Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Echinodermata

Disarticulated plates from the Cambrian Stage 3 of well-mineralised echinoderms are the first evidence of their appearance. They are represented by a few specimens in the Burgess Shale and are not found in the Chengjiang deposits, which suggests that the conditions of their normal habitat didn’t overlap with the type of conditions that were conducive to preservation of the Burgess Shale type. It has been argued (Clausen et al., 2010) that the presence of echinoderms in carbonate sequences from the Cambrian Stage 2 through to Stage 5 of preferential divergence in carbonate environments. The many small plates that comprise the echinoderm skeleton of tend disarticulate after death, though even a plate that is isolated can be referred with some confidence to Echinodermata because of the unique stereo texture of their plates. Many forms of the early echinoderm groups, that mainly appeared between Cambrian Stage 3 and the Middle Ordovician, lacking the familiar pentameral arrangement of ambulacral regions present in extant forms, some of those groups possessing only 1, 2, 3 or 4 ambulacra. Therefore Echinodermata shows the common metazoan pattern involving achieving a broad range of different morphologies, at the body sub-plan level, early in its history. Questions have been raised as to how to handle many of these early disparate groups taxonomically, as many of them are not known to be very diverse, have fossil records that are relatively short, and are relatively rare. The solution, under a Linnaean taxonomic approach, has been to recognise the disparities by assigning distinctive morphological groups to taxonomic levels showing morphological differences similar in magnitude to those seen in extant echinoderm groups. More than 20 different classes of echinoderm have been recognised, under this approach, during the Cambrian and Ordovician (Sprinkle, 1980; Sprinkle & Kier, 1987), though there are only 5 extant classes. The systematic practice of many decades ago is reflected in the large number of echinoderm classes that are relatively short-lived, many of which had few species. The modern view of early arthropod evolution contrasts starkly with the earlier approach. According to the present view just as many distinctive morphologies have been discovered among arthropods from the Cambrian, though there are only a few Linnaean classes or orders that have been defined. It seems likely that during the Palaeozoic there were as many as 30 echinoderm clades that were morphologically distinct (Sumrall & Wray, 2007).

Subsequently, the taxonomy of echinoderms has been given a cladistic treatment that has the aim of tracing the patterns of branching of their lineages as novel morphologies appeared and accumulated instead of identifying their disparate groups (Paul & Smith, 1984; Smith, A.B., 2005; Smith, A.B. & Peterson, 2002). The record of the early echinoderms is incomplete and the authors¹ suggest their known record may be of not much use for a detailed interpretation of patterns among body plans. A model that is more general, of the evolutionary change among the distinctive clades of echinoderms has resulted from a comparative study of the development of ambulacral geometry. All except Crinoidea among the extant echinoderm classes have 5 ambulacra that terminate at the central mouth, which is a fully pentameral symmetry. The crinoids and all fossil echinoderm groups that have 5 radiating ambulacra, have only 3 that reach the mouth, 2 of which split during development to produce the 5 of adult specimens (like the edrioasteroid Camptostroma, therefore they can be described as having a 2-1-2 pattern instead of a 1-1-1-1-1 pattern. Fossil echinoderm groups that have fewer than 5 ambulacra in the adult are not even pentameral and are interpreted (Sumrall & Wray, 2007) as having reduced from the more primitive pattern by various patterns of loss of ambulacra during early developmental stages. The losses of ambulacra have been correlated with important body change differences such as those that are characteristic of many disparate fossil echinoderm groups. The spotty fossil record makes this evolutionary model difficult to test, but according to the authors¹ the generality of this model make it appealing. It seems clear that reductions of ambulacra have also occurred within minor branches of some major clades. And this tends to complicate the phylogeny even further. The bodyplan sequence of appearance in skeletonised forms does not match sequential losses of ambulacra, though this is not sufficient to falsify the hypothesis of Sumrall & Wray when the incompleteness of the early echinoderm fossil record is taken into account.

The earliest Cambrian echinoderms have been assigned to Helicoplacoidea (Durham & Caster, 1965).  They are known from only 4 small-bodied genera (mostly 4-7 cm) from stage 3 that were recovered from western North America. The 3 genera that are best known have somewhat fusiform to top-shaped bodies, with a skeleton of imbricating plates that have a stereom texture that are arrayed in spiral columns. The columns are mostly interambulacral, but ambulacral columns, which indicate the presence of a water-vascular canal system, are shown by a few columns that have plates that are much smaller. There are 2 ambulacral columns that spiral down from the apex of the larger end to about the wider parts of the test, at which point they join; the single ambulacral column that results continues to spiral down towards the smaller end though it terminates before it reaches the base. The clearest description has been given of the ambulacral structures (Durham, 1993), working with a large collection, concluding that the mouth was at the larger end. It had previously been suggested that the mouth was at the junction of the 3 ambulacral columns (Paul & Smith, 1984). An oral framework has not been detected at either site, though the authors¹ suggest that this may be because of the collapsed, deformed state of the fossils. It would be suggested that the 3-canal system evolved to the 2-1-2, 5 canal system present in the early echinoderms by the doubling of each of the paired ambulacra, if the mouth were at the junction.

In rocks (mostly in the Kinzers Formation, Pennsylvania) of early Stage 4 2-1-2 pentaradial groups are the earliest known. A group which is often treated as a class is the Edrioasteroidea, which were circular in outline and lay on their dorsal surface, in some cases being apparently attached to the substrate; they have been assumed to have been suspension feeders. Camptostroma, the basal edrioasteroid, which has been interpreted as a basal form, and has been recovered from the Kinzers Formation, and it has been hypothesised to represent the deepest known branch of all the pentaradial echinoderms (A.B. Smith, 1988). A later branching may have resulted in the appearance of 2 echinoderm clades that produced the disparate pentaradial groups of the Palaeozoic, 1 of which was characterised by attachment stalks. The authors¹ suggest that Stromatocystites, representing 1 of these major clades may be ancestral to all living nonstalked echinoderm classes. The model of Sumrall & Wray, if correct, would imply that the last common ancestor arose following the evolution of the 1-1-1-1-1 pattern. Lepidocystis, another echinoderm from the Kinzers Formation has been hypothesised to represent a branch that given rise to stalked clades that are extinct such as Eocrinoidea and the Crinoids. Crinoids are pentaradial in the 2-1-2 fashion, that have ambulacra that extend up their feeding arms (as is the case in Lepidocystis); (A.B. Smith, 1988). They were all stalked and attached to the substrate for most of their existence, by a stem-like organ that was well-skeletonised, crinoids without stalks evolving much later. However, it has been hypothesised that a second lineage also branched off the edrioasteroids, which later evolved a stalk, diversifying into the Eocrinoidea, an extinct class. The arm-like structures of crinoids were not invested by coeloms, and did not bear ambulacra; such arms being called brachioles. If the suggested phylogenetic hypothesis is correct Edrioasteroidea and Eocrinoidea are both paraphyletic, though new fossil discovery are required to test these hypotheses.

Another enigmatic group of echinoderms, the carpoids, that lack pentaradial symmetry, is first known from Stage 5 in Bohemia. Though 2 of the 3 main groups have been suggested to “exhibit bilateral tendencies,” in Ken Caster’s phrase (Ubaghs & Caster, 1967, S583); and according to the authors¹ there are no other echinoids that are remotely like them. The Stylophora, a clade of carpoids, was at some time in the past proposed to be on the lineage of vertebrates (Jeffries, 1986). It has subsequently been shown that carpoids are a branch of echinoderms that are no more closely related to chordates than any other echinoderm group, though the architecture of carpoids is clearly outside the borders of all other known echinoderms, and the time of the first appearance of carpoids is uncertain.

Sources & Further reading

1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado