Ancient fossil may be the first bryozoan with a hard skeleton

A newly discovered fossil could offer insights into the development of the first mineralised skeletons.

Though skeletons reinforced with calcium carbonate and other minerals are widespread in the animal kingdom today, over 540 million years ago they were yet to come into existence. This changed during a period known as the Cambrian, when most of the major animal groups that exist today first appeared in the fossil record.

Part of the reason these fossils suddenly appear is due to the evolution of hard skeletons during this time, as these fossilise much more easily than soft tissue.

However, one of the oldest groups of animals, the bryozoans, were until recently only known from the Ordovician Period which followed the Cambrian, until researchers reinterpreted the soft-bodied fossil Protomelission gatehousei as a bryozoan.

Dr Paul Taylor, a scientific associate at the Museum who was involved in the description of this species, is now a co-author on a paper discussing what could be another Cambrian bryozoan, and potentially the earliest bryozoan with a hard outer skeleton.

'It is a good time to be researching Cambrian bryozoans,' he says. 'It's something of a renaissance.'

'I think finding Protomelission has opened a flood gate for bryozoan research and renewed interest in examining Cambrian rock for bryozoans. While bryozoans in the Cambrian are unlikely to be common or diverse, they were the foundations for a much larger diversification in the succeeding period.'

The findings of the new study were published in the journal Science Advances.

When did skeletons evolve?

Before the end of the Ediacaran Period, and the beginning of the subsequent Cambrian, what we would now call skeletons did not exist. Some animals, like the lancelets, stiffened parts of their body with cartilage, and limited mineralised structures existed in some early species, but were otherwise generally absent.

During the Cambrian explosion, when life rapidly diversified into a variety of new forms in a short period of time (evolutionarily speaking), mineralised skeletons became much more widespread, and gave rise to a variety of shells, bone and other structures such as teeth.

As well as allowing the formation of a variety of new body parts, mineralised skeletons also allowed for new behaviours and ecosystems to form. For instance, the first animal-constructed reefs came into being for the first time.

Though some scientists suggest the first reefs may have been made by a mysterious organism called Cloudina, this is controversial. Instead, it is more commonly agreed that the first animal reefs were formed by creatures known as archaeocyathan sponges at some point during the Cambrian.

Among the residents on these early reefs would likely have been the bryozoans, who acquired a mineralised skeleton during this time. The calcium carbonate skeletons of these colonial animals provide a substrate for other forms of life to settle and grow on while the bryozoans filter feed.

While P. gatehousei has been dated to the Cambrian Period, it has no hard exoskeleton, which today is a common feature of the majority of bryozoans. The Nevadan fossils, however, may represent the earliest of these animals to develop such a structure.

'These bryomorph fossils are found on the same geological level of archaeocyathan reefs,' Paul says. 'These sponges are found in the Early Cambrian, but as you get into the later parts of the Period it looks as if the bryomorphs were possibly colonising the reef.'

If confirmed as a bryozoan, the researchers believe that these animals probably lived as part of reef ecosystems containing echinoderms, the group of animals that includes today’s starfish and sea cucumbers, as well as trilobites.

These bryozoans are thought to have been sclerobionts, meaning that they needed to live on the surface of hard substrate. When the reef was damaged and their substrate ripped apart, their rigid skeleton kept them together as they were buried in the sediment and began to fossilise.

However, many of the finer surface details would have been lost as the animals were washed out of the reef. The poor preservation of these fossils has meant they have been difficult to identify.

How were the Nevadan bryomorph fossils identified?

Excavations taking place near the ghost town of Gold Point in Nevada have previously uncovered the remains of an ancient reef system in rocks known as the Harkless Formation. It preserves the remains of one of the last episodes of reef building in this area of the world during the early Cambrian.

The layers above this reef contain what appear to be small domes of around five millimetres in size. While their poor condition means the scientists cannot be completely certain of their identity, the researchers believe it most likely they are bryozoans. If confirmed, they would be tens of millions of years older than the next known mineralised bryozoan.

'Early fossils aren't always well preserved, and may not have much morphological complexity, so it can be really hard to know what they are,' Paul explains.

'Elsewhere in the fossil record, fossils that vaguely resemble this have been attributed to groups including algae, cyanobacteria, sponges and corals but after eliminating those which didn't fit, the best remaining option was that it was most likely a bryozoan.

'As the skeleton is very badly preserved, however, it has lost all of its original textures and structure.'

To confirm its identity, the researchers would want to see a laminated wall structure, which is a key signature of bryozoans. In addition, they would want to look for the ancestrula, the founding individual which the rest of the bryozoan colony develops from.

While none of the specimens are well-preserved enough to see these features, Paul hopes that the publication of this paper will lead to the discovery of better preserved bryomorphs which could confirm their identity.

'Papers like this draw people's attention to a particular kind of fossil that has been neglected to a large extent and possibly misunderstood,' he says. 'Once people have seen it, they can go out on excavations or into museum collections with fresh eyes and start looking for these tiny fossils.'