The abrupt appearance of Cambrian life forms in the Cambrian Period of Earth history continues to provide us with spectacular evidence of sophistication. New research on fossils recovered from petroleum exploration drill cores assigned to the Deadwood Formation of western Canada documents “a cryptic but significant diversity of Cambrian crustaceans”. Previously, palaeontologists have had hints of these animals from the nonmineralised remains of minute organisms (<2mm). The new finds are of disarticulated body parts that are unambiguously crustacean, representing branchiopods, copepods and ostracods. They are part of an assemblage known as SCFs (small carbonaceous fossils). They show many signs of modernity.
“The fresh taphonomic perspective of SCFs provides the only direct evidence for sophisticated particle-handling in larger-bodied Cambrian arthropods. This characteristically crustacean-type ecology at the interface of micro- and macroscopic nutrient cycling has otherwise been loosely inferred from overall body form and the proxy record of phytoplankton diversification. The detailed adaptations described here represent the acme of Cambrian differentiation within appendages, an alternative (and potentially correlative) measure of evolving arthropod complexity to the larger-scale tagmosis that has been the focus of previous studies. In part, the new fossils reinforce a picture of early origination and subsequent conservation in crustacean form and function.”
Fossil branchiopod-type limbs from the Middle Cambrian Deadwood Formation. (for detailed explanation and the PNAS source, go to Figure 3 here)
Branchiopod-type mandibles have been recovered that have overall shapes and ornamentation that are “conspicuously similar to those of branchiopod crustaceans”. There are “striking similarities” with mandibles of “various extant anostracan branchiopods”. These body parts are considered to be “adapted for enhanced food-grinding efficiency”. The inference from the size of these parts is that these organisms had a body size of at least 10-15 mm and formed part of a more complex ecosystem than is normally envisaged for the Cambrian:
“The presence in the first and second morphotypes of a moderately sized posterior tooth and an asymmetric “tooth-groove” system points to an ecology of mixed benthic scraping and suspension feeding, as opposed to more exclusive predation or suspension feeding.”
Some of the most beautifully preserved body parts are the filter plates of branchiopods that are essentially odern in form. Together with other material, the authors have reconstructed the branchiopod crustacean.
It has “a long series of filtering thoracic appendages, [and] an overall body length of at least several millimeters is likely for the more articulated arrays, although a centimetric body size is suggested by isolated filters constructed from substantially larger setae. A mixed scraping/filtering ecology (rather than a wholly planktic mode of life) is suggested by the juxtaposition of filter plates and saw-toothed armatures.”
Copepod-type mandibles have been recovered where there are many “close matches” with living species.
“In particular, the prominent projecting seta is comparable in form and position to the potentially homologous “dorsal seta” (sometimes a pair of setae) found in every major order of nonparasitic copepods [i.e., Calanoida, Cyclopoida, Platycopioida, Misophrioida, Harpacticoida and Mormonilloida.”
Ostracod-type mandibles closely resemble those belonging to both ostracod subgroups – the Myodocopa and Podocopa.
“The complexity and form of the gnathal edge appear to be shared in particular with halocyprid myodocopes, some of which express a similar suite of characters including a raised toothed blade with adjacent long setae, an intermediate region with a hook-shaped spine, and a protruding grinding surface.”
In their discussion, the researchers draw attention to the important evidences of modernity in the fossil material. It is customary to refer to stem-group and crown-group organisms, with the latter representing life forms that have reached a mature phase of evolutionary development.
“[t]he Deadwood fossils risk being assigned to inappropriately derived positions because of their “modern” appearance but disarticulated condition. Therefore, we conservatively assign them to comparatively inclusive clades, identifying crown groups via a synapomorphy shared with a subset of the crown (. . .) Taken together, our results provide unambiguous evidence for a substantial branching by the Late Cambrian of within-crown (pan)crustacean lineages – a largely cryptic component of the Cambrian “explosion”.”
The distinction between stem-group and crown-group forms draws heavily on the presupposition of evolutionary transformation from simpler “primitive” precursors. However, there are many evidences that are not a comfortable fit within this theoretical framework. The Deadwood assemblage is a case in point. All the groups identified demonstrate modernity of form extending back to the Middle Cambrian. Evidence for stem-group characters does not exist. Sophisticated structures are apparent in the first appearance of these organisms.
“In part, the new fossils reinforce a picture of early origination and subsequent conservation in crustacean form and function.”
Interestingly, this research points to an ecological (rather than an evolutionary) theoretical framework for understanding the fossil data. The authors refer to an “unanticipated ecologic turnover”. This is from their concluding paragraph:
“At the same time, however, the small carbonaceous record provides evidence for unanticipated ecologic turnover. In the modern oceans, branchiopods are represented by a just a few species of small, secondarily marine cladocerans; larger forms, comparable in size to those of the Deadwood (up to ~15 mm or more) and Mount Cap (~50 mm), are now entirely nonmarine. Furthermore, modern free-living copepods are almost all much smaller than the ~5- to 10-mm (plus) Deadwood taxon. In the modern world, visual predators – especially teleost fish – drive down body size in planktic freshwater crustacean communities and strongly constrain the complex behaviors and distribution patterns of krill, a group that shares with the Cambrian branchiopods the attributes of centimetric body size, marine habitat, and (by convergence) thoracic filtering. Significantly, the Deadwood and Mount Cap fossils reveal a contrasting pattern of crustacean distribution in the comparatively “unescalated” Cambrian biosphere.”
The scenario, then, is one of adaptation to changing environments, where organisms are affected by environmental factors (including predation). The adaptations do not result in evolutionary novelties, but to changes in size, lifestyles and ability to thrive in waters of different salinity. These changes are not surprising, given the phenomenon of phenotypic plasticity. If some want to describe this as “evolution”, then they should note that they are working with a concept that does not begin to explain the origin of branchiopods, copepods and ostracods.
Exceptionally preserved crustaceans from western Canada reveal a cryptic Cambrian radiation
Thomas H. P. Harvey, Maria I. Velez, and Nicholas J. Butterfield
PNAS, Published online before print January 17, 2012, doi: 10.1073/pnas.1115244109
Abstract: The early history of crustaceans is obscured by strong biases in fossil preservation, but a previously overlooked taphonomic mode yields important complementary insights. Here we describe diverse crustacean appendages of Middle and Late Cambrian age from shallow-marine mudstones of the Deadwood Formation in western Canada. The fossils occur as flattened and fragmentary carbonaceous cuticles but provide a suite of phylogenetic and ecological data by virtue of their detailed preservation. In addition to an unprecedented range of complex, largely articulated filtering limbs, we identify at least four distinct types of mandible. Together, these fossils provide the earliest evidence for crown-group branchiopods and total-group copepods and ostracods, extending the respective ranges of these clades back from the Devonian, Pennsylvanian, and Ordovician. Detailed similarities with living forms demonstrate the early origins and subsequent conservation of various complex food-handling adaptations, including a directional mandibular asymmetry that has persisted through half a billion years of evolution. At the same time, the Deadwood fossils indicate profound secular changes in crustacean ecology in terms of body size and environmental distribution. The earliest radiation of crustaceans is largely cryptic in the fossil record, but “small carbonaceous fossils” reveal organisms of surprisingly modern aspect operating in an unfamiliar biosphere.
See also:
Butterfield, N. J. and Harvey, T. H. P. (2012) Small carbonaceous fossils (SCFs): a new measure of early Paleozoic paleobiology. Geology, 40(1). 71-74.