Functional morphology of the prosoma of Baltoeurypterus tetragonophthalmus (Fischer) (Chelicerata: Eurypterida)

Abstract

The prosomal morphology of Baltoeurypterus tetragonophthalmus (Fischer) from the Baltic Silurian is redescribed and reconstructed. The first eurypterid labrum and new secondary sexual characters of Baltoeurypterus are described. The radially-arranged coxae of Baltoeurypterus were capable of adduction and abduction for food mastication, but not promotor-remotor movements for locomotion. Joint diagrams are presented for the first time for an extinct arthropod. Promotion and remotion of the limbs occurred about subvertical trochanteral pivots, as in all other chelicerates except xiphosurans. Baltoeurypterus probably walked in a “slow” gait; a method of choosing possible gaits for extinct arthropods is outlined. Swimming in Baltoeurypterus was effected by means of a rowing action of the posterior limb pair, which is provided with complex joints for collapsing the paddle during the recovery stroke. The limb arrangement and joint mechanisms of Baltoeurypterus are intermediate between those of the xiphosurans and the arachnids. It is possible that a sister relationship exists between the eurypterids and some arachnid groups, which would render Merostomata and Arachnida unnatural assemblages.

Abnormal xiphosurids, with possible application to Cambrian trilobites

“…Furthermore, paleontologists have been intrigued by the morphological similarities of L. polyphemus and fossil xiphosurids like Yunnanolimulus luopingensis Zhang et al, 2009 (Guanling Formation, China, Triassic;Hu et al, 2017), Mesolimulus walchi (Desmarest, 1822) (Solnhofen Limestone, Germany, Jurassic; Sekiguchi and Sugita, 1980;Smith and Berkson, 2005) and Limulus darwini Kin and Błaże-jowski, 2014 (Sławno Limestone, Kcynia Formation, Poland, Late Jurassic;Błażejowski, 2015). Finally, L. polyphemus is a useful modern analogue for exploring how extinct gnathobase-bearing arthropods consumed food, including large eurypterids (Selden, 1981;Poschmann et al, 2016), Sidneyia inexpectans Walcott, 1911(Zacaï et al, 2016Bicknell et al, 2018b;Bicknell and Paterson, 2018), and Alacaris mirabilis Yang et al 2018.…”

The Early Devonian eurypterid Grossopterus overathi (Gross, 1933) from Overath, Germany

“…Rawlinson's (2000) conclusions on an evolutionary trend towards cutting pterygotid chelicerae have also been noted above. Although no direct evidence (in the form of gut contents or disjecta) for eurypterids preying on fish is known, putative eurypterid coprolites from the Hagshaw Hills of Scotland contain disarticulated fish fragments (Selden 1984), supporting Romer's general argument.…”

“…Tenninology follows Stprnner (1955) and Tollerton (1989) and also Selden (1981) for the appendages and Braddy & Dunlop (1997) for the genitalia. Schriel (1933) described the stratigraphic and tectonic setting of the Overath fossils.…”

“…In an unpublished study, Rawlinson (2000) investigated the functional morphology of the chelicerae of pterygotid eurypterids, identifying two main functional types: cutters and crushers based on their relative mechanical advantages (see Selden 1984 for details). Cutters were characterised by a mechanical advantage of less than 0.1, the presence of an enlarged principal tooth on the fixed ramus, and the inclined orientation of the proximal tooth on the free ramus.…”

“…Functional morphological studies of pterygotid chelicerae suggest that they were designed for the rapid capture of fast-moving prey (Selden 1984) and fish would be an obvious candidate. Selden also noted how the saw-like teeth on the chelicerae of the pterygotid Acutiramus resemble fish-descaling implements and thus might have been adapted specifically to deal with vertebrates.…”