Allometric ontogenetic changes in two Late Jurassic gastrodorid hermit crabs (Crustacea, Decapoda, Anomura) from central Europe

Extinct anomurans are known mostly by their carapaces. Regions in the fossil carapace are well delimited by grooves and, by comparison with extant hermit crabs, they can be referred to the underlying internal organs. In this paper we examine the allometric effects in ontogeny of two Jurassic hermit crabs of an extinct genus Gastrodorus: G. neuhausensis von Meyer, 1864 and G. bzowiensis sp. nov. described herein. Both species occur in Oxfordian and Kimmeridgian strata in southern Poland, but the former species only in southern Germany. Samples of carapaces of both species consisted of carapaces of a wide size range, illustrative of the various ontogenetic stages. A reduced major axis was applied for assessment of allometry. The most pronounced positive allometric effect in both species concerned the posteriormost region of the cephalothoracic shield, which became disproportionally more elongated than the overall carapace length (positive allometry) as overall size increased. Statistically significant differences between the two species concern the cardiac region, which gained in length with size in G. neuhausensis (positive allometry) and became proportionally narrower in the new species (negative allometry). Thus it may be assumed that overall growth rate of the heart region was less in the latter species. Additionally, new terminology of carapace regions previously introduced to pagurids and based on morphology of recent representatives is applied to the gastrodorids.http://zoobank.org/urn:lsid:zoobank.org:pub:E64D2515-BD0D-4A78-83A7-D35C5F9FA3E5

已灭绝的异尾下目（Anomura）类群大多仅通过其头胸甲（carapace）为人所知。化石头胸甲的区域通过沟槽清晰界定，结合现生寄居蟹的对应结构，可将这些区域对应至其内部器官。本文针对已灭绝腹足寄居蟹属（Gastrodorus）下的两种侏罗纪寄居蟹开展个体发育中的异速生长效应研究：分别是1864年由冯·迈耶命名的G. neuhausensis von Meyer, 1864，以及本文新描述的G. bzowiensis sp. nov.。两种化石均发现于波兰南部的牛津阶（Oxfordian）与启莫里阶（Kimmeridgian）地层中，而G. neuhausensis仅见于德国南部。两类物种的头胸甲样本涵盖了宽泛的尺寸范围，完整展现了各阶段的个体发育历程。本研究采用约化主轴法（reduced major axis）评估异速生长关系。两种物种中最为显著的正异速生长（positive allometry）效应均出现在头胸甲的最后侧区域：随着整体体型增大，该区域相较头胸甲总长度出现不成比例的伸长。两物种间存在统计学意义显著差异的区域为心区（cardiac region）：G. neuhausensis的心区长度随体型增大呈正异速生长，而新种的心区则随体型增长呈比例变窄（negative allometry）。据此可推测，新种的心区整体生长速率低于前者。此外，本研究将此前基于现生类群形态学提出、适用于寄居蟹科（Paguridae）的头胸甲区域新术语体系，应用至腹足寄居蟹类（gastrodorids）。http://zoobank.org/urn:lsid:zoobank.org:pub:E64D2515-BD0D-4A78-83A7-D35C5F9FA3E5