7-3 Morphometric analyses suggest that regional size differences in high-latitude Devonian brachiopods are unaffected by environment type (Paraná and Cape basins)

Session: Undergraduate Research, Part I (Posters)

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During the Devonian, the reduction of general size in the brachiopod genus Orbiculoidea is often attributed to transgressive-regressive cycles and widespread anoxia. These cycles are associated with a series of biotic events, that is, periods of abiotic changes linked to major biological impacts. In polar marine ecosystems, the most abrupt change is expressed in the Paraná Basin (Brazil) by the Middle Devonian Kačák Event (Eifelian-Givetian), where rapid dwarfism (~90% decrease in general body size) is observed in many surviving brachiopod species and clades (Lilliput Effect). From this same period, an adjacent basin, the Cape Basin (South Africa), has shown no signs of these effects occurring, which may be interpreted as geographic isolation between the neighboring basins. Two other transgressive-regressive anoxic pulses within the Paraná Basin are recorded before the Middle Devonian. The Choteč Event (late Emsian) is one of them. This event reflects widespread hypoxic or anoxic conditions sparked by transgressive-regressive cycles, which are associated with body size reduction. However, the impacts of the Choteč Event on the morphology of marine groups are still unknown. To address this gap, we used the abundant brachiopod genus Orbiculoidea to identify the impacts of the Choteč Event in the two polar marine basins. For that, we (1) measured and analyzed the shape variation of Brazilian and South African orbiculoids using a morphometric analysis, and (2) quantified environmental influences on morphology through a Principal Component Analysis (PCA). Data were collected from literature and museum specimens. Landmarks were constructed for each side of the shell (dorsal/ventral) using the StereoMorph package in R. From this dataset, PCA results indicate that size variation in ventral and dorsal valves has non-significant correlations with environment type, species, and region. Subsequent T-test analyses indicate that Orbiculoidea baini individuals from the Paraná Basin expressed a significant difference in general size (~37% smaller) compared to O. baini from the Cape Basin. Hence, although there is a size difference among basins, our results demonstrate that these variations cannot be attributed to environmental or taxonomic differences. Future research on Orbiculoidea morphometrics should incorporate more detailed morphological features rather than strictly general size (length and width), which may prove more effective for distinguishing populations than size alone.

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