550-million-year-old fossils just solved a centuries-old 'skeleton' mystery

Determining the appearance of the earliest creatures to develop skeletons was impossible - until now.
Sade Agard
Earliest animals with skeletons are discovered as well-preserved "soft tissues" fossils
Earliest animals with skeletons are discovered as well-preserved "soft tissues" fossils

Luke Parry and Guangxu Zhang/Xiaodong Wang 

Scientists have resolved a centuries-old mystery by determining the appearance of the earliest animals to create skeletons - thanks to an extremely well-preserved collection of fossils found in eastern Yunnan Province, China.

The discoveries were recently published in Proceedings of the Royal Society B, and complete a crucial gap in our understanding of Earth's evolutionary history.

"This really is a one-in-million discovery"

Around 550–520 million years ago, during the so-called Cambrian Explosion, the first animals to develop robust, hard skeletons suddenly appeared in the fossil record. Most of these early fossils are only hollow tubes ranging from a few millimeters to several centimeters in length.

It was only possible to determine what kind of animals generated these skeletons due to the preservation of their soft parts. Until now, no fossil allowed for the identification of these animals as members of one of the main groups of organisms that are still alive today.

"This really is a one-in-million discovery. These mysterious tubes are often found in groups of hundreds of individuals, but until now they have been regarded as 'problematic' fossils because we had no way of classifying them," said corresponding author Dr. Luke Parry, Department of Earth Sciences, University of Oxford, in a press release.

"Thanks to these extraordinary new specimens, a key piece of the evolutionary puzzle has been put firmly in place."

The study is the first to demonstrate that Gangtoucunia was an ancient jellyfish

550-million-year-old fossils just solved a centuries-old 'skeleton' mystery
Gangtoucunia diagram showing body parts.

These characteristics are only present in contemporary jellyfish, anemones, and their close cousins (known as cnidarians), which are soft-bodied animals rare in the fossil record. The research demonstrates that these primitive creatures were among the first to develop the hard skeletons that make up a large portion of the fossil record.

According to the researchers, Gangtoucunia looked similar to modern scyphozoan jellyfish polyps, with a rigid, tubular structure anchored to the underlying substrate. The tentacle mouth would have extended outside the tube but could have been retracted inside the tube to avoid predators.

The tube of Gangtoucunia was made of calcium phosphate. This hard material also makes up our own teeth and bones, unlike living jellyfish. Over time, it has become less common for animals use this material to construct their skeletons.

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Soft tissues were exceptionally preserved thanks to anaerobic (oxygen-poor) conditions

The fossil was discovered at a location in Kunming, eastern Yunnan Province, China, in the Gaoloufang neighborhood. The existence of microorganisms that ordinarily break down soft tissues in fossils is constrained in this environment by anaerobic (oxygen-poor) conditions.

Four specimens of Gangtoucunia aspera with soft tissues still intact, including the stomach and mouthparts, are part of the new collection of 514-million-year-old fossils. These demonstrate that this species had a mouth bordered by a ring of 5 mm long smooth, unbranched tentacles.

According to the experts, these tentacles were probably used to sting and catch prey, such as tiny arthropods (insects). The fossils also demonstrate that Gangtoucunia had an interior cavity-partitioned blind-ended gut that filled the length of the tube and was only open at one end.

The new specimens debunk a common theory about Gangtoucunia's origin

"A tubicolous mode of life seems to have become increasingly common in the Cambrian, which might be an adaptive response to increasing predation pressure in the early Cambrian," revealed coauthor Xiaoya Ma in the same press release.

Significantly, the new specimens debunk a theory that Gangtoucunia was related to annelid worms (earthworms, polychaetes, and their relatives). It is now clear that Gangtoucunia's body had smooth skin outside and a longitudinally divided gut. On the other hand, Annelids have segmented bodies with transverse body partitioning.

Although the fossil demonstrates that Gangtoucunia was an ancient jellyfish, it is still possible that other early tube-fossil species had completely different appearances.

A significant component of the evolution puzzle has been solidly established

This study underscores how vital soft-tissue preservation is to understanding these extinct species. Such discoveries then aid in our understanding of the nature of life on Earth millions of years ago, and help us comprehend how Earth has evolved since then.

The full study was published here.

Abstract:

Exoskeletal dwelling tubes are widespread among extant animals and early fossil assemblages. Exceptional fossils from the Cambrian reveal independent origins of tube dwelling by several clades including cnidarians, lophophorates, annelids, scalidophorans, panarthropods and ambulacrarians. However, most fossil tubes lack preservation of soft parts, making it difficult to understand their affinities and evolutionary significance. Gangtoucunia aspera (Wulongqing Formation, Cambrian Stage 4) was an annulated, gradually expanding phosphatic tube, with occasional attachments of multiple, smaller juveniles and has previously been interpreted as the dwelling tube of a 'worm' (e.g. a scalidophoran), lophophorate or problematicum. Here, we report the first soft tissues from Gangtoucunia that reveal a smooth body with circumoral tentacles and a blind, spacious gut that is partitioned by septa. This is consistent with cnidarian polyps and phylogenetic analysis resolves Gangtoucunia as a total group medusozoan. The tube of Gangtoucunia is phenotypically similar to problematic annulated tubular fossils (e.g. Sphenothallus, Byronia, hyolithelminths), which have been compared to both cnidarians and annelids, and are among the oldest assemblages of skeletal fossils. The cnidarian characters of G. aspera suggest that these early tubular taxa are best interpreted as cnidarians rather than sessile bilaterians in the absence of contrary soft tissue evidence.

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