The fossil record of pycnogonids (sea spiders) is sparse, to say the least. Until recently, there were only 5 named species from 3 localities: one Silurian specimen, some larval instars from the late Cambrian and the rest are Devonian. The fossil record between these Palaeozoic Systems and the present was quiet.
However, a French team has broken the silence by announcing some spectacularly preserved specimens from Jurassic rocks. They point out that the discovery "fill[s] a 400Myr gap of knowledge in the evolutionary history of this enigmatic group of marine arthropods."
So, what can now be said about their "evolutionary history"? The authors write: "They reveal very close morphological and functional (locomotion, feeding) similarities with present-day pycnogonids." Bear in mind that there are over 1100 species of modern pycnogonids, so variation is extensive within this group of animals. The implication is that the fossil record of sea spiders reveals stasis: minor variations about a central theme.
The authors also say that there are "marked differences with all Palaeozoic representatives of the group". However, whilst it has been suggested that a greater diversity of body plans existed among the Palaeozoic pycnogonid taxa, it has not been obvious what is primitive and what is derived. The Devonian forms have suggested this diversity, but the geologically earlier Silurian specimen was placed "near the base of the pycnogonid crown group". This implies that stasis continues back into the Palaeozoic, with various specialized forms emerging as localized variant species.
Here is yet another life form, stretching from the lower Palaeozoic to the present, that displays stasis in its morphology with relatively minor differences over time. Why is it that the dominant feature (stasis) gets so little attention, when "evolutionary history" gets so much?
New sea spiders from the Jurassic La Voulte-sur-Rhone Lagerstatte
Charbonnier, S., Vannier, J. & Riou, B.
Proceedings of the Royal Society B, October 2007, 274, 2555-2561 | doi 10.1098/rspb.2007.0848
Abstract: The diverse and exceptionally well-preserved pycnogonids described herein from the Middle Jurassic La Voulte Lagerstatte fill a 400Myr gap of knowledge in the evolutionary history of this enigmatic group of marine arthropods. They reveal very close morphological and functional (locomotion, feeding) similarities with present-day pycnogonids and, by contrast, marked differences with all Palaeozoic representatives of the group. This suggests a relatively recent, possibly Mesozoic origin for at least three major extant lineages of pycnogonids (Ammotheidae, Colossendeidae, Endeidae). Combined evidence from depositional environment, faunal associates and recent analogues indicate that the La Voulte pycnogonids probably lived in the upper bathyal zone (ca 200m). Our results point to a remarkable morphological and ecological stability of this arthropod group over at least 160Myr and suggest that the colonization of the deep sea by pycnogonids occurred before the Jurassic.
Jaggard, V. Photo in the News: New Sea Spider Fossils Found, National Geographic News, August 16, 2007.
Siveter, D.J. et al., A Silurian sea spider, Nature, 431, 978-980 (21 October 2004)
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