Ontogenetic development captured in amber - the first record of aquatic representatives of Isopoda in Cretaceous amber from Myanmar

Mario Schädel,Joachim T. Haug,Matúš Hyžný

doi:10.1590/2358-2936e2021003

Mario Schädel, Joachim T. Haug + Show 1 more

Open Access

https://doi.org/10.1590/2358-2936e2021003

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Abstract

Two fossils from Burmese amber are the subject of this study. The specimens differ in size; yet, they appear to be conspecific because of the profound morphological similarity. The fossils are interpreted as representatives of Isopoda, more precisely of the group Cymothoida, due to the presence of a triangular basipod of the uropod. Cymothoida comprises parasitic forms of Isopoda as well as many other types of feeding-habits. The morphology in the studied fossils suggests that they are not representatives of any of the parasitic ingroups of Cymothoida. Since there are no other findings of Isopoda from the Cretaceous with the same morphological features, the fossils at hand are described as a new species - Electrolana madelineae sp. nov. The smaller specimen lacks well-developed walking appendages on trunk segment seven; it can thus be interpreted as a manca stage (immature) individual. The systematic affinity and the functional morphology of the herein described fossils, as well as three seed shrimps (Ostracoda) in close proximity to one of the specimens, and the presence of pyrite in the amber piece points towards an aquatic lifestyle and a preservation in moist conditions. In addition, we review the fossil record of immature forms of Isopoda.

Highlights

General backgroundThe majority of representatives of Isopoda are marine, with a high diversity of body shapes and fulfilling various ecological functions (Wägele, 1989; Brusca and Wilson, 1991; Brandt and Poore, 2003; Poore and Bruce, 2012)
Since there are no other findings of Isopoda from the Cretaceous with the same morphological features, the fossils at hand are described as a new species – Electrolana madelineae sp. nov
There are morphological differences between the two studied fossil specimens: 1) The antennae are proportionally longer, more slender and consist of a larger number of f lagellum elements in the larger specimen (Fig. 3A vs. Fig. 4C). 2) The trunk appendage 7 is only well developed in the larger specimen (Fig. 3A–C vs. Fig. 4D). 3) The posterior margin of the pleotelson is more rounded and less triangular in shape in the larger specimen (Fig. 2A vs. Fig. 5A). 4) The distal ends of the uropods are more acute and less rounded in the larger specimen (Fig. 3F vs. Fig. 4A)

Summary

Introduction

The majority of representatives of Isopoda are marine, with a high diversity of body shapes and fulfilling various ecological functions (Wägele, 1989; Brusca and Wilson, 1991; Brandt and Poore, 2003; Poore and Bruce, 2012). Oniscidea is the only lineage within Isopoda that successfully managed to establish a full terrestrial lifestyle, with more than 4,000 species (Brusca et al, 2001; Schmalfuss, 2003). The oldest fossil record of the group Isopoda in general reaches back into the Middle Pennsylvanian (Late Carboniferous, about 300 million years old) of Illinois (Hesslerella shermani Schram, 1970 from Mazon Creek). Cretaceous amber from Myanmar (Cenomanian, about 99 million years old; Shi et al, 2012) includes the oldest record of terrestrial representatives of Isopoda. The diversification of the terrestrial lifestyle must have occurred earlier, as the presence of several oniscidean lineages in Burmese amber suggests (Broly et al, 2015; Poinar, 2018; Ross, 2019; Yu et al, 2019)

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