Digestive and appendicular soft-parts, with behavioural implications, in a large Ordovician trilobite from the Fezouata Lagerstätte, Morocco

Abstract

Trilobites were one of the most successful groups of marine arthropods during the Palaeozoic era, yet their soft-part anatomy is only known from a few exceptionally-preserved specimens found in a handful of localities from the Cambrian to the Devonian. This is because, even if the sclerotized appendages were not destroyed during early taphonomic stages, they are often overprinted by the three-dimensional, mineralised exoskeleton. Inferences about the ventral anatomy and behavioural activities of trilobites can also be derived from the ichnological record, which suggests that most Cruziana and Rusophycus trace fossils were possibly produced by the actions of trilobites. Three specimens of the asaphid trilobite Megistaspis (Ekeraspis) hammondi, have been discovered in the Lower Ordovician Fezouata Konservat-Lagerstätte of southern Morocco, preserving appendages and digestive tract. The digestive structures include a crop with digestive caeca, while the appendages display exopodal setae and slight heteropody (cephalic endopods larger and more spinose than thoracic and pygidial ones). The combination of these digestive structures and the heteropody has never been described together among trilobites, and the latter could assist in the understanding of the production of certain comb-like traces of the Cruziana rugosa group, which are extraordinarily abundant on the shallow marine shelves around Gondwana.

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

“…Using these criteria invalidates the recent claim of the presence of a crop in a specimen of Megitaspis ( Ekaraspis ) hammondi from the Fezouata Shale of Morocco (Gutiérrez‐Marco et al . ). Indeed, this putative crop was described as: (1) running medially under the posterior two‐thirds of the glabella; (2) narrowing progressively backwards; and (3) being flanked laterally by digestive glands.…”

The oldest known digestive system consisting of both paired digestive glands and a crop from exceptionally preserved trilobites of the Guanshan Biota (Early Cambrian, China)

“…Gutiérrez-Marco and colleagues [ 27 ] interpreted the combination of a crop and digestive glands in Megistaspis (Ekeraspis) hammondi to be a new “type 3” trilobite digestive system. However, we suspect that at least some previous classifications may have been misled by taphonomic variabililty.…”

“…Even if they are digestive glands, the fact that the specimen represents a very early growth stage (early meraspid) has kept open the possibility that the crop decreased in relative size over ontogeny, in which case the two-type model based on adult morphologies would still be valid [ 7 , 20 , 23 ]. The second is the remains of the digestive system in an adult specimen of the trilobite Megistaspis (Ekeraspis) hammondi from the early Ordovician Fezouata Lagerstätte [ 27 , fig 1b-d]. In this specimen, the putative crop is narrow, taking up only one-quarter of the width of the glabella, similar in width to the alimentary tract spanned by the gut diverticula.…”

A new xandarellid euarthropod from the Cambrian Chengjiang biota, Yunnan Province, China

“…The extensively preserved digestive system of L. deletres potentially offers additional insights into diet and palaeoecology. The overall architecture, with a midline gut producing lateral, unbranched diverticula that are transversely wider towards the front of the body, recalls that of various trilobites (e.g., Chatterton et al 1994;Lerosey-Aubril et al 2012, 2017Gutiérrez-Marco et al 2017). Many extant arthropods with extensive diverticula are fluid feeders (i.e., predators with rich but intermittent intake of food or fluidised food), and the evolution of complex digestive systems has more generally been linked to the rise of predatory and scavenging behaviour (e.g., Vannier et al 2014).…”