Evolution does not work in mysterious ways.
The basics (small variations leading over dozens of generations to larger changes) are simple:
- wider / narrower (skull, body, feet, etc.)
- taller-larger / smaller-shorter
- longer (more ribs) / shorter (fewer ribs)
- longer limbs / shorter limbs
- larger skull / smaller skull
- longer preorbital region / longer postorbital region
- longer neck / shorter neck
- sharp claws / rounded claws
- etc. / etc.
are extinct baseball star, Babe Ruth, and extant comedian/commentator, Jon Stewart, graphically demonstrating #1 on the above list, wider / narrower in the skull shape. Both are male members of the species Homo sapiens.
one can add to this list include various perforations or fenestrae (which have several and often convergent origins and disappearances:
- fenestra between the naris and orbit (antorbital fenestra)
- fossa surrounding antorbital fenestra
- one or more fenestrae between the orbit and occiput
- fenestra in the mandible
- occipital fenestrae expand over braincase
- acetabulum perforated or not
And once fenestrae are formed:
- Loss of lower temporal arch
- Loss of upper temporal arch
- Loss of both
the size and shape of various bones and their processes compared to other bones and you have yourself a long character list. Enough of these (150+) provide a good matrix of characters and character states that can produce the menagerie of reptiles found in the large reptile tree, now numbering 566 taxa for 228 characters.
The wider / narrower and smaller / larger dichotomies
can also be seen in the variety of specimens attributed to Proterosuchus and Chasmatosaurus (Fig. 2, Broom 1903). Some paleontologists (Welman 1998, Ezcurra and Butler 2015) consider these taxa congeneric. They think this variety constitutes an ontogenetic series. On the other hand, the large reptile tree recovered these taxa in distinct nodes and clades. Narrower-skulled forms nest together. So do wider-skulled forms and they lead to other even more distinct taxa, including some once again tiny forms. The tall-skulled proterosuchids do not lead to more derived taxa.
The smallest taxon
shown here (Fig. 2), Youngoides romeri, leads to euparkeriids and then to a long list of archosauriforms including dinosaurs, crocs and birds. This last common ancestor of proterosuchids and euparkeriids (all archosauriforms) also had a small antorbital fenestra.
Have a great weekend!
Keep those cards and letters coming.
Broom R. 1903. On a new reptile (Proterosuchus fergusi) from the Karroo beds of Tarkastad, South Africa. Annals of the South African Museum 4: 159–164.
Ezcurra MD and Butler RJ 2015. Post-hatchling cranial ontogeny in the Early Triassic diapsid reptile Proterosuchus fergusi. Journal of Anatomy. Article first published online: 24 APR 2015. DOI: 10.1111/joa.12300
Welman J 1998. The taxonomy of the South African proterosuchids (Reptilia, Archosauromorpha). Journal of Vertebrate Paleontology 18 (2): 340–347.