Fish Out of Water 241From the primitive forms like Ichthyostega and Acanthostega, the tetrapods began a great
radiation of more advanced terrestrial forms in the Carboniferous. Some, like Greererpeton
(figs. 10.6 and 10.10) had long fishy bodies not much different from those of Acanthostega.
Their limbs and shoulder bones were considerably more advanced and terrestrial than those
of Acanthostega, yet they still had fishlike features, such as the lateral line canals. By the mid-
Carboniferous, we find that tetrapods have branched into many different lineages, including
the big flat-bodied flat-skulled temnospondyls (fig. 10.1), the more delicate lepospondyls
(some of which became legless and converged on snakes and apodans), and the anthraco-
saur lineage that leads to amniotes. And, by the middle Carboniferous, we find the first true
amniotes as well. We will discuss them in the next chapter.
The “Frogamander”
Creationists frequently taunt scientists by pointing to an image of something as specialized
as a frog and saying that there is no way they could imagine a transitional fossil between
frogs and other amphibians. But in 2008, a fossil was announced that put this question to rest
(Anderson et al. 2008). Formally named Gerobatrachus hottoni, it was dubbed “frogamander”
FIGURE 10.10. Drawing of the evolutionary transition
of the bones from the skull, shoulder girdle, and
forelimb, from fully aquatic rhipidistians like
(A) Eusthenopteron to the slightly more tetrapod-
like (B) Panderichthys to the more advanced (C)
Acanthostega and concluding with a fully terrestrial
tetrapod, (D) Greererpeton. The fishlike elements
of the shoulder girdle, such as the cleithrum and
anocleithrum, are gradually reduced, while the
clavicles (our “collarbones”) become the largest
bone in the shoulder girdle. Meanwhile, the major
bones covering the gill slits (shaded here in gray) are
reduced and then lost, the eyes shift backward as the
snout expands, and the cheek regions are reduced.
(From Clack 2002: fig. 6.4; used with permission)
CleithrumCleithrumCleithrumCleithrumAnocleithrumAnocleithrumClavicleClavicleClavicleClavicle(A)(D)(C)(B)