Tuesday 30 October 2012

The first Arthropod?

The Arthropods are one of the most abundant and diverse groups of animals alive today, and the fossil record suggests this has been the case for most of the last 500 million years. However the origin of the group remains obscure. There are a wide variety of Arthropods in early Cambrian faunas, though these are strange, unfamiliar forms, as well as members of groups thought to be related but outside the group, such as the Lobopodians and Anomalocarians, but the precise nature of the earliest Arthropods is still highly debatable.

In a paper published in the Proceedings of the Royal Society Series B: Biological Sciences on 10 October 2012, David Legg of the Department of Earth Sciences and Engineering at Imperial College London and the Department of Earth Sciences at the The Natural History MuseumMark Sutton of the Department of Earth Sciences and Engineering at Imperial College London, Gregory Edgecombe of the Department of Earth Sciences at the The Natural History Museum and Jean-Bernard Caron of the Department of Natural History (Palaeobiology Section) at the Royal Ontario Museum and the Department of Ecology and Evolutionary Biology at the University of Toronto describe a new fossil from the Burgess Shale Lagerstätten (rich fossil find) in British Columbia which they believe may illustrate the form of the earliest Arthropods.

The new fossil is named Nereocaris exilis, where Nereocaris  means 'Nereo's crab' (Nereo was a Titan, form Greek mythology, often depicted with a fish's tail), and exilis means slender. It is a member of a group known as the 'Bivalved Arthropods', which have bilaterally flattened, elongate bodies with their limbs held within the valves. 

Nereocaris exilis is a 142 mm Bivalved Arthropod with stalked lateral eyes and a single rod-shaped median eye; a bivalved carapace with a postero-dorsal keel; the limbs are entirely encased within this carapace. Behind the carapace is an elongate, tail-like abdomen, composed of about 60 ring-like segments, terminated by a telsion (final group of segments, with small median and elongate lateral processes.

Reconstruction of Nereocaris exilis. Legg et al. (2012).

Based upon their interpretation of Nereocaris, Legg et al. have created a cladistic analysis (family tree, but with maths) of the earliest Arthropods, and the order in which they gained the features typical of the group. Working on the assumption that the Anomalocarians are outside the true Arthropods, this suggests that Nereocaris is the most primitive arthropod yet discovered.

Cladistic reconstruction of the origin of the Arthropods, based upon the accumulation of typical Arthropod features. (1) Development of compound eyes (some groups have subsequently lost, and even re-evolved these). (2) Appearance of Arthropod-like jointed limbs. (3) Appearance of such limbs on the trunk, Branched limbs appear on the trunk, segmented, mineralized exoskeleton. (4) Specialized cephalic (head) appendages. (5) Development of distinct dorsal and ventral sides to the body segments (as opposed to the simple ring-like segments that had occurred before). (6) Reduction in number and greater specialization of the limbs. (7) Modification of appendages into distinct antennae. (8) Appearance of mandibles capable of 'chewing' food. Legg et al. (2012).

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