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Placoderm Primer

This post has been a long time in coming but hopefully I’ll be able to start posting regularly after this and get a steady stream of updates going. Here, finally, is a primer on placoderms since I’m likely to be posting about them more.

Placoderms are an entirely extinct group of armoured fishes that were most common in the seas of the Devonian Period (419-459 mya). They were an impressively diverse group of organism containing both ray-like bottom feeders (Rhenanids) and giant predators (Dunkelosteus). Placoderms also include some of the smallest known vertebrates (Minicrania lirouyii ) [1] with a head and thoracic shield of only 20mm) and the largest vertebrates to have evolved up to the Devonian (Titanichthys; probably maxed out at over 5 meters long). Despite their diversity and dominance for over 40 million years no placoderms survived past the end of the Devonian for reasons unknown. So this is a whole branch of life that originated, diversified, and went extinct just as vertebrates were starting to move onto land!

There are nine orders of placoderms currently recognized [2] Stensioellida, Pseudopetalichthyida, Petalichthyida, Ptyctodontida, Acanthothoraci, Rhenanida, Antiarchi, Phyllolepida, and Arthrodira with the first two being poorly known. They were long thought to be a monophyletic group but recent discoveries have turned the old relationships on its head, especially the discovery of a mid-Silurian vertebrate, Entelognathus primordialis, that has a combination of characters seen in placoderms and bony fishes (osteichthyes) [3]. I might do a separate post just about that strange beast sometime. Anyway, the different orders of placoderms are now thought to be paraphyletic with Antiarchs as the most basal, arthrodires as intermediate, and ptyctodontids as the sister-group to the remaining jawed vertebrates [4], figure 1 although this is still in flux.

In terms of morphology placoderms as a whole have an ossified dermal skeleton, that is the bones form from the dermal layer near the surface of the skin rather than internally like most of the bones in our own bodies (Interestingly most of our skull bones are dermal in origin). This external skeleton is commonly referred to as armor and it protects the soft internal body of the fishes including the internal skeleton which is rarely preserved and appears to be largely composed of cartilage. The armor of the placoderm is typically broken down into four groups of armor plates that are tightly connected to each other: head, cheek, thoracic, and ventral armors (figure 2). In antiarchs the pectoral fins are enclosed by jointed appendages reminiscent of arthropods.

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The head shield in most arthrodires consists of seven paired plates (postnasal, central, preorbital, postorbital, paranuchal, marginal, and postmarginal) and three median plates (rostral, pineal, and nuchal). In some primitive arthrodires the rostral and pineal plates are fused (Buchanosteidae) to form a rostropineal and in other forms the postnasals appear to have been lost. The head shield is connected to the thoracic shield by a hinged contact between the paranuchal and the anterior dorsal lateral. The thoracic shield is composed of five to seven plates in arthrodires (median dorsal, anterior dorsal lateral, anterior lateral, posterior dorsal lateral, posterior lateral, interolateral, and spinal). The interolateral and spinal plates are lost in independent lineages, coccosteids and aspinothoarcids (surprise surprise) respectively. Depending on the species the thoracic armor is articulated to varying degrees with the ventral armor which would have protected the soft underbelly. This part of the armor consists of four plates (anterior median, posterior median, anterior ventral lateral, and posterior ventral lateral) which are typically found in isolation or fragmented. Finally, the cheek shield is connected to the head shield either loosely with the preorbital or intimately with both the preorbital and lateral portion of the head shield. The cheek shield consists of three plates (suborbital, postsuborbital, and submarginal).

I’ll stop there for now. I’ve updated the placoderm occurrence database so I might do something about placoderm diversity next. I also have a paper on Titanichthys under review so maybe more on that soon too!

References

1. Zhu, M. & P. Janvier. 1996. A small antiarch, Minicrania lirouyi Gen. et sp. nov., from the Early Devonian of Qujing, Yunnan (China), with remarks on antiarch phylogeny. Journal of Vertebrate Paleontology 16:1-15.

2. Denison, R.H. 1978. Handbook of Paleoichthyology: Placodermi. Gustav Fischer Verlag,             Stuttgart, New York, 128 pp.

3. Zhu, M., X. Yu, P. E. Ahlberg, B. Choo, J. Lu, T. Qiao, Q. Qu, W. Zhao, L. Jia, H. Blom, and Y. Zhu. 2013. A Silurian placoderms with osteichthyan-like marginal jaw bones. Nature 502:188-193.

4. Davis, S.P., J.A. Finarelli, and M.I. Coates. 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486:247-250.

5. Dunkle, D.H. and P.A. Bungart. 1947. A new genus and species of arthrodiran fish from the Upper Devonian Cleveland Shale. Scientific Publication of the Cleveland Museum of Natural History 8:103-117.

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What is a fish?

Since a lot of what I’ll be posting will have to do with placoderms and other marine animals I thought it would be a good idea to explore what the word ‘fish’ actually means. What makes something a fish? The question is not as simple as it first appears, we all know in general what we mean when we refer to fish in conversation but this is partially due to the fact that the overwhelming majority of fish today belong to the actinopterygians (ray-finned fishes) which includes everything from gars to goldfish. All these fish share a similar body with paired fins, a tail fin, gills, a bony skeleton, and swim bladders. So let us take the idea that fish are water-dwelling organism with fins, scales, gills, and who are poikilothermic (their body temperature fluctuates with the temperature of their environment). This covers most of the organisms you would refer to as fish in normal conversation but what about eels that don’t have the paired fins or scales? Or the even stranger (and uglier) lampreys and hagfishes that also lack jaws in addition to scales and fins? There are even some fish that can regulate their body temperatures both by more active circulation (Salmon sharks)1 or by producing antifreeze proteins in their blood2. For every rule defining fish there are many exceptions to counter them. The reality is that the group we refer to as ‘fish’ is a grouping of human convenience for swimming organisms in the water, which usually possess scales and gills.

A quick side note I feel obligated by a former professor to pass on! Fish is the proper plural when referring to multiple individuals of the same species while fishes is used when referring to a group of individuals containing more than one species.

Today if we consider only the kinds of fish with scales, paired fins, and gills (teleosts) they are the most diverse group of vertebrates on the planet. If we look at the evolutionary history of marine organisms similar to teleosts there was an even greater diversity in the past. The figure below is from a review of fishes over the last 500 million years and I would highly recommend giving it a read if you have any interest in ichthyology (reference is at the end of the post). The first thing you might notice is that all terrestrial vertebrates (including you dear reader!) are descended from the sarcopterygians (lobe-finned fishes) and thus a technically a fish. Those swim bladders I mentioned earlier are primitively lungs so it is not just land animals that have lungs but the majority of vertebrates. In fact you are more closely related to a goldfish or salmon than either of those is to a shark.

Image

Figure 1. A phylogeny of fishes, extant and extinct from Friedman and Sallan 2012.

It is also probably clear that there a large number of entirely extinct lineages of fishes. Everything from the anapsida to Osteoraci, excluding the eel-like unarmored conodonts, are historically referred to as ostracoderms or agnathans. They are primitive armored fishes that lack any jaws and had their greatest diversity in the late Silurian to early Devonian (~420 mya) hundreds of millions of years before the dinosaurs walked the land and before any vertebrates walked the land for that matter. The many lineages of ostracoderms are a fascinating array of creatures that has only recently received renewed attention for understanding the evolution of all jawed vertebrates. Unfortunately, there is still relatively little known but they more posts about them will probably appear as new publications come out.

Moving into the gnathostomes (vertebrates with jaws) one of the earliest branches were the paraphyletic acanthodians (spiny sharks), which are known from mostly fragmentary remains. The lines including the acanthodians lead on to both chondrichthyes (sharks and rays) as well as teleosts (ray-finned fish, lobe-finned fish, and terrestrial vertebrates). The other branch led to the placoderms (armor skin) which were a highly diverse group of fishes with armor around their heads and part of the trunk. Below are reconstructions of the many forms of placoderms which dominated the Devonian seas before becoming extinct at the end of that period. Placoderms include the earliest known instance of live birth4 and included the largest vertebrates to have ever evolved up to the end of the Devonian likely greater than five meters in length.

Image

Figure 2. The placoderms Coccosteus from Wikipedia.

The next post will be a primer on placoderms before I really start to delve in on specific hypotheses, publications, or theories. I’ll attempt to answer reasonable comments in a timely manner and be glad to answer any questions.

References

1Goldman, K. J., S. D. Anderson, R. J. Latour, and J. A. Musick. 2004. Homeothermy in adult

salmon sharks, Lamma ditropis. Environmental Biology of Fishes 71:403-411.

2Fletcher, G. L., C. L. Hew, and P. L. Davies. 2001. Antifreeze proteins of teleost fishes. Annual

Review of Physiology 63:359-390.

3Friedman, M. and L. C. Sallan. 2012. Five hundred million years of extinction and recovery: a

Phanerozoic survey of large-scale diversity patterns in fishes. Palaeontology 55:707-742.

4Long, J. A., K. Trinajstic, G. C. Young, and T. Senden. 2008. Live birth in the Devonian Period.

Nature 453:650-652.

What am I doing?

Hello world!

I’ve been brainstorming about starting a blog for a while but kept putting it off for another day but no more! For most of my life I’ve been interested in paleontology and that’s what I’m going to write about for the most part on this blog. I can’t say I have a very specific focus on any particular group but there are a few that will probably be the majority of post to start with at least.  This blog was in large part inspired by SVPOW which has been an enormous amount of fun for me to follow over the past two years. 

I’m currently a master’s student working on evolutionary patterns in graptolites (if you don’t know what those are don’t worry I’m sure they’ll appear here from time to time) and will be continuing on to a PhD in the fall. My undergraduate career was spent working on placoderms from the Cleveland, Ohio area and I’ve really gotten fond of them but I don’t have much opportunity to work on them where I am now so I’m partially using this blog as a way to keep myself involved in that literature by trying to get other people interested in them as well.

It’s my hope that this blog will help me work on a couple things. First, helping me improve my scheduling and time-management which could definitely use some work. Second, to help me learn how to write and portray ideas more effectively. And third, to get some ideas bouncing around my head somewhere easily accessible so other people can learn, provide feedback, and maybe answer some of the questions I haven’t been able to.

I’ve already alluded to the fact that this blog will probably cover some graptolites and definitely placoderms. Both groups are from the Paleozoic and I’m expecting most of my posts on specific groups or organisms to be from that time. However, I also was introduced to paleontology like most people through dinosaurs and so they’ll creep into discussion too I have no doubt. But most things in the fossil record younger than the Cretaceous just don’t interest me as much for whatever reason and so this blog will almost exclusively be Before the Bolide.