Body-size scaling of metabolic rate in the trilobite Eldredgeops rana

We infer the body-size scaling slope of metabolic rate in a trilobite by applying a cell-size model that has been proposed to explain metabolic scaling in living organisms. This application is especially tractable in fossil arthropods with well-preserved compound eyes because the number and size of eye facets appear to be useful proxies for the relative number and size of cells in the body. As a case study, we examined the ontogenetic scaling of facet size and number in a ∼390-Myr-old local assemblage of the trilobite Eldredgeops rana, which has well-preserved compound eyes and a wide body-size range. Growth in total eye lens area resulted from increases in both facet area and number in relatively small (presumably young) specimens, but only from increases in facet area in large (presumably more mature) specimens. These results suggest that early growth in E. rana involved both cell multiplication and enlargement, whereas later growth involved only cell enlargement. If the cell-size mod...本研究借助已被提出用于解释现生生物代谢缩放规律的细胞大小模型（cell-size model），推断三叶虫的代谢率随体型变化的缩放斜率。对于保存有完好复眼的节肢动物化石而言，该模型的应用尤为可行，因为复眼小眼面（eye facets）的数量与大小可作为体内细胞相对数量与大小的有效替代指标。作为案例研究，我们针对一具距今约3.9亿年的爱尔德里奇三叶虫（Eldredgeops rana）当地化石组合，展开了小眼面大小与数量的个体发育缩放（ontogenetic scaling）研究；该样本不仅保存有完好的复眼，且涵盖了较宽的体型范围。体型相对较小（推测为幼年个体）的样本中，复眼总晶状体面积的增长源于小眼面面积与数量的同时增加；而体型较大（推测为更成熟个体）的样本中，该增长仅来自小眼面面积的提升。上述结果表明，爱尔德里奇三叶虫的早期生长同时伴随细胞增殖（cell multiplication）与细胞增大（cell enlargement），而后期生长仅依赖细胞增大。若该细胞大小模型……