Comprehensive analysis and reinterpretation of Cenozoic mesofossils reveals ancient origin of the snapping claw of alpheid shrimps

Abstract

Alpheid snapping shrimps (Decapoda: Caridea: Alpheidae) constitute one of the model groups for inferences aimed at understanding the evolution of complex structural, behavioural, and ecological traits among benthic marine invertebrates. Despite being a super-diverse taxon with a broad geographical distribution, the alpheid fossil record is still poorly known. However, data presented herein show that the strongly calcified fingertips of alpheid snapping claws are not uncommon in the fossil record and should be considered a novel type of mesofossil. The Cenozoic remains analysed here represent a compelling structural match with extant species of Alpheus. Based on the presence of several distinct snapping claw-fingertip morphotypes, the major radiation of Alpheus lineages is estimated to have occurred as early as 18 mya. In addition, the oldest fossil record of alpheids in general can now be confirmed for the Late Oligocene (27–28 mya), thus providing a novel minimum age for the entire group as well as the first reliable calibration point for deep phylogenetic inferences.

Preliminary note: alpheid shrimp (Decapoda, Caridea, Alpheidae) snapping claw fingertips from the early Pliocene of Tuscany (central Italy)

“…The studied specimens share strictly morphological characters and shape with the selected sample reported and illustrated by Hyžný et al (2017), mainly with the morphotypes reported as "triangular with a short hook and convex margin" (Hyžný et al 2017, fig. 3a-c); "trian gular with a long hook and convex margin" (Hyžný et al 2017, fig. 3d-f); and "triangular with a long hook and con cave margin" (Hyžný et al 2017, fig.…”

“…As pointed out by feldMann et al (2019: 283), Oligorostra (fixed finger tips) and Oligosella (movable finger tips), both coming from the same locality, could represent the same species. The shape of the finger tip and the presence of only one or two setal pores on the lateral surfaces exclude the belonging of the studied spec imens to the abovementioned genera having a row of setal pores on the distal margin, alternate in size between small and large in Alpheus and regular in size in Oligosella and with just one large setal pore near the distal end in Oligo rostra (CiaMpaglio & Weaver 2008;Hyžný et al 2017Hyžný et al , 2018feldMann et al 2019). We cannot exclude, however, the possible belonging of the studied strongly calcified claw fingertips to the fossil and extant genus Alpheus, based upon the results reported by Hyžný et al (2017: table 1) that all samples from the Cenozoic (late Oligocene, Mio cene, and Pleistocene) of United States (Alabama), Europe (France, The Netherlands, Austria, Poland, Slovakia, and Czech Republic), Africa (Egypt), and Asia (Japan) have been assigned to Alpheus sensu stricto.…”

“…Based upon morphological and chemical analyses of the fossil record of the Alpheidae by Hyžný et al (2017Hyžný et al ( , 2018, the studied specimens are herein interpreted as belonging to the strongly calcified claw fingertips of major chela of some representatives of alpheid snapping shrimps. According to Hyžný et al (2017Hyžný et al ( , 2018 the stud ied claw fingertips exhibit the "triangular morphotype with a short hook", suggesting that they are most likely movable fingertips.…”

“…Indeed, the family includes smallsized species with a poorly mineralized body cuticle and low fossilisation potential, and the only body structure usu ally common in the fossil record is the strong, wellcal cified fingertip. The most typical character of Alpheidae rafinesque, 1815 is the highly specialized major snap ping claw with strong fingertips, having different micro structures and chemical compositions, as for instance in Alpheus fabriCius, 1798 and Synalpheus spenCe bate, 1888 (Hyžný et al 2017) (Fig. 3).…”

“…Recently, Hyžný et al (2017) carried out a rigorous multi disciplinary and exhaustive analysis to support this hypo thesis, revising the specimens previously reported from the worldwide Cenozoic. Based on these new multi disciplinary observations, we report several specimens from the early Pliocene of three localities in Tuscany (cen tral Italy), herein interpreted as alpheid snapping shrimps' fingertips.…”