Pancrustacean Evolution Illuminated by Taxon-Rich Genomic-Scale Data Sets with an Expanded Remipede Sampling

The relationships of crustaceans and hexapods (Pancrustacea) have been much discussed and partially elucidated following the emergence of phylogenomic data sets. However, major uncertainties still remain regarding the position of iconic taxa such as Branchiopoda, Copepoda, Remipedia, and Cephalocarida, and the sister group relationship of hexapods. We assembled the most taxon-rich phylogenomic pancrustacean data set to date and analyzed it using a variety of methodological approaches. We prioritized low levels of missing data and found that some clades were consistently recovered independently of the analytical approach used. These include, for example, Oligostraca and Altocrustacea. Substantial support was also found for Allotriocarida, with Remipedia as the sister of Hexapoda (i.e., Labiocarida), and Branchiopoda as the sister of Labiocarida, a clade that we name Athalassocarida (=”nonmarine shrimps”). Within Allotriocarida, Cephalocarida was found as the sister of Athalassocarida. Finally, moderate support was found for Hexanauplia (Copepoda as sister to Thecostraca) in alliance with Malacostraca. Mapping key crustacean tagmosis patterns and developmental characters across the revised phylogeny suggests that the ancestral pancrustacean was relatively short-bodied, with extreme body elongation and anamorphic development emerging later in pancrustacean evolution.

Anamorphic development and extended parental care in a 520 million-year-old stem-group euarthropod from China

Extended parental care (XPC) is a complex reproductive strategy in which progenitors actively look after their offspring up to – or beyond – the first juvenile stage in order to maximize their fitness. Although the euarthropod fossil record has produced several examples of brood-care, the appearance of XPC within this phylum remains poorly constrained given the scarcity of developmental data for Palaeozoic stem-group representatives that would link juvenile and adult forms in an ontogenetic sequence. Here, we describe the post-embryonic growth of Fuxianhuia protensa from the early Cambrian Chengjiang Lagerstätte, and show parental care in this stem-group euarthropod. We recognize fifteen distinct ontogenetic stages based on the number and shape of the trunk tergites, and their allocation between the morphologically distinct thorax and abdomen. Our data demonstrate anamorphic post-embryonic development in F. protensa, in which tergites were sequentially added from a posterior growth zone. A life assemblage consisting of a sexually mature F. protensa adult alongside four ontogenetically coeval juveniles, constitutes the oldest occurrence of XPC in the panarthropod fossil record. These findings provide the most phylogenetically basal evidence of anamorphosis in the evolutionary history of total-group Euarthropoda, and reveal a complex post-embryonic reproductive ecology for its early representatives.

Anamorphic development of Apfelbeckia insculpta (L. Koch, 1867) (Diplopoda: Callipodida: Schizopetalidae)

An overview of the anamorphic development of Apfelbeckia insculpta is provided. As in other myriapods and arthropods, this period of the life cycle includes different stages that are separated by molts. Based on an earlier description of post-embryogenesis of A. insculpta and on our data, we describe ten stadia that occur after juveniles of our focal species hatch from the egg. Each molt is accompanied by the addition of podous and apodous pleurotergites, leg-pairs and ocelli. Thus, the numbers of these structures can be used as reliable criteria for the separation of post-embryonic stadia in A. insculpta. Adulthood is reached through teloanamorphosis, i.e., with the ninth and last molt individuals become sexually mature and achieve adulthood. Sexes can be distinguished from stadium VIII onward.