{"id":3929,"date":"2016-04-01T00:01:26","date_gmt":"2016-04-01T00:01:26","guid":{"rendered":"https:\/\/www.palaeontologyonline.com\/?p=3929"},"modified":"2016-03-29T17:45:21","modified_gmt":"2016-03-29T17:45:21","slug":"fossil-focus-cinctans","status":"publish","type":"post","link":"https:\/\/www.palaeontologyonline.com\/?p=3929","title":{"rendered":"Fossil Focus: Cinctans"},"content":{"rendered":"

by Imran A. Rahman<\/a>*1<\/sup><\/p>\n

Introduction:<\/h2>\n

The fossil record of early animals \u2014 which dates back at least to the Cambrian<\/a> period, more than 500 million years ago \u2014 is packed full of bizarre sea creatures that seem, at first glance, rather different from anything alive today. These include the armoured slug-like Wiwaxia<\/a><\/em>, the spiny worm-like Hallucigenia<\/a><\/em> and Earth\u2019s first big predator, Anomalocaris<\/a><\/em>. Collectively, these fossils were termed \u201cweird wonders\u201d by the evolutionary biologist Stephen Jay Gould<\/a>; they possess some, but not all, of the characteristics shared by their modern relatives, and so are crucial for understanding the early evolution of animal phyla<\/a>.<\/p>\n

This article focuses on a peculiar extinct group of Cambrian weird wonders called the cinctans, which look more like tennis racquets than any living animals! They are known exclusively from the middle part of the Cambrian, in rocks that are around 509 million to 497 million years old. Cinctans have a hard, mineralized skeleton made of calcium carbonate (calcite) with a distinctive porous microstructure called stereom<\/a>. This clearly identifies them as members of the echinoderms<\/a>, the major group that includes starfish and sea urchins. However, unlike starfish, sea urchins and all other living echinoderms, they do not have five-fold symmetry. In fact, apart from the skeleton, cinctans have no characters that unambiguously place them with echinoderms. As a result, important aspects of their palaeobiology are debated by scientists.<\/p>\n

Morphology:<\/h2>\n

Cinctan fossils range from a few millimetres to a few centimetres long. Like all echinoderms, they have a skeleton made of many interlocking calcite plates, which are organized into a flattened body (theca) and a stiff appendage (stele; Fig. 1). The theca is made of an outer ring of stout marginal plates (the cinctus) surrounding numerous small central plates (the upper and lower integuments). One narrow groove or an asymmetrical pair of such grooves (covered by many tiny plates) runs along the front of the theca, ending at a circular opening that is thought to be the mouth. Next to this, there is a large opening at the midline of the theca (called the porta), which is enclosed by a large spoon-shaped plate (the operculum). In at least some species, a pyramid of small plates pierces the upper surface of the theca. The stele is a short, rigid structure at the back of the cinctus.<\/p>\n

\"Figure
Figure 1 \u2014 The key characteristics of a typical cinctan. (A) Upper view. (B) Lower view. (C) Front view. Modified from Zamora & \u00c1lvaro (2014<\/a>).<\/figcaption><\/figure>\n

Cinctan thecae come in many different shapes, from circular to oval and boot-shaped. Some are nearly symmetrical down the centre; others are completely asymmetrical (Fig. 2). There are other notable differences, including the number of plates in the cinctus and the stele, the size and position of protuberances on the marginal plates, and the relative length of the grooves along the front of the theca. All these features are important for recognizing cinctan species.<\/p>\n

\"Figure
Figure 2 \u2014 Different cinctan species. (A) Lignanicystis barriosensis<\/em>. (B) Asturicystis jaekeli<\/em>. (C) Sucocystis undata<\/em>. (D) Gyrocystis badulesiensis<\/em>. (E) Sucocystis quadricornuta<\/em>. (F) Trochocystites bohemicus<\/em>. (G) Trochocystites bohemicus<\/em>. Modified from Smith & Zamora (2009<\/a>).<\/figcaption><\/figure>\n

State-of-the-art imaging techniques have allowed palaeontologists to describe the morphology of select species in exceptional detail. For example, X-ray computed tomography<\/a> (CT) scanning was used to study the Spanish cinctan Protocinctus mansillaensis<\/em>, allowing researchers to create a computer model in which they could separate the fossil from the surrounding rock without having to destroy the fossil. The resulting 3D reconstruction revealed previously hidden aspects of the fossil\u2019s anatomy (Model 1). Such approaches are becoming increasingly common in palaeontological research.<\/p>\n