First African thylacocephalans from the Famennian of Morocco and their role in Late Devonian food webs

Abstract

thylacocephalans are enigmatic arthropods with an erratic palaeozoic and Mesozoic fossil record. in many of the few localities where they occur, they are quite abundant. this also holds true for the famennian thylacocephalan Layer in the Maider (eastern Anti-Atlas of Morocco), a small epicontinental basin hosting some strata with taphonomic properties of a conservation deposit yielding exceptionally preserved gnathostomes and non-vertebrates. in a thin argillaceous interval in the earliest middle famennian, thylacocephalans occur in such great numbers that they became eponyms of this unit. therein, we discovered a new taxon of thylacocephalans, Concavicaris submarinus sp. nov., which represent the oldest records of thylacocephalans from Africa. in the ct-imagery, the holotype of Concavicaris submarinus sp. nov. revealed anatomical details including its eyes, appendages and other soft parts. Sedimentary facies and faunal composition of the thylacocephalan Layer suggest that these animals populated the water column above the low-oxygen sea floor. Thus, thylacocephalans likely represented an important component of the diet of chondrichthyans and placoderms, which are quite common as well. The abundance of thylacocephalans in other conservation deposits like the Cleveland Shale (USA) and the Gogo formation (Australia) underline their pivotal role in Late Devonian pelagic food webs.

New Thylacocephala (Crustacea) assemblage from the Spathian (Lower Triassic) of Majiashan (Chaohu, Anhui Province, South China)

“…Microcaris has been considered a bottom dweller because its terraced ornamentation appears to be related to an endobenthic way of life (Briggs and Rolfe, 1983; Tintori et al, 1986). This is in contrast to a more recent interpretation that regards smooth thylacocephalans as nektonic organisms living near the water surface (Jobbins et al, 2020). However, considering that these narrow organisms could generate very low lift and had rather small swimming appendages (Jobbins et al, 2020), it is probable that thylacocephalans could not swim well enough to live far from the sea bottom.…”

“…This is in contrast to a more recent interpretation that regards smooth thylacocephalans as nektonic organisms living near the water surface (Jobbins et al, 2020). However, considering that these narrow organisms could generate very low lift and had rather small swimming appendages (Jobbins et al, 2020), it is probable that thylacocephalans could not swim well enough to live far from the sea bottom. Tintori et al (1986) pointed out that the depositional environment of thylacocephalans was different from their living environment (at that time, thylacocephalans were considered mainly epibenthic) as they were usually preserved in well-laminated, anoxic rocks.…”

“…However, this could also be a result of a biological process (such as predation) because thylacocephalan fragments are sometimes discovered in coprolites (Affer and Teruzzi, 1999). A recent study suggests that thylacocephalans could have been an important food source for large predators such as chondrichthyans and placoderms during the Late Devonian (Jobbins et al, 2020). Therefore, their huge population could also have served as the food source of fishes and ichthyosaurs during the Early Triassic.…”

A symmoriiform from the Late Devonian of Morocco demonstrates a derived jaw function in ancient chondrichthyans

“…For these reasons, we suggest that Ferromirum likely provides a glimpse of more general functional conditions in early sharks. Notably, in the Maïder Basin, Ferromirum is one of several chondrichthyans occurring within sediments rich in thylacocephalan crustaceans 23,46 (Fig. 7).…”

Upper Ordovician Thylacocephala (Euarthropoda, Eucrustacea) from Bohemia indicate early ecological differentiation

“…2018; Jobbins et al . 2020), using the paddle‐like appendages of the trunk for propulsion, while more robust, heavy, squat forms with strong raptorial appendages are believed to have been benthic or nekto‐benthic ambush predators and/or scavengers (Pinna et al . 1985; Rolfe 1985; Secretan 1985; Schram 1990; Fischer 2003; Vannier et al .…”