Nature Blog Network

Thursday, October 4, 2007

New Fossil Crustacean Pushes Back Arthropod Origins

Blogging on Peer-Reviewed Research
Finding any new fossil is rare. Finding invertebrate fossils is even rarer due to the squishy nature of most invertebrates. They just do not preserve well over time. Sometimes the wandering paleontologist, who must look very carefully under a field scope through dust and debris, can find parts of squishy invertebrates like scolodonts (polychaete jaws), coral rubble, carbonate shell cement, or maybe sea star or sponge spicules. On even rarer conditions, this careful wandering paleontologist may find imprints of the outlines of squishy invertebrates that have no hard parts, none whatsoever.

China has been an amazing hot spot for fossils in recent years. Entire ecosystems have been preserved. Careful, wandering paleontologists are able to move beyond describing the random individual to describing how some dinosaurs formed nests, embryonic fossils still in the egg, biometric trends in populations of fossil fish or molluscs, interspecific interactions and just about anything else. The Chinese paleontologists have been prolific, publishing widely on their discoveries, inviting western paleontologists on digs and getting the Chinese people and government serious about their fossil wealth. They do have a problem with fossil poachers, but the rate of discovery remains high and stiffer penalties are becoming more commonplace.
"Reconstruction of Y. dianensis on the basis of a larval stage with eight thoracomeres (developmental stage 4). The head limbs are drawn in median view and the trunk limbs in lateral view."-Zhang et al. 2007

Zhang et al. 2007 present a new find of a crustacean with some interesting morphology and even more interesting implications. Yicaris dianensis (above drawing) new species, is named for both the people, the Yi, and the place the fossils originated from, Dian. The suffix 'caris' refers to something that is a crustacean and the suffix 'ensis' refers to being from a place. What is remarkable to me is that 6 successional developmental stages are represented in pre-Cambrian strata. This is close to the hypothesized origin of the Arthropoda, specifically the Crustacea. So this form should theoretically be the closest ontologically to the earliest Crustacea.
"With such ontogeny data and having the age of the fossil as a time marker, it is possible to more precisely include ontogenetic evolutionary pathways in the reconstruction of relationships and ground patterns of stem species and monophyla. According to our analysis, Y. dianensis represents the first undoubted eucrustacean known from the Lower Cambrian (the single previously described Lower Cambrian putative eucrustacean species lacks eucrustacean characters) and can serve as a substantial tool for testing relevant character acquisition and phylogenetic hypotheses. This is of particular importance because crustacean phylogeny has gained new interest by recent studies using neurobiological, developmental, biological and molecular investigations."
Y. dianensis shares many features in common with modern day branchiopods and cephalocarids of which I won't go into detail. Some of the similarities are in the mouthparts and other structures involved in food uptake or feeding. The interesting character for this crustacean is called the epipod, or "above-foot", which are lobe-shaped osmoregulatory or respiratory outgrowths on the external limb bases of various Eucrustaceans (SEM below). This structure is derived from setae and many of the Eucrustacea ("true" Crustacea) still bear at least one epipod on some of its appendages. Some authors even believe insect wings are derived from epipods (here and here).

"Fragment of trunk (holotype; developmental stage 6); lateral view"-Zhang et al. 2007; ep=epipod

This all comes home to pushing back the time of arthropod origins as stated by Zhang et al.
Its stratigraphical position provides substantial support to the proposition that the main cladogenic event that gave rise to the Arthropoda was before the Cambrian
It was found in the Orsten layer, which is rich in Phosphatocopina, the sister group to Eucrustacea characterized by a phosphotatized cuticle. By using Lyell's principle of Uniformitarianism, we can assume that since the characters represented in Y. dianensis are present in the Lower Cambrian, Its ancestors possessed some or all of its characters. Hence arthropod origins are likely pre-Cambrian.


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