248 Evolution? The Fossils Say YES!
As we move up the sequence of anthracosaurs, we see these features accumulating one
by one in a mosaic evolution pattern (fig. 11.3). For example, the more primitive Solenodon-
saurus has the primitive eardrum notch but advanced jaw muscles. Gephyrostega also retains
the primitive eardrum notch and primitive vertebrae and limbs but has an advanced ankle
joint. Limnoscelis and Seymouria have lost the primitive eardrum notch and also have more
advanced, robust limbs. Diadectes, a pig-sized herbivore (the first herbivorous land verte-
brate known), is very close to amniotes, with the atlas and axis vertebrae in its neck and
strong limbs with advanced ankles, but it still retains the primitive eardrum notch.
Most of these anthracosaurs are apparently end-members of an extinct side branch rep-
resenting earlier lineages that were not preserved in the fossil record because the oldest
fossil that most paleontologists agree is an amniote comes from older beds of the Lower
Carboniferous. Nicknamed “Lizzie the lizard” by its discoverer, Stan Wood, this fossil is
officially known as Westlothiana lizziae (fig. 11.4) and comes from the famous East Kirkton
beds of Scotland (Smithson et al. 1994). Slightly later, true amniote fossils known as Hylo-
nomus come from the Middle Carboniferous beds of Joggins, Nova Scotia (about 15 million
years younger than “Lizzie”). These fossils show a dramatic change from the more primitive
tetrapods. They are small (about 20 centimeters long, or the size of a typical lizard) rather
than the much bigger dog-sized and even pig-sized anthracosaurs. They were also delicate
and slender, with a very long trunk and tail, long slender limbs and toes, and relatively small
heads in comparison to most anthracosaurs. Their vertebrae are robust and fused into the
arches above the spinal column, a typical feature of amniotes. Westlothiana and Hylonomus
had deep skulls with effective jaw muscles and no eardrum notch in the back of the skull.
FIGURE 11.4. The oldest known true amniote, Westlothiana lizziae, showing both (A) the actual specimen and
(B) a reconstruction of its skeleton. (Reproduced by permission of the Royal Society of Edinburgh and R. L.
Carroll; from Smithson et al. 1994)
(A)1 cm(B)