Deciphering carpoids: Fossil ‘problematica’
Imran A Rahman (UK)
Fossils can provide unique insights into the history of life on Earth, which stretches back nearly four billion years. Unfortunately, the closest living relatives of fossil organisms are sometimes unclear and, therefore, their evolutionary significance may not (yet) have been fully realised. Such fossils are called ‘problematica’ – the bizarre-looking carpoids are a classic example of a problematic fossil group.
Carpoids are small, flattened, marine fossils, ranging from a few millimetres to a few centimetres in length. They possess a hard, mineralized skeleton composed of calcium carbonate (calcite) and often display one or more slender appendages (arms, stems or tails). The group existed during the Palaeozoic Era, from the Middle Cambrian (about 510 million years ago) to the Late Carboniferous (about 310 million years ago), and its fossils are found today in marine rocks throughout the world. Within the UK, carpoids are known from County Tyrone (Northern Ireland), Ayrshire (Scotland), the West Midlands (England) and South Wales.
The first carpoid fossils were reported in the late 1850s, when the palaeontologists Joachim Barrande and Elkanah Billings described material from Bohemia and Canada, respectively. Subsequent discoveries in the mid-to-late nineteenth century, and especially in the twentieth, greatly increased the known diversity of the group, and there are now over 180 recognised species divided into four sub-groups (cinctans, ctenocystoids, solutes and stylophorans). However, over 150 years of study, however, carpoids continue to defy palaeontologists’ best efforts to make sense of them.
Stylophorans
No group more clearly exemplifies the controversy surrounding carpoids than the stylophorans. First discovered in 1858 by Billings, stylophorans are characterised by an asymmetrical body (or theca) and a long, tripartite appendage. The appendage is the focal point of many debates. It is envisaged as a feeding arm or a muscular, locomotory tail. If it is taken as an arm, then the mouth must have been located nearby, defining this end as the front of the animal.
Alternatively, if the appendage is interpreted as a tail, then the mouth was presumably located at the opposite margin of the animal and the appendage marked the rear end. Consequently, it is unclear which end is the front and which the back in stylophorans, a fundamental issue for palaeontologists!
The disagreement does not stop here though. Some stylophorans exhibit an arc-like series of special body openings that have been considered to be gill slits, that is, structures which aid feeding and/or respiration in several groups of modern marine animals, including fish. This interpretation is important, as it offers a guide to the biological affinities of stylophorans, as well as their mode of life. However, it is not accepted by all scientists.
The only way to confidently address these problematic points is to consider the soft parts (that is, muscles, ligaments and internal organs) of the animals. Alas, the hard calcite skeleton is all that remains in carpoid fossils. As a result, most reconstructions of their internal anatomy are highly speculative and difficult to justify.
Evolutionary position
Because of their cryptic morphology (epitomized by stylophorans), carpoids are hard to classify and there is heated discussion over their placement in the animal evolutionary tree. It is widely accepted that they are deuterostomes – a major animal group that includes chordates (vertebrates, sea squirts and lancelets), hemichordates (acorn worms and sea angels) and echinoderms (starfish, sea urchins and the like).
However, determining the relationships of carpoids with other deuterostomes has proven controversial – originally regarded as primitive echinoderms, carpoids have subsequently been reinterpreted as ‘advanced’ (derived) echinoderms closely related to sea lilies, and even as primitive chordates and hemichordates. Discriminating between these radically different theories is essential to accurately reconstruct the early evolution of animals.
Solving the enigma
A critical step towards understanding the evolutionary history of life is to decipher all of the fossils – including ‘problematica’ – that document this period. The carpoid fossil record is difficult to read. However, there are signs that, by employing sophisticated new analytical techniques, it may be possible to resolve some of the issues surrounding this notorious extinct group. For example, X-ray micro-CT systems – variants of medical CT scanners – have been used to ‘see’ inside fossils, revealing structures that appear to support the idea that stylophorans possessed gill slits.
Moreover, by examining the genes of modern animals and the microscopic structure of fossils, it was possible to determine that the skeleton of carpoids is identical to that of echinoderms, implying that carpoids are echinoderms. Nevertheless, while computational and molecular methods may provide valuable insights into the enigmatic carpoids, new fossils remain key to unlocking their mysteries. Discovering such specimens requires not only a great deal of luck, but also the assistance of fossil collectors worldwide – I entreat you all to get hunting!
Further reading
Rahman, I.A. 2009. Making sense of carpoids. Geology Today 25: 34–38