Wednesday 12 October 2016

A Devonian Nursery

A cluster of mantid hatchlings on the underside of a leaf
Some animal parents devote little care to their young. Many mantids will simply glue their clutch of eggs to the underside of a leaf and leave them to it. Hatchlings have to fend for themselves, and not just from predators, cannibalism amongst siblings is common.

Other animals are rather more attentive to their broods' needs. African bullfrog males will watch over pools containing developing tadpoles, even going so far as to dig canals to larger bodies of water if the nursery pool is in danger of drying up. Nursery-type behaviours have also been observed in the fossil record from a diverse array of organisms ranging from dinosaurs to sharks.

Recently one such nursing site has been uncovered in 360 million year old rocks in a quarry in Belgium. It is special as it is the oldest known instance of multiple species, specifically of placoderms, using a common nursery. Placoderms were an ancient lineage of fish which flourished during the Silurian and Devonian. They were the first vertebrates to develop jaws, which encouraged a wide range of feeding possibilities and ecological niches. While their skeletons were composed of cartilage, they were covered in plates of body armour.

A reconstruction of the immative placoderms and the
Strud nursery. Order of species is same as in the text
'These sorts of juvenile-only assemblages are rare in the fossil record,' said Dr Ted Daeschler from the Academy of Natural Sciences of Drexel University, 'We are quite sure that the juvenile-only placoderm assemblage is not the result of sorting of small material by water currents because there are larger skeletal elements of other kinds of fish. We believe this points to a nursery.'

Discovered near Strud, the site consisted of a multitude of largely complete skeletons of immature fish species Turrisaspis strudensis, Grossilepis rikiki and Phyllolepis undulata alongside the elements of the adults. The paucity of adults suggests that the site was only used for egg laying and/or live birth as opposed to parental care, yet was well protected from predators. It was shallow and contained slow-moving, sheltered water and was further protected by large, spiny plants, fossils of which were found alongside the placoderms.

'By studying the past, with the ability to see a moment in time and changes through time, we are better able to understand ecosystems and the organisms that live in them today,' said Sebastien Olive also from Drexel University. 'Geologists say that the present is the key to understanding the past. But we can also say that the past is the key to understanding the future.'

Today many mating, nesting and nursery sites in multiple ecosystems are threatened by human activities. An understanding of their mechanics in response to past conditions is key to effective direction for current and future conservation schemes.