Between 500 million and 200 million years ago, vertebrates slowly but surely evolved to include more and more complex physical features. The earliest vertebrates like the Ostracoderms possessed the most "primitive" bodies. By the time the Pterosaurs dominated the skies, vertebrates had developed many features that we still see in the animal kingdom today. (No. 1 on the list represents the early vertebrates; No. 7 represents more advanced morphology.)
1. VERTEBRAL COLUMN and BRAINCASE
2. GNATHOSTOMES (Jaws)
Sharks, Rays: Chondrichthyians
Ray-finned Fishes: Actinopterygians
3. TETRAPODS (Four Limbs)
Always the same structure: one bone, two bones, many bones (“One too many”): femur or humerus; tibia/fibula or radius/ulna; wrist/fingers or ankle/toes
4. AMNIOTES (Watertight Egg)
5. SAUROPSIDS (A pair of openings in the palate [roof of mouth])
Turtles, Pareiasaurs, Procolophonids
6. DIAPSIDS (Sauropsids) (A pair of openings behind the eyes for jaw muscle attachments)
Plesiosaurs, Lizards, Ichthyosaurs
7. ARCHOSAURS (A pair of openings between eye and nose: antorbital opening)
Modern reptiles inhabit every continent with the exception of Antarctica. Several living subgroups are recognized:
• Testudines (turtles, terrapins and tortoises): approximately 400 species
• Sphenodontia (tuatara from New Zealand): 2 species
• Squamata (lizards, snakes, and worm lizards): over 9,600 species
• Crocodilia (crocodiles, gavials, caimans, and alligators): 25 species
One of the distinguishing features of all squamata is the hemipenis in the male. All other male reptiles have a single penis; all lizards and snakes have a pair of penises (aka. HEMIPENES) https://en.wikipedia.org/wiki/Hemipenis
RUTIODON CAROLINENSIS (Late Triassic)
Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. The cephalaspids were more advanced than the heterostracans in that they had lateral stabilizers for more control of their swimming.
Cephalaspidomorphs are a group of jawless fishes named for Cephalaspis of the osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day.
KOSKINONODON (aka. BUETTNERIA): Koskinonodon was originally named as Buettneria back in 1922, however it was later realized that Buettneria had already been used to name a genus of katydid (bush cricket).
DISCLAIMER: I do not represent the Museums mentioned on this website. My photos, links, and comments are personal points of view that do not necessarily reflect any official position by the administration of those Museums or their curators.