The parade of the craniates Part 2

Tetrapoda Amphibia Limbs with digits Fossil Dipnoans Amphibia Limbs with digits In basal forms – labyrinthine folding in dentine of teeth (Labyrinthodont) Icthyostegalia of Greenland

Amphibia Older extinct groups Modern Group Labrynthodonts – Devonian Temnospondyli – Lower Carboniferous Looked a lot like modern frogs Anthracosauria Microsauia – Upper Carb. Looked a lot like modern caecilians Modern Group Lissamphibia*

Lissamphibia Apoda (= Gymnophiona) – caecilians Burrowing Limbless

Urodela (= Caudata) Tailed Sirens, Amphiuma, Salamanders, Hellbenders

Anurans (= Salientia) Frogs, toads, tree toads Dwarf tree toad (Indonesia)

Brief embryology of amniotes For proper embryonic development: Requirement of aqueous environment Solutions to the problem Fish, Lissamphibians – many species – eggs laid externally in HOH Terrestrial orgs (Amniotes) Amphibian ancestors of reptiles (~250mya) Retain HOH requirement Sequester it in a sac – the amnion All vertebrate embryos require an aqueous environment for development. * Fish and amphibians lay their eggs in water. * Terrestrial animals live in dry environments and have evolved two solutions to this problem: the shelled egg in birds and reptiles and the uterus in placental mammals. * The embryos of reptiles, birds, and mammals develop in a fluid-filled sac, the amnion. * These three classes of animals are referred to as amniotes due to the presence of the amnion around the embryo.

The amniote egg Allows wandering from existing water. Carries own food and water The four sacs of the amniote egg: Yolk sac – nutritive proteins Amnion – fluid bathing embryo Allantois – embryonic metabolism wastes deposited Chorion – selectively permeable membrane (allows interaction with external environment)

For comparison

Day 1 Chick

Day 4 Chick

Early cat embryo - amnion still around embryo, placenta opened

The problem of waste

Skull Fenestra Issue The temporal region (the roof of the skull) is formed from the parietal (Par), squamosal (Sq), postorbital (PO), and jugal (Ju) bones. Four patterns are distinguished, based on the number and position of temporal openings.  The ancestral anapsid pattern, found in turtles, has no openings. The synapsid pattern, found in the ancestors of mammals, has a single low lateral opening formed at the junction of the PO, Sq, & Ju bones (the arch formed by the Sq / Ju connection evolves into the zygomatic arch of Mammalia). The diapsid pattern has a second opening at the junction of the PO, Sq & Par bones. The true diapsid pattern is seen today only in Rhyncocephalia (tuatara: Sphenodon); further reduction of the jugal in Archosauria (crocs and birds) and Lepidosauria results in a single temporal opening. (A fourth pattern, euryapsid, is found only in an extinct group of marine sauropsid reptiles, and consists of a single high lateral opening).

diapsid synapsid anapsid

Amniota Reptiles and synapsids Reptilia Anapsids – Chelonians Diapsids – all other living reptiles Archosauia – (ie dinos and birds) Lepidosauria – (lizards, snakes (squamates), tuatara (rhynchocephalia))

Synapsids Pelycosaurs Therapsida Extant Mammals

Mammalia Subgroup of Therapsids End of the Triassic Characteristics: synapsid skull Hair Mammary glands (not in monotremes) Nipples Single dentary bone articulating with squamosal Three bones in middle ear Muscular diaphragm separating thoracic and abdominal cavities Sweat glands Absence of adult cloaca (except monotremes) Heterodont dentition (except in toothed whales) Only two sets of teeth (milk and permanent) Biconcave, circular, anucleate red blood cells No fourth aortic arch Sound-collecting lobe (auricle) Cerebral cortex highly developed As a group: wildly morphologically diverse