The holotype and only known specimen of the eurypterid (Chelicerata: Eurypterida) Grossopterus overathi (Gross, 1933) from the Early Devonian (Siegennian) of Overath, north-west Germany is redescribed. Based on comparisons with other eurypterid taxa we interpret G. overathi as having a well-preserved type B genital appendage, which exhibits two apomorphic character states: (1) a furca fused into a single, spatulate plate and (2) marginal serrations near the distal end of the appendage. Grossopterus is assigned to the family Hughmilleriidae, but its carapace resembles that of the larger Slimoniu acuminatu (Salter, 1856) (Slimoniidae). A provisional phylogenetic analysis resolves Grossopterus as the sister group of (Slimonia + Pterygotoidea).The large pterygotids, in particular, have been suggested as significant predators on early vertebrates and hypotheses about eurypterid-fish co-evolution are reviewed, in particular Romer's proposal that dermal annour in fish evolved in response to eurypterid predation.Key words: Devonian, north-west Germany, Overath, Eurypterida, Grossopterus, Genital appendage, fish, co-evolution. ZusammenfassungDer Holotyp -das einzig bekannte Exemplar -des Eurypteriden (Chelicerata: Eurypterida) Grossopterus overathi (Gross, 1933)
The holotype and only known specimen of the eurypterid (Chelicerata: Eurypterida) Grossopterus overathi (Gross, 1933) from the Early Devonian (Siegennian) of Overath, north-west Germany is redescribed. Based on comparisons with other eurypterid taxa we interpret G. overathi as having a well-preserved type B genital appendage, which exhibits two apomorphic character states: (1) a furca fused into a single, spatulate plate and (2) marginal serrations near the distal end of the appendage. Grossopterus is assigned to the family Hughmilleriidae, but its carapace resembles that of the larger Slimoniu acuminatu (Salter, 1856) (Slimoniidae). A provisional phylogenetic analysis resolves Grossopterus as the sister group of (Slimonia + Pterygotoidea).The large pterygotids, in particular, have been suggested as significant predators on early vertebrates and hypotheses about eurypterid-fish co-evolution are reviewed, in particular Romer's proposal that dermal annour in fish evolved in response to eurypterid predation. ZusammenfassungDer Holotypdas einzig bekannte Exemplardes Eurypteriden (Chelicerata: Eurypterida) Grossopterus overathi (Gross, 1933) aus dem friihen Devon (Siegennian) von Overath i m nordwestlichen Deutschland wird wieder beschrieben. Basierend auf Vergleichen mit anderen Eurypteriden-Taxa interpretieren wir G. overathi als Trager eines Typ B-Anhanges, wobei zwei apomorphe Merkmalszustande vorliegen: (1) die Furca ist zu einer einzigen, spatelformigen Platte verschmolzen, und (2) der Seitenrand nahe des distalen Endes des Genitalanhanges ist gezahnt. Grossopterus wird zur Familie der Hughmilleriidae gestellt, aber der prosomale Dorsalschild ahnelt der groljeren Slimoniu acuminata (Salter, 1856) (Slimoniidae). Eine vorlaufige phylogenetische Analyse zeigt Grossopterus als Schwesteruppe von (Slimonia + Pterygotoidea) auf. Besonders die groljen Pterygotiden wurden als signifikante Pradatoren von friihen Vertebraten gehandelt, und Hypothesen zur Eurypteriden-Fisch Koevolution werden iiberpriift, speziell Romers Vorschlag, dass die dermale Panzerung von Fischen als Antwort auf die Pradation durch Eurypteriden evolvierte.
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