Although there have been many studies of biological patterns along latitude, less attention has been paid on how latitude might affect the large-scale spatial patterns of ornament among shelled invertebrate organisms. Based on a newly designed set of four quantitative metrics of shell ornaments and the measurement and quantification of shell ornaments from 2654 Changhsingian (Late Permian) brachiopods (representing 702 species) spanning 80° south to 30° north in paleolatitude, this paper analyzes the relationship between the degree of shell ornament development of Changhsingian brachiopods and paleolatitude. For the first time, we report the presence of statistically significant latitudinal gradient patterns in the development of a multitude of brachiopod shell ornament types for the Permian, generally characterized by the tendency of an increased proportion of brachiopods with more complicated and stronger ornament towards the paleotropics. The gradient pattern is manifested not only in the individual types of ornament (e.g., radial ornament, body spines) but also pronounced when all the studied ornament types are integrated and analyzed together. Two scenarios are offered to account for the latitudinal gradient of Changhsingian brachiopod shell ornaments. First, in consistency with most previous similar studies, the latitudinal gradient of brachiopod shell ornament is explained by G. Vermeij's escalation theory, in that shell ornaments are regarded as defence apparatuses against predation, and that shell ornament of Changhsingian brachiopods is interpreted to have become stronger and more elaborate in lower paleolatitudes as an adaptive response to the progressively increased predation pressure towards the paleotropics. Second, the latitudinal gradient pattern of Changhsingian brachiopod shell ornaments could also be explained by the varied calcification rate in the oceans as a function of the latitudinal temperature gradient. The CaCO3 solubility is known to increase in colder, high-latitude areas. This means it is more difficult for marine organisms to extract calcium carbonate to generate their shells in higher latitudes, consequently resulting in less developed ornament for shelled marine benthic organisms living in colder water habitats. This second interpretation potentially complements and extends the G. Vermeij's escalation theory in explaining the observed latitudinal gradients of Changhsingian brachiopod shell ornaments, but the theory itself requires both experimental and empirical testing with respect to living shelled marine invertebrates and their production of shell ornament.
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