Reptile Evolution -- May 13, 2019

Updates

 

September 5, 2020: Archelosauria -- coined in 2015 to evoke its two subclades, the Archosauria and the Chelonia. The clade unites archosaurs (birds and crocodilians) with turtles. Genetic studies: chelonians are the sister group to birds and crocodiles. In addition, there are about 1,000 ultra-conserved elements in their genome that are unique to turtles and archosaurs, but which are not found in lepidosuars (tuatara, lizards, and snakes).  It was found that Archelosauria was grouped within Sauria, as the sister branch to lizards and Sphenodon. So, we're getting close with regard to classification of turtles, or we are there! By the way, this would also fit with the "red gene." Turtles and birds have the "red gene." Crocodiles also probably had it but lost it. Snakes and lizards would not have it (the "red gene").

September 5, 2020: from bcmbiol, 2012:

Our analyses based on the CAT-GTR + G model placed the divergence between turtles and archosaurs around the Permian-Triassic boundary at a mean of 255 Mya (range 274 to 233 Mya), the separation of crocodilians and birds in the Upper Triassic with a mean of 219 Mya (249 to 186 Mya), and the most recent common ancestor (MRCA) of living turtles (corresponding to the separation between Pleurodira and Cryptodira) in the Upper Jurassic with a mean of 157 Mya (207 to 104 Mya) depending on whether amino acids or nucleotides are considered.

Note: the Permian Mass Extinction was 252 mya so the divergence between turtles and archosaurs occurred right across the "extinction event."

Reminder: The Cryptodira and the Pleurodira are distinguished from each other by the way they retract their necks. Turtles placed into the Cryptodira retract their neck in a vertical plane, whereas members of the Pleurodira (side-necked turtles) retract their necks in a horizontal or sideway plane.

Original Post
From wikipedia, reptiles:

Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic metabolisms.

So defined, the group is paraphyletic, excluding endothermic animals like birds and mammals that are descended from early reptiles.

A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors. So defined, Reptilia is identical to Sauropsida.

Though lots of reptiles today are apex predators (top of the food chain), many examples of apex reptiles have existed in the past. Reptiles have an extremely diverse evolutionary history that has led to biological successes, such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.

Reptiles: paraphyletic --  descended from a common evolutionary ancestor or ancestral group, but not including all the descendant groups.

Reptiles in the traditional sense are a paraphyletic group comprising all non-avian and non-mammalian amniotes.

So, praphyletic with reptiles:

• non-avian amniotes
• non-mammalian amniotes

It looks like the three paraphyletic amniotes:

• reptilia
• aves
• synapsida (mammals)

The tetrapoda separated into two groups:

• amniotes
• amphibia (so you can see why I suggest the amphibians had 15 minutes of fame; they transitioned to land where the adults thrive but except for frogs pretty much disappeared)

The vertebrates separated into two groups; this is a pretty big deal -- land vs sea:
tetrapods
pisces (fish)

The paraphyletic reptiles, evolved this way:
aminotes (reptilia and synapsdia); aves "burrowed" through the reptilian paraphylum

After synapsidia broke off from the amniotes, the amniotes "evolved" into Sauropsida

Sauropsida = reptiles not including birds

Testudina (turtles) broke off from Sauropsida; the latter "evolved" into diapsida.

Lepidosauria broke off from diapsida, the latter "evolved" into archosauria (the ruling lizard -- dinosaurs)

Crocodylia broke off from the ruling lizards, the Archosauria. Crocodylia would have been the end of the line had birds (Aves) not survived as extant dinosaurs.

Even though crocodylia would have been the end of the line, the lepidosauria and the testudina would have still been with us:

• lepidosauria: lizards, snakes, tuataras
• testudinata: all tetrapods with a true turtle shell 
• amphisabaenians: legless lizards; look very much like snakes or earthworms 

So, when you go through all of this, reptiles had a great run, but at the end of the day, came close to dying out.

Their 15 minutes of fame continues, unlike the amphibians, but only as long because they compete in the mammalian niche.

The survivors:

• fish
• birds
• mammals

Ice age planet: it really looks like the "ice age" planet -- earth -- drove the evolution of warm-blooded animals to replace cold-blooded animals.