Modularity in the trilobite head consistent with the hypothesized segmental origin of the eyes

Vargas-Parra, Ernesto E; Hopkins, Melanie J.

doi:10.1111/ede.12418

Cited by 3 publications

(2 citation statements)

References 55 publications

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“…Although many of the major metazoan body plans and clades were established at this time, bivalves did not undergo an increase in taxonomic, morphologic and functional diversity until the Ordovician, in contrast with several other similarly ranked groups. This macroevolutionary lag may result from a delayed acquisition of key anatomical novelties or a lower degree of morphological modularity than in the more segmented clades that underwent Cambrian radiations [53,54]. Even in their Ordovician diversification, bivalves failed to show a burst of functional disparity relative to taxonomic diversity, although this was not a non-adaptive radiation either.…”

Section: Discussionmentioning

confidence: 99%

Cambrian origin but no early burst in functional disparity for Class Bivalvia

et al. 2023

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Both the Cambrian explosion, more than half a billion years ago, and its Ordovician aftermath some 35 Myr later, are often framed as episodes of widespread ecological opportunity, but not all clades originating during this interval showed prolific rises in morphological or functional disparity. In a direct analysis of functional disparity, instead of the more commonly used proxy of morphological disparity, we find that ecological functions of Class Bivalvia arose concordantly with and even lagged behind taxonomic diversification, rather than the early-burst pattern expected for clades originating in supposedly open ecological landscapes. Unlike several other clades originating in the Cambrian explosion, the bivalves' belated acquisition of key anatomical novelties imposed a macroevolutionary lag, and even when those novelties evolved in the Early Ordovician, functional disparity never surpassed taxonomic diversity. Beyond this early period of animal evolution, the founding and subsequent diversification of new major clades and their functions might be expected to follow the pattern of the early bivalves—one where interactions between highly dynamic environmental and biotic landscapes and evolutionary contingencies need not promote prolific functional innovation.

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Section: Discussionmentioning

confidence: 99%

Cambrian origin but no early burst in functional disparity for Class Bivalvia

et al. 2023

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“…Still more research used trilobite morphometric data to explore specific morphological or behavioural evolutionary questions. For example, Vargas-Parra and Hopkins (2022) tested patterns of modularity in the trilobite cephalon, and hypotheses relating to the developmental origins of the eye. Drage (2022) used traditional multivariate morphometric analyses to test for an association between body proportions and exoskeleton moulting behaviour across Trilobita, while Drage et al (2023) tested the same hypothesis on an intraspecific scale, using a large dataset of Estaingia bilobata Pocock, 1964.…”

Section: Introductionmentioning

confidence: 99%

Distinct causes underlie double-peaked trilobite morphological disparity

Drage,

Pates

2024

Preprint

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Trilobite cephalic morphology impacted the autecology of individuals, and is critical for high- and low-level taxonomic assignments. Disparity in trilobite cephalon shape varied through time and was integral to the occupation of a diversity of ecological niches. To fully appreciate trilobite cephalic evolution, we must understand how this disparity varies, and what factors control cephalon morphometry. We explore the disparity of trilobite cephala through the Palaeozoic, and analyse the associations between cephalic morphometry and taxonomic assignment and geological Period, using a dataset of 983 2D trilobite cephalon outlines. Elliptical Fourier transformation visualised as a Principal Components Analysis suggests significant differences in morphospace occupation for order and Period groups, and comparisons of disparity measures also suggest significantly different disparities between the groups. Trilobite cephalic disparity peaks in the Ordovician and Devonian. The Cambrian–Ordovician expansion of morphospace occupation appears a result of radiation to new niches, and thus all trilobite orders were established by the late Ordovician. In comparison, the morphospace expansion from the Silurian to Devonian seems solely a result of within-niche diversification rather than novel niche occupation. However, analyses interrogating the regions of morphospace occupied, including centroid distances, average pairwise shape comparisons and Linear Discriminant Analysis demonstrate that, except for the order Harpida and the Cambrian and Ordovician Periods, order and geological Period could not be robustly predicted for an unknown trilobite. Further, Kmeans clustering analyses suggest the total dataset naturally subdivides into only seven groups that do not correspond with taxonomic orders, though Kmeans clusters do decrease in number through the Palaeozoic, aligning with findings of decreasing disparity.

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Modularity in the trilobite head consistent with the hypothesized segmental origin of the eyes

Vargas-Parra

Hopkins

2022

Evolution and Development

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The trilobite head served multiple functions and was composed of several fused segments. Yet, the underlying organization of the trilobite head, and whether patterns are conserved across trilobites, remains unclear. Modeling the head as being composed of modules, or subunits that vary and thus have the potential to evolve semi-independently can reveal underlying patterns of organization.Hypotheses of modular organization based on the comparative developmental biology of arthropods were evaluated using geometric morphometrics. Twodimensional (semi)landmark datasets collected from the cranidia of two Ordovician trilobite species, Calyptaulax annulata (Phacopida) and Cloacaspis senilis (Olenida sensu Adrain, 2011) were analyzed. The degree and pattern of modularity were assessed using the covariance ratio (CR), which compares the covariation within putative modules to the covariation between them, and the fit of different models was compared using an effect size measure derived from the CR. When treating the eyes as a distinct module, the best modular hypothesis identified for C. annulata

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Modularity in the trilobite head consistent with the hypothesized segmental origin of the eyes

Cited by 3 publications

References 55 publications

Cambrian origin but no early burst in functional disparity for Class Bivalvia

Cambrian origin but no early burst in functional disparity for Class Bivalvia

Distinct causes underlie double-peaked trilobite morphological disparity

Modularity in the trilobite head consistent with the hypothesized segmental origin of the eyes

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