (Late Triassic) Turtles from farther down the tree are generally similar, with: Of these, the earliest and best documented is Proganochelys (Late Triassic), well known from several specimens. It could not retract its legs, head, or tail into its shell. The neck and tail were studded with free-floating pieces of dermal armor. Nevertheless, it was turtle-like in overall appearance, with a full-blown shell and lacking marginal teeth.

    Proganochelys quenstedti
    It plesiomorphically displayed:

    What are turtles?

    Proganochelys, the prototypical ancestral turtle pretty much looks like a turtle: Weird and not especially similar to other amniotes. What are turtles, in fact? For twenty years, this has been the big debate in amniote systematics. Since HONR219d was last offered, the question has finally started to yield.