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Palaeontologists have been developing some highly sophisticated tools for analysing fossil specimens. Of particular interest are techniques that probe the details of soft tissue preservation. In the research considered here, the 30 mm specimen was found at the Chengjiang lagerstatte locality in southwest China. It had large, claw-like appendages on its head and many jointed legs. It is assigned to the arthropods and thought to be a probable extinct chelicerate. It is referred to as one of the megacherian (meaning "great hand") species with the genus name Alalcomenaeus. To analyse the soft tissues, a 3-D model of the specimen was produced using a CT-scanner and, at the same time, an X-ray microscope documented the distribution of selected chemical elements. In particular, iron has been found to map out the nervous system of the animal. The findings are spectacular.
This close-up of the head region of the Alalcomenaeus fossil specimen includes superimposed colors of a microscopy technique that reveal the distribution of chemical elements in the fossil. Copper shows up as blue, iron as magenta and the CT scans as green. The coincidence of iron and CT denote nervous system. The creature boasted two pairs of eyes (ball-shaped structures at the top). (Image: N. Strausfeld/University of Arizona, source here)
Living arthropods are classified into two major groups: the chelicerata (which include spiders, scorpions, mites and horseshoe crabs), and the mandibulata (which includes insects, crustaceans and millipedes). The new research locates the "great appendage arthropods" unambiguously in the chelicerata.
"We now know that the megacheirans had central nervous systems very similar to today's horseshoe crabs and scorpions," senior author Nicholas Strausfeld, a professor in the department of neuroscience at the University of Arizona in Tucson, said in a statement. (source here)
"The animal's brain consists of three fused ganglia, and blends into more ganglia that extend down the length of the animal's body. It has four eyes, each of which is served by just one optic lobe. That's a chelicerate layout - in mandibulates, the body ganglia would be more distinct and separated by long nerves, and there would be two to four optic lobes per eye." (Ed Yong, source here)
Evolutionary biologists are very fond of the terms "stem" and "crown" to describe where fossils fit into the tree of life. A "stem" fossil is supposed to have more transitional characters and the "crown" specimens are essentially modern. These great appendage arthropods were previously interpreted as "stem group chelicerates" or "stem-group arthropods". However, the neural architecture is essentially modern.
"Professor Strausfeld said: 'Greg plugged these characteristics into a computer-based cladistic analysis to ask, "where does this fossil appear in a relational tree?" 'Our fossil of Alalcomenaeus came out with the modern chelicerates." (source here)
This research is actually the second study of its type. The first was concerned with the neural structure of a mandibulate organism, published last year.
"Xiaoya Ma and Nicholas Strausfeld described the brain of a 520-million-year old animal called Fuxianhuia protensa. It consisted of three clusters of nerves (ganglia) that had fused together. (Source here)
"Nerves from the second ganglion reached into the creature's antennae, while nerves from the third one led into a pair of claws. Each of the animal's eyes was served by three further nerve bundles, known as optic lobes. "In other words, the specimen had a brain like that of a modern crustacean," says Strausfeld. Fuxianhuia was clearly an early relative of modern crabs, lobsters and shrimp - a relationship that was unclear from its body alone. [. . .] Fuxianhuia exemplified the mandibulate pattern." (Source here)
"No one expected such an advanced brain would have evolved so early in the history of multicellular animals," said Strausfeld, a Regents Professor in the UA department of neuroscience. [. . .] "In principle, Fuxianhuia's is a very modern brain in an ancient animal." (Source here)
So we have the interesting situation that both groups of arthropods have neural patterns that are essentially modern. Whereas morphology is used to argue the case for "stem" and "crown" organisms, complex specified information is more readily discerned in soft tissues: genetic systems, developmental gene regulatory networks and neural patterns. Research over the past decade has indicated that numerous core genes are common to a large number of phyla with the implication being that they preceded the Cambrian Explosion of animal phyla (here and here). The same applies to developmental pathways. The research considered in this blog shows that the neural pathways for arthropods must be dated at least to the Early Cambrian. This provides an additional dimension to the Cambrian Explosion phenomenon - in that an extraordinary accumulation of biological information is already in place by the Middle Cambrian.
A final point to note relates to the way iron mineralisation has preserved neural patterns, for reasons that are not altogether clear. It appears that nerve cells are more resistant to decay than other soft tissues:
"Strausfeld says that the nerves of invertebrates are dense and rich in fats, which makes them water-repellent. This, combined with their hard external skeleton, might have slowed the process of decay long enough for them to fossilise. Indeed, in an earlier study, Strausfeld's team buried marine worms in mud and put them under high pressure to simulate the start of fossilisation - and their nerves lasted while their muscles decayed." (source here)
Whilst there is a rationale for nerve tissue surviving longer than other tissues, it remains to be discovered why nerve tissue should take up iron mineralisation. Is there something about the nerve cells that attracts iron? All are agreed that soft tissue preservation requires rapid fossilisation, but we may find that nerve cells have to react with iron in solution very soon after burial if we are to preserve the neural ground pattern. Whatever the answers to such questions, soft tissue preservation is a clear pointer to fossilisation in a geological instant. Geologists (and others) are recognising that the principle of uniformitarianism promoted by Charles Lyell is a poor tool for interpreting the rock record, for many geological processes are abrupt and not gradual. Nevertheless, Lyell's legacy lives on in Darwinian evolution, where gradualism reigns supreme. Most Darwinians continue to stumble over the Cambrian Explosion and continue to predict that an extensive Precambrian fossil record will emerge with continued research. However, the fossil record that we do have, especially probed in detail as outlined in this blog, means that scenarios of "climbing Mount Improbable" savours more of a vivid imagination unconstrained by evidence. To understand why uniformitarianism is inappropriate for grappling with the Cambrian Explosion, go here. For a paradigm-shifting treatment of this whole issue, go here.
Chelicerate neural ground pattern in a Cambrian great appendage arthropod
Gengo Tanaka, Xianguang Hou, Xiaoya Ma, Gregory D. Edgecombe & Nicholas J. Strausfeld
Nature, 502, 364-367 (17 October 2013) | doi:10.1038/nature12520 (pdf here)
Preservation of neural tissue in early Cambrian arthropods has recently been demonstrated, to a degree that segmental structures of the head can be associated with individual brain neuromeres. This association provides novel data for addressing long-standing controversies about the segmental identities of specialized head appendages in fossil taxa. Here we document neuroanatomy in the head and trunk of a "great appendage" arthropod, Alalcomenaeus sp., from the Chengjiang biota, southwest China, providing the most complete neuroanatomical profile known from a Cambrian animal. Micro-computed tomography reveals a configuration of one optic neuropil separate from a protocerebrum contiguous with four head ganglia, succeeded by eight contiguous ganglia in an eleven-segment trunk. Arrangements of optic neuropils, the brain and ganglia correspond most closely to the nervous system of Chelicerata of all extant arthropods, supporting the assignment of "great appendage" arthropods to the chelicerate total group. The position of the deutocerebral neuromere aligns with the insertion of the great appendage, indicating its deutocerebral innervation and corroborating a homology between the "great appendage" and chelicera indicated by morphological similarities. Alalcomenaeus and Fuxianhuia protensa demonstrate that the two main configurations of the brain observed in modern arthropods, those of Chelicerata and Mandibulata, respectively, had evolved by the early Cambrian.
Chow, D. Ancient "Mega-Clawed" Creature Had Brain Like a Spider's, Scientific American (21 October 2013)