Animal Diversity: Chordata

Microsaurs represent a diverse group of fossil forms known from the Pennsylvanian to the
lower Permian. They share a number of skeletal features with caecilians, which may suggest
either a close relationship or convergence on an elongate body form specialized for
burrowing.
Anthracosaurs Anthracosauria is a small Paleozoic group, thought to be in direct line to the
amniotes. Their fossil record extends from the Mississippian to the Triassic.
Lissamphibians Living amphibians of approximately 2000 species may be grouped in three
orders: Apoda, Urodela and Anura.
a. ORDER 1. APODA (GYMNOPHIONA / CAECILIA).
Members of the order are pantropical in distribution. The caecilians are burrowing forms,
with worm like bodies, lacking limbs. The tail is very short suited to their mostly
terrestrial habits and the anus is almost terminal. The skull is solid and bony, again suited
for a burrowing lifestyle. The animals are blind, but carry special sensory tentacles.
Unlike other amphibians, some caecilians have dermal scales. Adults lack gills and gill
slits. The very small eyes are buried beneath the skin or under the skull bones. Because of
the presence of an intromittent organ in males, internal fertilization is assumed. In some
caecilians, eggs are laid, which hatch into free-living larvae. The eggs are large, yolky
and cleavage is meroblastic; they are laid on land in Ichthyophis, and the embryos
develop around the yolk sac, but often have long, plumed gills. The female guards the
eggs until the larvae hatch and move to the aquatic habitat. Other genera skip over the
aquatic larval stage and a few have specialized external gills. In still other genera, the
eggs are retained within the female, metamorphosis occurring before birth. Viviparity is
common in the aquatic form, Typhlonectes. Important Apoda families are as follows:
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