Hot on the heels of an arthropod with complex compound eyes from the Emu Bay Shale (in Australia) has come an even more dramatic discovery! The same lagerstatten has yielded some fossil eyes, attributed to Anomalocaris, that show just how much 'modernity' can be traced back to the Cambrian fauna.
"The number of ommatidia in the Anomalocaris eyes would almost certainly have greatly exceeded the count based on the exposed surface of the eye alone. [. . .] the total count could be substantially greater than the observed 16,000+ lenses. If this is indeed the case, few living arthropods have as many ommatidia, and these eyes would certainly have functioned with a high degree of acuity. [. . .] Throughout the geological history of Arthropoda, compound eyes have rarely exceeded this size; very large Siluro-Devonian pterygotid eurypterids and some Jurassic thylacocephalans represent some of the rare examples with eyes larger than those of Anomalocaris."
Anomalocaris and its compound eyes (source here)
Inevitably, this raises questions about the way the evolution of compound eyes has been presented in the past. Plots of ommatidia density vs geological time have been used to defend gradualism. However, most of the data was related to trilobites, which are dominated by benthic forms in the Lower Cambrian, diversifying to include nektonic species in the Late Cambrian and subsequent Periods. Now that the eyes of nektonic animals are being discovered and documented, the picture looks rather different. A whole range of eye designs were present during the Early-Middle Cambrian.
"The eyes of Anomalocaris expand the known diversity of visual adaptations in the early Cambrian: low-resolution organs with >100 ommatidia (eodiscoid trilobites), higher-resolution eyes with a distinct bright zone that might have functioned in low light, and very large eyes with a uniformly dense visual field adapted to bright environments."
It should always be remembered that eye complexity means nothing without neuronal processing capability. Light signals have to be transmitted, analysed and decoded as visual images. Modernity in these aspects must be inferred also.
"The very large size of anomalocaridid compound eyes and the visual acuity inferred from the elevated lensnumber and low interommatidial angles suggest that processing of visual information would have required the optic neuropils and brain to be of comparable complexity to crown-group (that is, modern) arthropods. In the crown group, two optic neuropils are reconstructed in the most recent commonancestor, transmitting to a protocerebrum with a median unpaired neuropil, the central body."
Discoveries like this create major challenges for advocates of Darwinian gradualism. Again and again, the source of novelty is pushed earlier into the undocumented past. Gradualism as a working concept is sustained, not by data, but by inference - but the gaps available for gradualist change are ever shrinking!
"Dense, hexagonal packing of ommatidia in compound eyes has been demonstrated to have been unequivocally present in Schinderhannes bartelsi, a Devonian species resolved as the immediate sister group to the arthropod crown group. The eyes of Schinderhannes resemble those of Anomalocaris in being large, stalked, having an ovoid outline of the visual surface, and a highly elevated number of lenses. The finding that Anomalocaris, resolved more basally than Schinderhannes in the arthropod stem group, possesses the same kind of ommatidial packing as in Schinderhannes and crown-group arthropods pushes the origin of compound eyes further down the arthropod stem group. As such, compound eyes evolved earlier than the origin of a hardened tergal exoskeleton and biramous trunk limbs (the latter characters being present in Schinderhannes but not anomalocaridids). We infer that the stalked eyes of all Radiodonta (that is, anomalocaridids) are arthropod-type compound eyes."
The same general analysis applies to all the evolutionary stories that are developed around the fossil record. As in so many research papers, the evolutionary comments are tacked on and are little more than table-talk. The last sentence of the abstract and the last sentence of the paper make the same point: the sophisticated visual system of Anomalocaris would have the consequence of enhancing selection pressures and "probably helped to accelerate the escalatory 'arms race' that began" with the Cambrian fauna.
However, justification of the 'arms race' and the way it is supposed to affect the course of evolution is left to the imagination. As an alternative scenario, consider an ecological perspective. The more we know of Cambrian faunas, the more we are finding evidence of ecosystems adapting to environmental change over time. The changes documented in the fossil record are better explained by reference to ecological concepts combined with the phenomenon of phenotypic plasticity.
Acute vision in the giant Cambrian predatorAnomalocaris and the origin of compound eyes
John R. Paterson, Diego C. Garcia-Bellido, Michael S. Y. Lee, Glenn A. Brock, James B. Jago & Gregory D. Edgecombe
Nature, 480, 237-240 (08 December 2011) | doi:10.1038/nature10689
Until recently, intricate details of the optical design of non-biomineralized arthropod eyes remained elusive in Cambrian Burgess-Shale-type deposits, despite exceptional preservation of soft-part anatomy in such Konservat-Lagerstatten. The structure and development of ommatidia in arthropod compound eyes support a single origin some time before the latest common ancestor of crown-group arthropods, but the appearance of compound eyes in the arthropod stem group has been poorly constrained in the absence of adequate fossils. Here we report 2-3-cm paired eyes from the early Cambrian (approximately 515 million years old) Emu Bay Shale of South Australia, assigned to the Cambrian apex predator Anomalocaris. Their preserved visual surfaces are composed of at least 16,000 hexagonally packed ommatidial lenses (in a single eye), rivalling the most acute compound eyes in modern arthropods. The specimens show two distinct taphonomic modes, preserved as iron oxide (after pyrite) and calcium phosphate, demonstrating that disparate styles of early diagenetic mineralization can replicate the same type of extracellular tissue (that is, cuticle) within a single Burgess-Shale-type deposit. These fossils also provide compelling evidence for the arthropod affinities of anomalocaridids, push the origin of compound eyes deeper down the arthropod stem lineage, and indicate that the compound eye evolved before such features as a hardened exoskeleton. The inferred acuity of the anomalocaridid eye is consistent with other evidence that these animals were highly mobile visual predators in the water column. The existence of large, macrophagous nektonic predators possessing sharp vision - such as Anomalocaris - within the early Cambrian ecosystem probably helped to accelerate the escalatory 'arms race' that began over half a billion years ago
Marshall, M. First top predator was giant shrimp with amazing eyes, New Scientist (7 December 2011)
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