Fossil Frog-amander Claimed to Be Missing Link
It looks like a frog with four equal legs and a tail; is it the ancestor of frogs and amphibians? It depends on whether you read the original paper or the popular press.
Live Science writer Jeanna Brynner is absolutely certain this puts a feather in Darwin’s cap: “the creature represents a transitional amphibian, sporting features of both frogs and salamanders.” The “walking frog” has fused bones in the ankle characteristic of salamanders, and a wide skull and large eardrum characteristic of frogs. She quoted Jason Anderson (U of Calgary) who described the five-inch Texas fossil, named Gerobatrachus hottoni, as “kind of an early frog-amander.” Brynner said the fossil “provides a marker of when frogs and salamanders went their separate ways along the evolutionary path toward modern forms.” She did, however, quote a team member who remarked that the divergence date according to the fossil’s position in the geological record occurred “much more recently than previous molecular data had suggested.”
What did the original paper in Nature claim?1 Right off the bat, Anderson et al remarked, “The origin of extant amphibians (Lissamphibia: frogs, salamanders and caecilians) is one of the most controversial questions in vertebrate evolution, owing to large morphological and temporal gaps in the fossil record.” This indicates that one alleged transitional fossil could only provide a partial solution at best. They did claim that G. hottoni “bridges the gap between other Palaeozoic amphibians and the earliest known salientians and caudatans from the Mesozoic,” but the paper revealed a number of questions and problems. First, the specimen appears to be from a juvenile. We don’t know what the adult form looked like. Frogs undergo a dramatic transformation from tailed tadpole to hopping adult. Second, only one specimen was found. Third, frogs and salamanders already share a great many traits, both being amphibians, and a good deal of diversity exists within both groups. Add to that the thousands of extinct species, and the result is a lot of leeway in where a specimen might fit into a big evolutionary picture.
The following excerpt illustrates the amount of wiggle room involved in inferring evolutionary relationships:
If our interpretations are correct, the preaxial pattern of digital development is either independently derived in Gerobatrachus and salamanders, or primitive in batrachians but reversed in frogs. Knowledge of development in fossil taxa is always inferential, especially when based on a single specimen, but our speculative hypothesis is testable with a more complete developmental series of either Gerobatrachus or another amphibamid. A preaxial pattern of digital development has recently been demonstrated in branchiosaurids, which are thought to be closely related to, if not included within, Amphibamidae (Fig. 4), but branchiosaurids lack ossified carpals and tarsals and thus it remains unknown if they possessed a basale commune [fused distal tarsals]. This observation, however, may support the possibility that preaxial development is primitive for batrachians (and more basal amphibamids), and will be the subject of future research.
A good deal of the interpretation was thus left to future research.
After the morphological analysis, the researchers performed a phylogenetic analysis. Human choice also affects the inferences here: scoring and rescoring results, tossing out instances deemed irrelevant or misleading, and selecting what software and algorithm to use. How much confidence, then, can be placed in their conclusion? “Thus, the available morphological evidence supports the hypothesis of a diphyletic origin of extant amphibians from Palaeozoic tetrapods, with a separate origin of the limbless, largely fossorial caecilians from within the lepospondyls, whereas Batrachia originates within Temnospondyli.”
The paper ended by estimating the divergence times of frogs and amphibians, given their hypothesis. They agreed that the fossil evidence and molecular evidence were off.
1. Anderson et al, “A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders,” Nature 453, 515-518 (22 May 2008) | doi:10.1038/nature06865; Received 23 October 2007.
We hope this little excursion into how evidence for Darwinism is manufactured was revealing. The popular press (especially Darwin-loving, creation-hating sites like Live Science) stretches hypothetical possibilities into confident truths. Get to the source; look at the raw data and see if the inference is justified.
Look how much human tinkering was required to get the data to fit their inference. Even if one accepts their dating scheme the match is imperfect and subjective. Without the dating assumption, the fit is almost completely arbitrary. Any number of relationships could be hypothesized between living and extinct individuals. Besides, this is just a claim about a missing link within the amphibians. We don’t see transitional forms between the higher-level taxa. Where are the clam-frogs, echino-flies, and sponge-worms? (No fair mentioning lionfish and scorpionfish – those are just fish.)
The bottom line: data don’t jump up and draw Darwin’s tree of life. Evolutionary trees are manufactured by certain people with biases, agendas and world views. If you asked G. hottoni if it was a Tree frog, it would just say ribbit.