Page 12 - Amphibian

The bare bones

Amphibians have simple skeletons with fewer bones than
other modern vertebrates (animals with backbones) and many
INSIDE OUT
This 19th-century, half-inside, half-outside, fewer than their fishy ancestors. This highlights an evolutionary
anatomical preparation of an edible frog trend, or overall change, in amphibians toward a reduced number of
clearly shows the bony framework that lies
beneath the soft outer tissues. bones in the skull and spine (made up of vertebrae). But caecilians
are an exception to this rule. Caecilians have as many, or more,
bones in their skull and far more bones in their spine than
salamanders. Evolutionary change within the amphibian
Orbit
(eye socket) family has gone in two directions, beginning from a basic
ancestral salamander-type skeleton. The frog-type
skeleton has a broad head, large
eye sockets, a short spine (usually
One of no ribs), no proper tail vertebrae,
four digits,
or toes and long, hind leg bones. The
caecilian skeleton, on the other
hand, has a small, almost tubular
skull, no (or very tiny) eye sockets, a
large spine with ribs, and no legs at
all. Comparing skeletons of modern
amphibians helps to place fossils,
like Triadobatrachus (pp. 8–9), in
their correct evolutionary position
Juvenile specimen of
a young Japanese – as an early prefrog type.
salamander,
prepared c. 1900

Left
front Pre­sacral, or
foot shortened,
Transverse vertebra
Siren’s process, joining
skull with Top row of vertebra to rib
mouth front teeth
open Rib
barely
Palate, or roof formed
of mouth
Siren’s skull with
mouth closed

WHAT IS A SIREN?
Sirens are eel-like amphibians found in the south-
eastern part of the U.S. They remain in the larval stage
throughout their lives and have external gills and small
front legs but no hind legs. Because of this unique
combination of features, some scientists regard sirens
(pp. 46–47) as a separate, fourth group of amphibians;
others think they are just very odd-looking
salamanders. Sirens do not have an arch around their
eye orbits, their teeth are different from those of frogs
and salamanders (teeth on the lower jaw are on an
inner bone), and their jaws have a horny covering. The
way in which sirens fit in with other amphibians is
uncertain, but their tail vertebrae look like those of the
extinct Diplocaulus (pp. 8–9).

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