FOSSILS IN THE PHYLOGENY OF ISOPOD CRUSTACEANS (ARTHROPODA)

The relationships of fossils to extant taxa is an recurring problem in the phylogeny of the order Isopoda. This speciose and morphologically diverse crustacean order is found in all environments from the deepest abyss to mountaintops, and has a fossil record beginning in the Carboniferous, around 325 my. As such, the suborder's origin appears late in the diversification of the Malacostraca, a major subclass of the Crustacea. To determine the impact and placement of fossils in the phylogeny and classification of the Isopoda, complete or nearly complete fossil species were incorporated into a morphological dataset of extant isopods. The earliest fossil isopod, Hesslerella shermani, is classified in the Phreatoicidea, a Gondwanan freshwater suborder. Because of their early origins and possession of several plesiomorphic features relative to other isopods, phreatoiceans have been placed basally in the isopod cladogram. The current analysis, however, finds that Hesslerella is in the crown-group, which has substantial apomorphic specialisations. Recent molecular and morphological analyses place phreatoiceans more distally within the isopod cladogram, implying that ancestral isopods have arisen much earlier. Their presumptive sister group, the suborder Asellota, one of the dominant crustacean taxa in the deep sea, appears to be more basally derived. A recent Bayesian analysis, using some of these fossils for calibration, dated the origin of the Isopoda from the late Silurian to early Devonian. This evidence argues that the ancestral isopod appeared substantially earlier than indicated by the fossil record. Among the fossil isopod taxa studied, several were placed into extant groups; the Jurassic-Cretaceous Brunnaega is classified among scavengers in the modern family Cirolanidae. Others, such as the Mesozoic cosmopolite Archaeoniscus, have uncertain affinities, although the current analysis places it with extant sphaeromatoid isopods (marine "pill bugs"). The fossils studied did not substantially change the outcome of the morphological analysis, but they have significant implications for the dating of the origins of the Isopoda and its subclades.