What Is a Blastoid? Small Bud-Shaped Fossils With Elaborate Plumbing

A blastoid consists of a rounded body a centimetre or two across, built from a small and fixed number of interlocking calcite plates arranged with strict five-part symmetry, sitting on a stalk that attached it to the seafloor. Five petal-shaped areas radiate from the top, each carrying a food groove that was lined with numerous slender appendages extending into the water to capture particles. Those appendages, called brachioles, are delicate and usually detached, so most fossils show only the body with the petals marked on it. A mouth sits at the centre of the top and openings around it served for waste and reproduction. The constancy of the plate arrangement across the group is striking, since the same small number of plates in the same positions appears in every member, which makes the fossils easy to recognise and hard to confuse with anything else.

The interior contains the group's most distinctive and least understood feature:

• •Hydrospires, thin folded sheets of calcite hanging inside the body beneath the petals
• •An arrangement that greatly increases surface area within a small volume, which is what such folding always achieves
• •A connection to openings at the top through which water could pass
• •Respiration as the leading interpretation, with the folds acting as gills and water drawn across them
• •Alternative or additional roles proposed in feeding and in reproduction, since the openings are shared
• •No equivalent structure in any living echinoderm, which is why the interpretation rests on inference rather than comparison

The reconstruction is of an animal fixed by a stalk, holding its body above the sediment and extending a crown of fine appendages into passing water to filter particles. That places them in the same ecological role as sea lilies, which are related, still living, and use a comparable approach with different anatomy. They occur in large numbers in some deposits, and beds composed substantially of their remains and of the stalk fragments of related animals are common in rocks of the appropriate age, which indicates dense populations in favourable conditions. Their requirements were the standard ones for this way of life, namely a firm substrate, moving water and clear enough conditions that fine appendages would not be clogged. The shape of the body varies between species in ways that relate to how strong the current was, with more streamlined forms in higher energy settings.

Certain Carboniferous limestone units in the central United States are famous for producing these fossils in enormous numbers, weathering out of the rock whole and being collected by the thousand, and they are among the most widely distributed fossils in teaching and reference collections for that reason. The abundance reflects both a genuinely dense living population and preservation conditions that favoured the compact bodies, since the plates interlock tightly and hold together after death while the delicate appendages and the stalk fall apart immediately. Comparable deposits occur in Europe and elsewhere in rocks of similar age. Collecting is straightforward where the rock weathers, since the fossils separate from the matrix naturally, and complete specimens with the appendages preserved are rare enough to be notable. Their small size and distinctive shape also make them useful for identifying rocks of the appropriate age in the field.

The group appeared in the Ordovician, remained modest for a long period, and became abundant and diverse in the Carboniferous, which is when most of the familiar fossils were formed and when the group reached its greatest variety. It then declined and disappeared entirely at the end of the Permian, in the same extinction that removed the rugose corals and a great deal else, leaving no descendants. That trajectory, a long quiet existence followed by a burst of diversity and then complete removal, is common among Palaeozoic groups and is part of why the end-Permian event is treated as the most consequential boundary in the fossil record. Their absence afterwards left the filter-feeding role to the sea lilies, which survived in reduced numbers and recovered, and to other groups entirely, which is the usual pattern of replacement after a major extinction.

A bud-shaped body of interlocking plates in strict five-part symmetry, on a stalk, with five petals carrying grooves and fine appendages that filtered particles from the water. Inside sit folded calcite sheets that increase surface area enormously and have no equivalent in any living animal, most likely gills. The group peaked in the Carboniferous and vanished completely at the end of the Permian.