The golden orb-weaver spider features in newly reported research and provides an exciting insight into past ecosystems. Today, these animals adorn tropical rainforests, with giant females of Nephila maculate (legs spanning up to 20 cm), and small males (just a few centimetres across). However, the fossil record of the Nephilidae family is meagre. The earliest example of the genus Nephila comes from the Eocene (considered to be about 34 Ma) and the earliest example of the family Nephilidae is a male from the Cretaceous (considered to be 130 Ma). The newly reported fossil golden orb-weaver spider is a giant female with a leg span of about 15 cm.
"Here, we report the largest known fossil spider, Nephila jurassica sp. nov., from Middle Jurassic (approx. 165 Ma) strata of Daohugou, Inner Mongolia, China. The new species extends the fossil record of the family by approximately 35 Ma and of the genus Nephila by approximately 130 Ma, making it the longest ranging spider genus known."
Nephila jurassica (Credit: Royal Society Biology Letters, P. Selden et al. Source here)
Since all known male fossil nephilids are of normal small size, the large size of the female indicates that sexual dimorphism characterised the Middle Jurassic population (although, as the authors say, confirmation of this awaits the discovery of a male). Of particular interest are some of the fine details preserved in the tuffaceous sedimentary source rock. These include the pedipalps, spinnerets, setal brushes and trichobothria.
"You see not just the hairs on the legs but little things like the trichobothria which are very, very fine. They're used to detect air vibrations. There's a very distinct group of them and they're a very distinct size which is typical of this genus, Nephila," Professor Selden explained.
So this particular living fossil exhibits stasis at the genus level and raises again the issue of what can be learned from the phenomenon of stasis. A previous blog expressed some frustration at Neodarwinian evolutionists who file stasis in a box that says: no environmental change, no selection pressures, no evolution. The problem with this is that so many potentially interesting questions are never asked - and the result is an impoverished science. However, there are evolutionary biologists who think differently, and it is worth considering what an alternative perspective on stasis might look like.
For over a decade, Eric Davidson has been championing the concept of developmental gene regulatory networks (dGRNs) which control ontogeny of the body plan. More than most biologists, he is aware of the significance of different paradigms and how they affect the way we approach the phenomenon of stasis since the Cambrian Explosion. He introduces his latest paper in this way:
"Never in the modern history of evolutionary bioscience have such essentially different ideas about how to understand evolution of the animal body plan been simultaneously current. Of the many different aspects of evolution, we are here to be concerned with how the developmental mechanisms generating the body plan architectures recognized in Linnaen systematics at the level of phylum and class evolve, and how these mechanisms have been maintained, often since the Cambrian or Ordovician. Ideas about the nature of the underlying evolutionary mechanisms, and what to do to study them, generally associate with one of several paradigmatic views."
Davidson considers that the neo-Darwinian paradigm has failed to deliver on its promises and has pursued unjustified assumptions that are in tension with the real world.
"[I]ts fundamental concepts are largely irrelevant to the process by which the body plan is formed in ontogeny. In addition it gives rise to lethal errors in respect to evolutionary process. Neo-Darwinian evolution is uniformitarian in that it assumes that all process works the same way, so that evolution of enzymes or flower colors can be used as current proxies for study of evolution of the body plan. It erroneously assumes that change in protein coding sequence is the basic cause of change in developmental program; and it erroneously assumes that evolutionary change in body plan morphology occurs by a continuous process. All of these assumptions are basically counterfactual."
The phenomenon of stasis is not a quirk of history (i.e. no drivers for transformation) but rather, it requires a genetic explanation. For that, we need to look beyond neoDarwinism to develop a regulatory systems biology.
"The answers lie in the architecture of dGRNs and the developmental logic they generate at the system level, far from micro-evolutionary mechanism. While adaptive evolutionary variation occurs constantly in modern animals at the periphery of dGRNs, the stability over geological epochs of the developmental properties that define the major attributes of their body plans requires special explanations rooted deep in the structure/function relations of dGRNs."
More, of course, needs to be said. The point I am making here is that if stasis is data, we need to take body plans seriously, and also those distinctives associated with Class, Order and Family levels of taxonomy. Some scholars approach these issues open to the thought that body plans really are planned and that intelligent design is the underpinning concept providing integration and coherence. Others do not advocate intelligent design, but are nevertheless committed to working with body plans, developmental logic and regulatory systems. Regarding the science, there is common ground. More comment on Davidson's paper will appear in a future blog.
A golden orb-weaver spider (Araneae: Nephilidae: Nephila) from the Middle Jurassic of China
Paul A. Selden, ChungKun Shih and Dong Ren
Biology Letters, October 23 2011, 7:775-778 | doi:10.1098/rsbl.2011.0228 [Open access]
Abstract: Nephila are large, conspicuous weavers of orb webs composed of golden silk, in tropical and subtropical regions. Nephilids have a sparse fossil record, the oldest described hitherto being Cretaraneus vilaltae from the Cretaceous of Spain. Five species from Neogene Dominican amber and one from the Eocene of Florissant, CO, USA, have been referred to the extant genus Nephila. Here, we report the largest known fossil spider, Nephila jurassica sp. nov., from Middle Jurassic (approx. 165 Ma) strata of Daohugou, Inner Mongolia, China. The new species extends the fossil record of the family by approximately 35 Ma and of the genus Nephila by approximately 130 Ma, making it the longest ranging spider genus known. Nephilidae originated somewhere on Pangaea, possibly the North China block, followed by dispersal almost worldwide before the break-up of the supercontinent later in the Mesozoic. The find suggests that the palaeoclimate was warm and humid at this time. This giant fossil orb-weaver provides evidence of predation on medium to large insects, well known from the Daohugou beds, and would have played an important role in the evolution of these insects.
Evolutionary bioscience as regulatory systems biology
Eric H. Davidson
Developmental Biology, 2011, in press | doi:10.1016/j.ydbio.2011.02.004
Abstract: At present several entirely different explanatory approaches compete to illuminate the mechanisms by which animal body plans have evolved. Their respective relevance is briefly considered here in the light of modern knowledge of genomes and the regulatory processes by which development is controlled. Just as development is a system property of the regulatory genome, causal explanation of evolutionary change in developmental process must be considered at a system level. Here I enumerate some mechanistic consequences that follow from the conclusion that evolution of the body plan has occurred by alteration of the structure of developmental gene regulatory networks. The hierarchy and multiple additional design features of these networks act to produce Boolean regulatory state specification functions at upstream phases of development of the body plan. These are created by the logic outputs of network subcircuits, and in modern animals these outputs are impervious to continuous adaptive variation unlike genes operating more peripherally in the network.
Fossilised spider 'biggest on record' By Jonathan Amos, BBC News (20 April 2011)
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