Data from: Gut evolution in early Cambrian trilobites and the origin of predation on infaunal macroinvertebrates: evidence from muscle scars in Mesolenellus.

Trilobites are particularly common Cambrian fossils, but their trophic impact on the rapidly evolving marine ecosystems of that time is difficult to assess, due to uncertainties on how diverse their feeding habits truly were. Gut anatomy might help to constrain inferences on trilobite feeding ecology, but preservation of digestive organs is exceedingly rare. Muscle scars on the glabella, known as ‘frontal auxiliary impressions’ (FAIs), have been interpreted as evidence of the evolution of a pouch-like organ with powerful extrinsic muscles (i.e. a crop) in some trilobites. Here we describe FAIs in Mesolenellus hyperboreus from Cambrian Stage 4 strata of North Greenland, which represents the oldest example of such structures and their first report in the Suborder Olenellina. Mesolenellus FAIs suggest that the crop in trilobites was clearly differentiated from the rest of the digestive tract, and essentially located under a hypertrophied glabellar frontal lobe. Reviews of the digestive anatomy of trilobite sister-taxa and the glabellar morphology of the oldest-known trilobites suggest that the gut of the trilobite ancestor was an essentially simple tract (i.e. no well-differentiated crop) flanked laterally by numerous midgut glands. A crop first evolved in the Cambrian in groups like olenelloids and (later) paradoxidoids. Using ichnological evidence, we hypothesize that the emergence of olenelloids yields evidence for the evolution of predatory inclinations in a group of arthropods originally dominated by surface-deposit-feeders. By allowing the exploitation of a rapidly developing food source, infaunal animals, the diversification of feeding strategies in trilobites might partially explain their unparalleled evolutionary success.

三叶虫（Trilobites）是寒武纪尤为常见的化石类群之一，但由于其真实摄食习性的多样性尚存诸多未知，学界难以评估它们对彼时快速演化的海洋生态系统所产生的营养级影响。肠道解剖结构或可为三叶虫摄食生态学的相关推论提供约束依据，但消化器官的化石保存情况极为罕见。头鞍（glabella）上的肌肉痕被称为“额辅印痕（frontal auxiliary impressions，FAIs）”，此前学界将其解读为部分三叶虫演化出带有强劲外肌的囊状器官——即嗉囊（crop）——的证据。本文记述了产自北格陵兰寒武纪第4阶（Cambrian Stage 4）地层的北极大中球三叶虫（Mesolenellus hyperboreus）的额辅印痕，这是此类结构迄今已知的最古老化石实例，也是油栉虫亚目（Suborder Olenellina）内首次报道此类结构。北极大中球三叶虫的额辅印痕表明，三叶虫的嗉囊与消化道其余部分存在明确分化，且主要位于膨大的头鞍前叶下方。通过对三叶虫姊妹群（sister-taxa）的消化解剖结构与已知最古老三叶虫的头鞍形态开展回顾性研究，结果显示三叶虫祖先的肠道结构相对简单（即无明确分化的嗉囊），肠道两侧分布有大量中肠腺（midgut glands）。嗉囊最早于寒武纪在油栉虫类等类群中演化出现，后续盘形虫类（paradoxidoids）也演化出了该结构。结合遗迹学证据（ichnological evidence），本文提出假说：在原本以表面沉积物食性为主的节肢动物类群中，油栉虫类的出现为掠食倾向（predatory inclinations）的演化提供了佐证。通过开发利用快速涌现的底内动物（infaunal animals）作为食物资源，三叶虫摄食策略的多样化或可部分解释其无与伦比的演化成功。