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Throughout their long history, trilobites occupied various ecological niches, colonizing a wide variety of marine environments. However, the paleoecology of this group is mostly based on shelf–slope environments and less is known about their distribution in marginal environments. To understand how trilobite communities respond to a deltaic influence, we studied changes in the taxonomic composition and structure of a diverse and well-known Lower Ordovician olenid-dominated fauna from the Argentine Cordillera Oriental along a delta–marine gradient. Cluster analysis revealed two distinct associations, and ordination analysis revealed a clear biotic gradient within each. The ecological structure and diversity trends of both associations follow a predictable response to a depth-related gradient. Impoverished communities with a highly nested structure characterize the lower offshore, whereas rich and even communities occur in the upper offshore. The trend towards higher diversity and greater taxonomic turnover in shallower environments corresponds to greater habitat heterogeneity. Towards the other extreme, only the ubiquitous genus Jujuyaspis was a successful colonizer in deltaic settings. This marked contrast with the more diverse and abundant assemblages of fully marine deposits indicates stressful physiological conditions in marginal-marine environments, where alternating and contrasting normal-marine to brackish-water conditions and high input of siliciclastic material were among the key factors controlling the distribution of these early trilobite communities.
Stratigraphic changes in the clustering of first or last taxon occurrences are a joint expression of evolutionary, ecological, taphonomic, and sedimentological processes. Sedimentation rates control the degree of sedimentary dilution and condensation and thus alter the time contained in a given thickness of sediment. However, it remains poorly explored quantitatively how distinct the stratigraphic patterns in the first and last occurrences can be under different deposition models with a constant thickness of accumulated sediment. Here, I present an algorithm that translates ecological or evolutionary signals between time and stratigraphic height. It is implemented for R Software as the package DAIME and complemented by tools to quantify the uncertainties associated with the construction of deposition models. By modeling the stratigraphic expression of the K/Pg extinction and an earlier extinction pulse potentially linked to Deccan volcanism on Seymour Island under varying sedimentation rates, I show that (1) clustering of last occurrences ∼ 250 kyr prior to the K/Pg boundary can be equally explained by a stronger earlier extinction pulse or prolonged intervals with reduced sediment accumulation rate, but (2) when the temporal variability in sedimentation rate is known, the most plausible extinction dynamics can still be identified. The approach is applicable for any type of information transported as a part of the sedimentary record (e.g., fossils or trace elements) or data derived from it (e.g., isotope ratios and rates of morphological evolution).
Tracks attributable to small ornithischian dinosaurs (thyreophorans and cerapodans) are generally rare in comparison with those representing large individuals. Here we report a presumed stegosaur track (ichnogenus Deltapodus) only 5.7 cm long originating from the Lower Cretaceous Tugulu Group of Xinjiang Province, China, co-occurring with the tracks of larger individuals. This track is only 15% as long as the type of Deltapodus curriei from the same locality. This is the smallest convincing example of a Deltapodus currently known. Reports of purported diminutive stegosaur tracks from the Jurassic of North America have been refuted. A review of well-known ornithischian track ichnogenera reveals that small tracks, less than 11.0–12.0 cm are rare, with only one previous report of a Deltapodus only 8.0 cm long. Most other reported tracks of these ichnogenera represent large individuals with footprint lengths mostly in the range of 30–50 cm. The scarcity of small ornithischian tracks contrasts with the relative abundance of small theropod tracks. The reasons for this paucity of small ornithischian tracks may be due to paleobiological (ontogenetic or paleoecological) or non-paleobiological (preservational) factors.
Actualistic studies are important for evaluating the fidelity of fossil assemblages in representing the living community. Poor live-dead (LD) fidelity in molluscan assemblages may result from transport-induced mixing. Large-scale mixing is more common in siliciclastic settings with a narrow shelf, high sedimentation rate, and those that are frequented by episodically high-energy events. Chandipur-on-sea, on the east coast of India has an optimal setting to promote such conditions. By studying the LD fidelity and modeling size-frequency distribution (SFD) of the fauna, we attempted to evaluate the contribution of “out-of-habitat” versus “within-habitat” mixing in developing the molluscan death assemblage. The correlation between the composition of live (LA) and death assemblages (DA) was insufficient; unlike LAs, the DAs do not show environmental partitioning in ordination space. A numerical simulation of the shell size frequency distribution (SFD) for DAs from LAs was compared with the observed SFD of the DAs. The results of this simulation indicate that DAs are not likely to be a product of within-habitat mixing. DAs probably received considerable input via regional transport, facilitated by frequent tropical cyclones affecting the coast of Odisha. Chandipur receives a large proportion of cyclones originating above 15°N, which causes a high degree of lateral transport and shell mixing between 15° to 21°N, explained by the high compositional similarity of species within this latitudinal extent. Our study highlights the significance of out-of-habitat transport in shaping the regional distribution of marine fossil assemblages, especially in storm dominated siliciclastic shallow-marine settings.
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