land offered no large animals to eat, and vertebrates had not yet evolved the
capacity to consume plants. Therefore, the only food consisted of inverte-
brates, including millipedes, centipedes, and the forerunners of insects.
The earliest known tetrapod was Acanthostega (Fig. 109), meaning “spine
plate.” It was essentially an aquatic animal with attached hands and feet. The
salamander-like body had large eyes on top of a flat head for spotting prey
swimming above as it sat buried in the bottom mud. It sported eight toes on
the front feet and seven toes on the rear feet, perhaps the most primitive of
walking limbs. The digits were sophisticated and multijointed. However,
because they were attached to an insubstantial wrist, the legs were virtually
useless for walking on the ground. The rest of the skeletal anatomy also sug-
gests acanthostega could not have easily walked on land. Instead, it probably
crawled around on the bottom of lagoons and used gills for respiration.
One of the earliest known amphibians was an ancient land vertebrate
called Ichth yostega,meaning “fish plate,” which lived its life half the time inFigure 108Amphibious
fish were the first tetrapods.
Figure 109
Acanthostegawalked on
the bottoms of lagoons
with legs having eight
toes.
Historical Geology