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.
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)!
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.