Turtles: Origins and phylogeny
All terrestrial vertebrates are result of a single evolutionary transition onto land
Tiktaalik – solid scull – anapsid. Root “aps” means opening, Anapsid is without opening.
Another feature of early tetropods is shoulder girdle outside ribs. (Include scapula [shoulder blade]
and clavicle [collar bone])
Another feature of early tetropods: dermal bone confined to skull.
There are 2 ways bones form in vertebrates: bone replaces cartilage (cartilagereplacement bone),
or bone forms in absence of cartilage (dermal bone)
Experiments in cranial fenestration
Fenestration evolved independently in different groups of vertebrates. Primitive condition is anapsid
– solid skull except orbit (for eye)
One group of descendants, the synapsids, have one opening, low down in cheek region
Another type, euryapsid, have single opening behind eye, but openings are in different places –
opening is higher up on euryapsid. No surviving euryapsids.
Diapsid – two openings. Basically have both syn and eury openings.
Modified diapsids: bottom bar lost, modified snake diapsid, modified diapsid that has lost the holes.
Diapsids include tuatara, lizards, snakes, crocs. And, maybe turtles? They may have secondarily
evolved anapsidlike skull.
Unusual body plans
Anapsidlike diapsid skull, edentate skull (lacks teeth), trilaminar armor (shell) with dorsal carapace
and ventral plastron, which in turn has three layers of vertebrae and ribs, dermal bony plates
(osteoderms, no cartilage precursor because dermal), and epidermal scutes (keratin). The limb
girdles are inside the rib cage.
Where do turtles come from?
Some have suggested captorhinomorphs – but these have an anapsid skull, probably not right
Pareiasaurs – also anapsid skull, kind of broad, thought they maybe had a shell, but probably not
There are some fossil turtles or transitional forms – Proganochelys = was oldest turtle until 2008.
One transitional feature: teeth!!! Otherwise, body plan fully evolved. But where did it come from?
In 2008, a new turtle fossil discovered – Odontochelys. Has marginal teeth (at jaw margins, not
further in center of mouth like proganochelys), fully developed plastron, carapace with expanded
ribs and neural plates, but no osteoderms. So, has some of carapace, some of shell, but not all of
Neural plates – there are a couple osteoderms inside neural plates
So from that fossil was generated an evolutionary scenario Diapsid ancestor
Anapsid skull (derived secondarily), plastron, expanded dorsal ribs, neural plates, pectoral girdle
inside ribs [Odontochelys]
Osteoderms (complete carapace), loss of marginal teeth [Proganochelys]
Loss of remaining teeth
In the same edition of Nature that published the research, a commentary suggested that the
interpretation was wrong – they say instead that Odontochelys actually evolved from
Proganochelys, that Proganochelys is more ancestral and Odontochelys lost its shell since it is an
aquatic turtle. Sort of like extant aquatic softshell turtles.
In 2010, evolutionary relationships