109 — Copulation in antiarch placoderms and the origin of gnathostome internal fertilization

John Long’s book, The Dawn of the Deed, has been on my to-read list for ages. I got very slightly closer to following through on that read when I heard him speak on one of my favorite podcasts, Palaeocast, and even closer still when I read this paper which discusses the evolutionary origins of internal fertilization. (In fact, Long notes in this episode of Palaeocast — which was recorded on-site at the Burgess Shale — that the studies in Dawn of the Deed are out-of-date, so quickly has the field moved in the past few years.)

Here, Long et al explain that gnathostomes — jawed vertebrates, such as mammals, fish, birds, etc — began to reproduce using internal fertilization (that sex thing) early in the phylogenetic tree. One of the most ancient branches of gnathostomes — the placoderms, or plated fish — certainly exhibited signs of internal fertilization, including specialized male dermal “claspers” to latch onto a female, but in order to demonstrate that this feature exists in the gnathostome phylogeny rather than just the placoderm group it is necessary to show that these claspers (among other implementation signs of internal fertilization) pre-date the placoderm divergence from other gnathostomes.

This paper reviews fossil evidence that suggests that early ancestors of the placoderm tree utilized internal fertilization, which means that gnathostomes also likely had this capability.

Contrary to many previously held hypotheses, this suggests that external fertilization, utilized by many types of modern fish and amphibians, descended from internal fertilization, rather than the other way around. This is interesting because it means that the seemingly more complex internal fertilization method predates modern external fertilization…or else “…the substantial morphological evidence for placoderm paraphyly must be rejected.”