Understanding Echinoderm Evolution From Cambrian Fossils

Deciphering the early evolution of echinoderms with Cambrian fossils

Authors:

Zamora et al

Abstract:

Echinoderms are a major group of invertebrate deuterostomes that have been an important component of marine ecosystems throughout the Phanerozoic. Their fossil record extends back to the Cambrian, when several disparate groups appear in different palaeocontinents at about the same time. Many of these early forms exhibit character combinations that differ radically from extant taxa, and thus their anatomy and phylogeny have long been controversial. Deciphering the earliest evolution of echinoderms therefore requires a detailed understanding of the morphology of Cambrian fossils, as well as the selection of an appropriate root and the identification of homologies for use in phylogenetic analysis. Based on the sister-group relationships and ontogeny of modern species and new fossil discoveries, we now know that the first echinoderms were bilaterally symmetrical, represented in the fossil record by Ctenoimbricata and some early ctenocystoids. The next branch in echinoderm phylogeny is represented by the asymmetrical cinctans and solutes, with an echinoderm-type ambulacral system originating in the more crownward of these groups (solutes). The first radial echinoderms are the helicoplacoids, which possess a triradial body plan with three ambulacra radiating from a lateral mouth. Helicocystoids represent the first pentaradial echinoderms and have the mouth facing upwards with five radiating recumbent ambulacra. Pentaradial echinoderms diversified rapidly from the beginning of their history, and the most significant differences between groups are recorded in the construction of the oral area and ambulacra, as well as the nature of their feeding appendages. Taken together, this provides a clear narrative of the early evolution of the echinoderm body plan.

Crypto-helical Body Plan in Cambrian Paleozoic Gogiid Echinoderms

Crypto-helical body plan in partially disarticulated gogiids from the Cambrian of South China

Authors:

Lin et al

Abstract:

All living echinoderms have a pentaradial symmetry that is unique within the Bilateria. However, the Cambrian origin of echinoderm radial/pentaradiate symmetry is a long-standing problem. During the Cambrian (542-488 Ma), gogiids were the most common stalked echinoderm characterized by an “irregularly” plated body. Based on recently discovered material from the Balang Formation (Cambrian Series 2), eastern Guizhou, China, three unusual, partially disarticulated specimens of Guizhoueocrinus have clear evidence for a helical body plan. This helical plating is only evident in partially disarticulated specimens, thus a crypto-helical body construction is present. Crypto-helical construction in a gogiid raises the possibility of a phylogenetic connection among helicoplacoids, gogiids, and Helicocystis. The crypto-helical body construction may be an important evolutionary innovation among pre-radiate echinoderms.

A Little Experimental Paleontology: Tumbling Echinoderms

Experimental tumbling of echinoderms – taphonomic patterns and implications

Authors:

1. Przemysław Gorzelak (a)
2, Mariusz A. Salamon (b)

Affiliations:

a. Department of Biogeology, Institute of Paleobiology, Polish Academy of Sciences, Twarda Str. 51/55, 00–818 Warsaw, Poland

b. University of Silesia, Faculty of Earth Sciences, Department of Palaeontology and Biostratigraphy, Będzińska Str. 60, 41–200 Sosnowiec, Poland

Abstract:

Despite a wide array of published actualistic studies on echinoderm taphonomy the detailed pattern of decay and disarticulation of their skeletons is still not well understood. Here we provide results of tumbling experiments using a rotating barrel filled with artificial seawater and medium-sized quartz sand to mimic physical forces experienced by echinoderms during trasportation in high-energy conditions. In particular, we determined semi-quantitatively transportation-induced rates and patterns of damage and disintegration of freshly killed ophiuroid, asteroid and crinoid skeletons that were not allowed to decay initially. Our experiments showed that echinoderm specimens disintegrated in a characteristic sequence toward an increase of the degree of disarticulation, abrasion and roundness or thinness of echinoderm ossicles. The sequence of disintegration in crinoids began with the partial disintegration of distal arms after 2 hrs (a time equivalent to ~ 1 km of transport). The initial split of ophiuroid and asteroid arms and crinoid cirri occurred after 24 hrs (~ 12 km) and complete destruction of the asteroid mouth and ophiuroid disc area occurred after 72 hrs (~ 36 km). The duration of transport necessary to promote initial fragmentation in asteroid and ophiuroid arms and crinoid cirri into isolated ossicles was 120 hrs (~ 60 km). The complete disarticulation of crinoid, ophiuroid and asteroid arms and crinoid cirri occurred after 312 hrs (~ 156 km) and 408 hrs (~ 204 km), respectively. Although it has been argued that the quality of preservation can be a poor index of post-mortem transport, echinoderms allowed limited initial decay in the presence of rapid and relatively constant physical disturbance, an approximation of the distance of transport can be made.

Our data demonstrate that articulated ossicles can remain for several days, sufficient time for long (even a few hundred km) transporation. This finding illustrates that articulated echinoderm remains do not necessarily imply low energy and highlights the importance of a reliable discrimination of autochthonous and allochthonous components of fossil echinoderm assemblages. Application of isolated fossil echinoderm ossicles in e.g. paleoenvironmental and paleoecological reconstructions may lead to serious misinterpretations and should be supplemented by observations of abrasion traces.