Science Literature

Placoderm fish, although extinct since the end of the Devonian, have surprised researchers on two occasions during the past year. The first surprise was the discovery of embryos with an umbilical cord in the ptyctodontids. These animals are a small group of placoderms that are regarded as "phylogenetically basal to the largest group, the Arthrodira". The second surprise came when embryos were also detected inside fossil specimens belonging to the Arthrodira.

"These new finds confirm that reproduction by internal fertilization and viviparity was much more widespread in the earliest gnathostomes than had been previously appreciated."

Incisoscutum ritchiei (source here)

The ptyctodont find stimulated the search "for signs of internal fertilisation in other placoderm fish", including samples from the Arthrodira.

"In two of the fossils' body cavities were smaller fish, originally presumed to be a well-preserved meal. But based on the position of the tiny fish and the similarity to the ptyctondontid finds, the group concluded that the tiny arthrodire fish were actually embryos. "Finding that first embryo was the key, the Rosetta stone for knowing what to look for," Long said."

Further analysis led to the identification of bony elements coming off the pelvic fins. These were interpreted as "similar to the claspers that males sharks insert into females to fertilize eggs". Sharks all have internal fertilisation and 55% are live bearers. The Rosetta Stone analogy is a striking one: when the researchers found the first embryo, they knew that if they looked systematically, they were likely to find more of what previously had been interpreted as food.

The surprised reactions have come because placoderms have been regarded as stem gnathostomes. They are considered ancestral to acanthodians, chondrichthyans and osteichthyans. However, as Ahlberg explains, external fertilisation by spawning is considered to be the mode of reproduction for evolutionary ancestors of gnathostomes.

"The ancestral mode of reproduction for osteichthyans seems to be external fertilization. The distribution of live-bearing among living vertebrates strongly suggests that internally fertilizing live-bearers are unlikely to give rise to externally fertilizing spawners, so we would not expect the osteichthyan stem lineage, or the gnathostomer stem lineage below it, to contain a segment characterized by live-bearing."

In other words, internal fertilization and live-bearing are associated with derived characters that are understood to have evolved from an ancestor with an external fertilization/spawning reproduction behaviour. Long et al. comment:

"Once viviparity develops, the complex physiological requirements that accompany live-bearing generally prohibit reversal back to external spawning, although there are rare cases of reversal from live-bearing to egg-laying in squamate reptiles."

Thus, placoderms as a group cannot be regarded as stem gnathostomes, for at least two members of the placoderm group cannot be ancestral to osteichthyans. So, a major rethink of placoderms is needed, as Ahlberg explains:

"Ideas about the origin of gnathostomes are currently in a state of flux. [. . .] A new analysis by Brazeau suggests that placoderms may not be a natural group at all, but a 'paraphyletic array' spread out along the gnathostome stem."

Clearly, gnathostome phylogeny is in the melting pot and placoderms are providing some crucial evidence relating to viviparity among the earliest jawed vertebrates. This is not what evolutionary theorists predicted. They expected primitive before derived, but two placoderm groups give them derived before primitive. One of the co-authors, a palaeontologist at London's Natural History Museum (which houses the fossils) said: "We expected that these early fishes would show a more primitive type of reproduction, where sperm and eggs combine in the water and embryos develop outside the fish". The search is now on for a paraphyletic structure for the placoderms and for an ancestral group that is definitely not viviparous, with antiarchs being the prime candidate.

Evolutionists are very reluctant to acknowledge when their predictions are falsified. Instead, they have cultivated the ability to say the new finds have broadened their thinking and that it places their understanding "on a much firmer footing". Indeed, the new finds "may prove to have far-reaching implications for our understanding of early vertebrate evolution". The reality is that these new finds do not confirm evolutionary predictions. Instead, there are now modified evolutionary predictions (as yet untested) that attempt to accommodate the data within an evolutionary framework. This is the well-documented behaviour of Kuhnian "normal" science.

I am reminded of a conversation with a friend of mine whose PhD was in fish development. He came to the conclusion that fish do not provide a Darwinian story of evolutionary transformation. These recent finds seem to confirm his assessment.

Devonian arthrodire embryos and the origin of internal fertilization in vertebrates
John A. Long, Kate Trinajstic & Zerina Johanson
Nature 457, 1124-1127 (26 February 2009) | doi:10.1038/nature07732

Evidence of reproductive biology is extremely rare in the fossil record. Recently the first known embryos were discovered within the Placodermi, an extinct class of armoured fish, indicating a viviparous mode of reproduction in a vertebrate group outside the crown-group Gnathostomata (Chondrichthyes and Osteichthyes). These embryos were found in ptyctodontids, a small group of placoderms phylogenetically basal to the largest group, the Arthrodira. Here we report the discovery of embryos in the Arthrodira inside specimens of Incisoscutum ritchiei from the Upper Devonian Gogo Formation of Western Australia (approximately 380 million years ago), providing the first evidence, to our knowledge, for reproduction using internal fertilization in this diverse group. [. . .]

See also:

Cressey, D. Fossils reveal early evidence for penetrative sex, Nature, 25 February 2009 | doi:10.1038/news.2009.122

Ghose, T. Early fish had live birth, The Scientist blog, 25th February 2009

Tyler, D. Live birth in a supposedly primitive Devonian fish, ARN Literature blog (4 June 2008)