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Victor Bach

Placoderm Ancestry

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Placodermi is a clade of ancient Gnathostomes that are famous for their hardened bony-plated skulls and for dominating the ocean ecosystems of the Devonian era. Placoderms have been considered one of the most basal Gnathostomes, being a sister clade to Eugnathostomata from which bony fish (Osteichthyes), cartilaginous fish (Chondrichthyes) and Acanthodians emerged from. It was thus initially though that Placoderms and bony fish evolved bony skulls separately while cartilaginous fish never evolved them at all.

This was all believed to be true until the discovery of Entelognathus in 2013, a Placoderm from the Silurian era with a jaw structure very different from that of other Placoderms. While most Placoderms had a beak-like jaw (composed of gnathal plates), Entelognathus instead had jaw features consistent with that of bony fish like a maxilla, premaxilla and dentary. The implications were huge, as it redefined our understanding of ancient fish evolution. It is now (to my knowledge) believed that both cartilaginous and bony fish evolved from a Placoderm ancestor with bony jaws. Cartilaginous fish lost these traits in favor instead of a cartilaginous mandibular arch (known as the platoquadrate and Meckel's cartilage) while bony fish retained them and perfected them over time. This means that all extant Eugnathostomes (including sharks, rays, lizards, birds, mammals, etc.) are technically (from a cladistic point of view) Placoderms, with Eugnathostomata being a sister clade to Placoderm subtaxa like Arthrodira (which includes the awesome Dunkleosteus) and Antiarchi. 

This is extremely relevant for the construction of the PEP as it means that Placoderms actually never went extinct, but instead diversified to become one of the most successful clades of Earth's life history. And yes, that also means you and I are actually very evolved Placoderms.

Placoderms have also been found to be some of the earliest species to show evidence of live birth, with Incisoscutum being found with unborn fetuses inside it and a female Materpiscis being found with a dead youngling still attached to its mother through a fossilized umbilical cord. They also seem to be the earliest clade of fish to have been reproducing through internal sexual fertilization, having claspers like extant cartilaginous fish. This also implies that internal fertilization may have been the norm with Gnathostomes and that cartilaginous fish retained the trait instead of evolving it independently, while bony fish lost the trait until Tetrapods eventually evolved it again when they left aquatic ecosystems.

This is an example of how our understanding of evolution keeps changing, sometimes radically, as new discoveries come around. Feel free to add any new findings and share thoughts about the evolution of our Placoderm ancestors. I attached a few papers and images about Entelognathus, Placoderm evolution and claspers for those interested in learning directly from the sources.

entelognathus 3 by gogosardina.jpg

entelognathus 2 by gogosardina.jpg

Placoderm claspers.jpg

Long (2016) The First Jaws.pdf Zhu et al. (2013) A Silurian Placoderm with Osteichthyan-like Jaws.pdf Friedman & Brazeau (2013) A Jawdropping Fossil Fish.pdf

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A fascinating read, Victor. Read all of both posts. There are data fields for images (when the white screen fix is done). I'd say this is worthy of a blog. You are welcome/invited to make one or turn this into one. Let me or Borg know.

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I wouldn't be opposed to turn this into a blog, I am just not sure on how to proceed or what that would imply. My knowledge is limited (I am just a humble student that likes fish and, more specifically, sharks) and although I love to share this kind of information, I don't know if I would be able to share things of this relevance on a regular basis. But I am more than happy to post/share what I can find to be relevant to the PEP!

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A blog is so diverse and open in termns of length, depth, topic etc. I'm looking for interesting stories, information, news, research and related. It doesn't need to be a regular thing. I'm thinking of doing a weekly one with different contributors who can revisit or do a new blog anytime. So, you can put something together or use all or parts of what you posted, withy or without pics. And I'll post the blog with your name.

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