Tag Archives: placoderm

Placoderm Nurseries

Many sharks and bony fishes make use of protected shallow habitats for their young to grow in today. Typically these habitats provide protection from predators and abundant resources that rush downstream from the eroding highlands. This allows the maximum number of offspring to grow quickly before reaching adult size where they are less vulnerable to predation and can migrate out to deeper waters. Given how common this is today it would be expected that extinct organisms would have done this as well. Unfortunately, even if they were common we still might not be likely to find evidence of nurseries in the fossil record for two reasons. First, nurseries tend to be in shallow environments such as rivers and estuaries which are likely to be eroded as sea level rises and falls or continents collide erasing the environment. Second, identifying a nursery requires finding a location that is dominated by juvenile organisms and in most nurseries the juveniles are either eaten (no fossils left) or successfully reach adulthood and leave the nursery. So it’s only in the rare circumstance where there is a mass death of juveniles due to some catastrophe (at least from the organisms’ perspective) and then up to luck that the sediments aren’t eroded away for millions of years before they are discovered and sampled by paleontologists.

Despite these unlikely circumstances there are a number of likely nursery site of chondrichthyans (sharks and their relatives) in Triassic rocks of Kyrgyzstan (Fischer et al. 2011) and Pennsylvanian rock of Illinois (Sallan & Coates 2014). The longer ago sediments were formed the more likely they will have been destroyed by geological processes so it was exciting news this past week when a new study in interpreted a site in Belgium as a placoderm nursery (figure 1) from the Late Devonian (bonus points for being placoderms!). I’ll detail some of the bits I find the most interesting here. The study (Olive et al. 2016) was published in PLoS One and thus is open-access so I encourage everybody to go read the full paper for themselves, it’s a light read at nine pages.

journal.pone.0161540.g002

Figure 1. From Olive et al. 2016 showing a reconstruction of the Strud nursery site. Scale bar =2cm. Illustration by J. Jacquot Hameon (MNHN, Paris).

 

The locality in Strud, Belgium would have been along the edge of the paleocontinent of Laurussia/Euramerica, a combination of North America and northern/western Europe, in the Late Devonian. It most likely represents the river deposits on an alluvial plain, a relatively calm environment, except in the case of flooding events. The authors report the recovery of 105 fragments of placoderms comprised of three species all of which were relatively small and showed other morphological correlates of juveniles based on previous studies of closely related placoderms (Werdelin & Long 1986; Deaschler et al. 2003). So what happened to these young placoderms that they weren’t eaten and yet they didn’t survive into adulthood? At Strud the most likely explanation is that these juveniles were in a pond or small tributary that dried up and became isolated from the main channel. So bad luck for the placoderms, good luck for paleontologists. What’s even more interesting though is that the Strud locality is the second known placoderm nursery of the Late Devonian. The other is in Tioga County, Pennsylvania (Downs et al. 2011) with similar species. In that case the hundreds of specimens are more complete and tend to be oriented in the same direction. Again, the explanation for their remains is that they were isolated in a shrinking body of water that also slowly became anoxic (aiding preservation) leading to a mass kill.

Both of these placoderm nurseries had large numbers of similarly aged individuals, and in the case of the Pennsylvania site there’s strong evidence that they represent a life assemblage, rather than one which has been time-averaged. So these sites probably show us that these placoderms had large numbers of offspring, though how exactly is up for debate. Some placoderms are known to have had live offspring (Ptyctodonts; Long et al. 2008) while there is questionable evidence of egg-laying behavior in other (Ritchie 2005; figure 2). Because of the number of similarly aged juveniles both nurseries most strongly support an egg-laying behavior in the species found there (primarily antiarchs). Given the position of placoderms as the outgroup to the rest of the jawed vertebrates their reproductive strategies can help us chart the evolution of reproductive strategies in Earth’s early history. It will be exciting to see if more of these nursery sites appear in other parts of the world outside of Luarussia and even earlier in fossil record. There are also tantalizing hints that large placoderm species (e.g. Dunkleosteus) might have used nearshore habitats for their juveniles as well (Daeschler & Cressler 2011)!

Cowralepis egg

Figure 2. Possible egg case of the plcaoderm Cowralepis. From Ritchie 2005. 

 

References

Daeschler, E.B. and W.L. Cressler III. 2011. Late Devonian paleontology and paleoenvironments at Red Hill and other fossil sites in the Catskill Formation of north-central Pennsylvania. Geological Society of America Field Guide 20:1-16.

Daeschler, E.B., A.C. Frumes, and C.F. Mullison. 2003. Groenlandaspid placoderm fishes from the Late Devonian of North America. Records of the Australian Museum 55:45-60.

Downs, J.P., K.E. Criswell, and E.B. Daeschler. 2011. Mass mortality of juvenile antiarchs (Bothriolepis sp.) from the Catskill Formation (Upper Devonian, Famennian Stage), Tioga County, Pennsylvania. Proceedings of the National Academy of Science Philadelphia 161:191-203.

Fischer, J., S. Voigt, J.W. Schneider, M. Buchwitz, and S. Voigt. 2011. A selachian freshwater fauna from the Triassic of Kyrgyzstan and its implication for Mesozoic shark nurseries. Journal of Vertebrate Paleontology 31:937-953.

Long, J.A., K. Trinajstic, G.C. Young, and T. Senden. 2008. Live birth in the Devonian period. Nature 453:650-652.

Olive, S., G. Clement, E.B. Daeschler, and V. Dupret. 2016. Placoderm assemblage from the tetrapod-bearing locality of Strud (Belgium, Upper Famennian) provides evidence for a fish nursery. PLoS One 11:e0161540.

Ritchie, A. 2005. Cowralepis, a new genus of phyllolepid fish (Pisces, Placodermi) from the late Middle Devonian of New South Wales, Australia. Proceedings of the Linnean Society of New South Wales 126:215-259.

Sallan, L.C. and M.I. Coates. 2014. The long-rostrumed elasmobranch Bandringa Zangerl, 1969, and taphonomy within a Carboniferous shark nursery. Journal of Vertebrate Paleontology 34:22-33.

Werdelin, L. and J.A. Long. 1986. Allometry in the placoderm Bothriolepis canadensis and its significance to antiarch evolution. Lethaia 19:161-169.

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.

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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.

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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.