Derek E. G. Briggs

Disparity as an evolutionary index; a comparison of Cambrian and Recent arthropods

Disparity is a measure of the range or significance of morphology in a given sample of organisms, as opposed to diversity, which is expressed in terms of the number (and sometimes ranking) of taxa. At present there is no agreed definition of disparity, much less any consensus on how to measure it. Two possible categories of metric are considered here, one independent of any hypothesis of relationship (phenetics), the other constrained within an evolutionary framework (cladistics). The Early Cambrian radiation was clearly a period of significant morphologic and taxonomic diversification. However, we question the interpretation of its first generation products as numerous body plans at the highest level. Four phenetic and two cladistic measures have been used to compare disparity among Cambrian arthropods with that in the living fauna. Phenetic methods assessing character-state variability and the amount of morphological attribute space occupied yield similar results for Cambrian and Recent arthropods. Assessments of disparity within a taxonomic framework rely on the identification of particular characters that delineate higher level body plans. This requires a phylogenetic interpretation, a cladistic investigation of hierarchical structure in the data. Both sets of arthropods fall within the same major clades, and within this cladistic framework the amount of character-state evolution in the two groups is comparable. None of these methods identifies markedly greater disparity among the Cambrian compared with the Recent taxa. Although measures of disparity are applied here to a consideration of the Cambrian radiation, the metrics clearly have a much wider potential for estimating macroevolutionary trends independently from existing taxonomic frameworks. Geometric morphometry is ideal for measuring morphological variety at lower taxonomic levels, but it requires the recognition of homologous landmarks in all the forms under comparison, or the identification of entire homologous structures. Conventional phenetics has much wider application as it can operate on data coded as discrete homologous character states (this facility is also a requirement of cladistics), which are a more appropriate basis for comparing disparity in markedly dissimilar forms.

Fossilization of soft-tissue in the laboratory

Some of the most remarkable fossils preserve cellular details of soft tissues. In many of these, the tissues have been replaced by calcium phosphate. This process has been assumed to require elevated concentrations of phosphate in sediment pore waters. In decay experiments modern shrimps became partially mineralized in amorphous calcium phosphate, preserving cellular details of muscle tissue, particularly in a system closed to oxygen. The source for the formation of calcium phosphate was the shrimp itself. Mineralization, which was accompanied by a drop in pH, commenced within 2 weeks and increased in extent for at least 4 to 8 weeks. This mechanism halts the normal loss of detail of soft-tissue morphology before fossilization. Similar closed conditions would prevail where organisms are rapidly overgrown by microbial mats.

Taphonomy of insects in carbonates and amber

Abstract The major taphonomic processes that control insect preservation in carbonate rocks (limestones, travertines and nodules) are biological: insect size and wingspan, degree of decomposition, presence of microbial mats, predation and scavenging; environmental: water surface tension, water temperature, density and salinity, current activity; and diagenetic: authigenic mineralisation, flattening, deformation, carbonisation. The major taphonomic processes that control the preservation of insects in fossil resins (amber and copal) are different, but can be considered under the same headings – biological: presence of resin producers, size and behaviour of insects; environmental: latitude, climate, seasonality, resin viscosity, effects of storms and fires, soil composition; and diagenetic: resin composition, insect dehydration, pressure, carbonisation, thermal maturation, reworking, oxidation. These taphonomic processes are geographically and temporally restricted, and generate biases in the fossil record. Nevertheless, where insects occur they may be abundant and very diverse. Taphonomic processes may impact on phylogenetic and palaeobiogeographic studies, in determining the timing of the origin and extinction of insect groups, and in identifying radiations and major extinctions. Taphonomic studies are an essential prerequisite to the reconstruction of fossil insect assemblages, to interpreting the sedimentary and environmental conditions where insects lived and died, and to the investigation of interactions between insects and other organisms.

Ordovician faunas of Burgess Shale type

The renowned soft-bodied faunas of the Cambrian period, which include the Burgess Shale, disappear from the fossil record in the late Middle Cambrian, after which the Palaeozoic fauna dominates. The disappearance of faunas of Burgess Shale type curtails the stratigraphic record of a number of iconic Cambrian taxa. One possible explanation for this loss is a major extinction, but more probably it reflects the absence of preservation of similar soft-bodied faunas in later periods. Here we report the discovery of numerous diverse soft-bodied assemblages in the Lower and Upper Fezouata Formations (Lower Ordovician) of Morocco, which include a range of remarkable stem-group morphologies normally considered characteristic of the Cambrian. It is clear that biotas of Burgess Shale type persisted after the Cambrian and are preserved where suitable facies occur. The Fezouata biota provides a link between the Burgess Shale communities and the early stages of the Great Ordovician Biodiversification Event.

TUZOIA: MORPHOLOGY AND LIFESTYLE OF A LARGE BIVALVED ARTHROPOD OF THE CAMBRIAN SEAS

Abstract For almost 30 years, paleontologists have analyzed evolutionary sequences in terms of simple null models, most commonly random walks. Despite this long history, there has been little discussion of how model parameters may be estimated from real paleontological data. In this paper, I outline a likelihood-based framework for fitting and comparing models of phyletic evolution. Because of its usefulness and historical importance, I focus on a general form of the random walk model. The long-term dynamics of this model depend on just two parameters: the mean (μstep) and variance (σ2step) of the distribution of evolutionary transitions (or “steps”). The value of μstep determines the directionality of a sequence, and σ2step governs its volatility. Simulations show that these two parameters can be inferred reliably from paleontological data regardless of how completely the evolving lineage is sampled. In addition to random walk models, suitable modification of the likelihood function permits consideration of a wide range of alternative evolutionary models. Candidate evolutionary models may be compared on equal footing using information statistics such as the Akaike Information Criterion (AIC). Two extensions to this method are developed: modeling stasis as an evolutionary mode, and assessing the homogeneity of dynamics across multiple evolutionary sequences. Within this framework, I reanalyze two well-known published data sets: tooth measurements from the Eocene mammal Cantius, and shell shape in the planktonic foraminifera Contusotruncana. These analyses support previous interpretations about evolutionary mode in size and shape variables in Cantius, and confirm the significantly directional nature of shell shape evolution in Contusotruncana. In addition, this model-fitting approach leads to a further insight about the geographic structure of evolutionary change in this foraminiferan lineage.

Controls on the formation of authigenic minerals in association with decaying organic matter: an experimental approach

Carcasses of the shrimp Crangon crangon were incubated in a marine medium under oxic conditions at 15°C which was inoculated with a consortium of sulfate-reducing, sulfide-oxidizing, and fermentative bacteria. These standard conditions were varied by adding sediment, omitting sulfate, adding glucose, omitting the inoculum, adding phosphate, and enhancing the buffer capacity. The chemical gradients generated by decay were monitored over a period of 29 days with O2, pH- and sulfide-microelectrodes. In most of the experiments oxygen was depleted, pH decreased and sulfide accumulated around the carcass within a week, creating steep chemical gradients, and decay was predominantly anaerobic. By 29 days maximum change in O2 concentration was from around 200 to 0 μM, in pH from 7.5 to 6.2, and in sulphide concentration from 0 to 5.6 mM. Although weight loss and general decay were least when only indigenous bacteria were present, only CaCO3 crystal bundles formed and there was no soft tissue preservation. In contrast, where decay and weight loss were more extensive anaerobic sulphate reduction was intense, pH decreased markedly, and some muscle tissue was replicated in CaPO4. The pH close to the decaying carcass seemed to determine whether CaCO3 or CaPO4 formed. Paradoxically, the exceptional preservation of soft-tissues in fossils requires elevated rather than restricted microbial activity as this leads to anaerobically driven authigenic mineral formation.

Plumage Color Patterns of an Extinct Dinosaur

Dinosaur Plumage Coloration and appearance provide important behavioral and evolutionary information in animals. However, for the most part, we do not know the coloration of fossil terrestrial animals. Li et al. (p. 1369, published online 4 February) have reconstructed the appearance of a theropod dinosaur by mapping features of its well-preserved feathers and comparing them with modern samples from birds. Feather color is partly determined by melanosome density and shape, and this information is preserved in a recently discovered fossil from China. The dinosaur was gray with white limbs and had a reddish crest and a speckled face. Comparison of melanosome shape and density between fossil feathers and modern ones reveals the appearance and color of a theropod. For as long as dinosaurs have been known to exist, there has been speculation about their appearance. Fossil feathers can preserve the morphology of color-imparting melanosomes, which allow color patterns in feathered dinosaurs to be reconstructed. Here, we have mapped feather color patterns in a Late Jurassic basal paravian theropod dinosaur. Quantitative comparisons with melanosome shape and density in extant feathers indicate that the body was gray and dark and the face had rufous speckles. The crown was rufous, and the long limb feathers were white with distal black spangles. The evolution of melanin-based within-feather pigmentation patterns may coincide with that of elongate pennaceous feathers in the common ancestor of Maniraptora, before active powered flight. Feathers may thus have played a role in sexual selection or other communication.

The role of the calcium carbonate-calcium phosphate switch in the mineralization of soft-bodied fossils

Authigenic minerals play an important role in the preservation of most soft-bodied fossils. The greatest detail is preserved in apatite (calcium phosphate) but its precipitation is usually inhibited by the high concentrations of HCO3- in aqueous settings. Nonetheless, investigations of soft-bodied biotas have revealed very early authigenic calcite crystal bundles in close association with phosphatized soft-tissues. This demonstrates that the geochemical controls on soft-tissue mineralization are dynamic and act on a very local scale. Direct comparisons with experimental results permit the conditions of fossilization to be inferred.

Phosphatization of soft-tissue in experiments and fossils

Soft-tissues phosphatized in laboratory experiments closely resemble fossil phosphatized soft-tissues, indicating that similar processes were involved. The smaller the aggregations of calcium phosphate particles precipitated the greater the fidelity of morphological preservation. The highest fidelity occurs where the bacteria themselves are not replicated even though precipitation is bacterially induced. While extensive phosphatization of larger carcasses, however, may necessitate the build-up of concentrations in the sediment beforehand, this is not the case for phosphatization of small quantities of soft-tissue. Mineralization of soft-tissue in the laboratory is not ‘instant’ but may take several weeks, or even months if decay is inhibited. The precipitation of associated calcium carbonate is controlled by shifts in pH in response to the decay process.

Exceptional fossil record: Distribution of soft-tissue preservation through the Phanerozoic

Preservation of soft-bodied fossil biotas (Konservat-Lagerstaten) that preserve traces of volatile nonmineralized tissues (readily degraded by bacteria) are not evenly spaced through geologic time. When compared to outcrop area, exceptional faunas appear to be over-represented in the Cambrian and Jurassic. These concentrations in time correspond to particular environments, indicating that controls on the distribution of exceptional faunas may have operated on a global scale. The reduction in the number of exceptional faunas after the Cambrian may reflect the evolution and diversification of deep bio- turbators. Specific conditions favoring stagnation and episodic burial were required to ensure preservation in younger rocks.