Fossil giants and surviving dwarfs. Arthropleurida and Pselaphognatha (Atelocerata, Diplopoda) : characters, phylogenetic relationships and construction
Kraus, O.; Brauckmann, C.
Verhandlungen des Naturwissenschaftlichen Vereins in Hamburg 40: 5-50
The late Palaezoic Arthropleurida were the most gigantic terrestrial arthropods ever known. According to the fossil record, they became extinct in the Lower Permian, but arguments are presented now to support the assumption that very small relatives survived: the Pselaphognatha (Eocene to Recent). They were not yet recognized as representatives of the Glade. The time distance between the assumed extinction of the Arthropleurida and the occurrence of the earliest known species of the surviving dwarfs has a dimension of roughly 255 million years. In order to ascertain this mode of a continuous existence of the Glade, characters of the Palaeozioc arthropod taxa Arthropleuridae, Eoarthropleuridae and a recently established "new order" Microdecemplicida were analyzed. A similar analysis was carried out by investigating comparable character expressions in the two extant subtaxa of the Diplopoda: Pselaphognatha and Chilognatha. A comparison of details drawn from palaeontology and neontology permitted the conclusion that Arthropleuridae and Eoarthropleuridae probably form a monophyletic unit called Arthropleurida. Arthropleurida share various characters with Pselaphognatha, e.g., (1) `trilobitoid' dorsal plates composed of syntergites and lateral pleural folds, (2) a thin, relatively weak and unmineralized cuticula, (3) irregular diplopody, (4) a "Y skeleton" in the prolateral side of the walking legs, combined with the formation of a `rosette plate', (5) special median sternites in combination with three additional lateral plates. Accordingly, Arthropleurida and Pselaphognatha together form a taxon called Penicillata (s. lat.). Whereas almost all Arthropleurida were true giants with body lengths up to appr. 2 m, their surviving relatives, the Pselaphognatha, scarcely exceed 3 mm. - In contrast to this, the Microdecemplicida remain enigmatic to some extent. Various characters described or reconstructed by the original authors are not comparable to any other extant representative of the Diplopoda (e.g., head capsule, constructional morphology of trunk diplosomites, presence of terminal filaments). However, there are some similarities with Oxyuropoda ligioides (Arthropoda inc. sed). Despite this, it is assumed that they could be related to the Chilognatha among diplopods. This proposal is primarily based on the presence of a three-toothed clypeolabrum and the presumed strict diplopody. They differ from what is now called Penicillata by the absence of paraterga plus pleurites and the presence of only one ventral plate on both sides of the unpaired sternite. Hence, a highranked taxon Arthropleuridea (sensu SHEAR) seems to be unjustified as a monophyletic unit; it is dissolved into the Penicillata (Arthropeurida + Pselaphognatha) and a possible separate unit formed by Microdecemplicida + Chilognatha. - The enormous size, especially of the Arthropleuridae, is discussed under functional and adaptational aspects and in correlation with semi-aquatic habitats. This includes a proposed explanation of the fact that the animals were not equipped with a stable and hence heavy exoskeleton. It is suggested that, as in caterpillars, stabilization was achieved by means of musculature and antagonistic hydraulics of the hemolymph of the body cavity. This could have caused the functional exclusion of stigmata and tracheae and their replacement by K plates; they obviously functioned as respiratory structures by plastron-breathing.