Erin E. Maxwell

The evolution and extinction of the ichthyosaurs from the perspective of quantitative ecospace modelling

The role of niche specialization and narrowing in the evolution and extinction of the ichthyosaurs has been widely discussed in the literature. However, previous studies have concentrated on a qualitative discussion of these variables only. Here, we use the recently developed approach of quantitative ecospace modelling to provide a high-resolution quantitative examination of the changes in dietary and ecological niche experienced by the ichthyosaurs throughout their evolution in the Mesozoic. In particular, we demonstrate that despite recent discoveries increasing our understanding of taxonomic diversity among the ichthyosaurs in the Cretaceous, when viewed from the perspective of ecospace modelling, a clear trend of ecological contraction is visible as early as the Middle Jurassic. We suggest that this ecospace redundancy, if carried through to the Late Cretaceous, could have contributed to the extinction of the ichthyosaurs. Additionally, our results suggest a novel model to explain ecospace change, termed the ‘migration model’.

Sawfishes and Other Elasmobranch Assemblages from the Mio-Pliocene of the South Caribbean (Urumaco Sequence, Northwestern Venezuela)

The Urumaco stratigraphic sequence, western Venezuela, preserves a variety of paleoenvironments that include terrestrial, riverine, lacustrine and marine facies. A wide range of fossil vertebrates associated with these facies supports the hypothesis of an estuary in that geographic area connected with a hydrographic system that flowed from western Amazonia up to the Proto-Caribbean Sea during the Miocene. Here the elasmobranch assemblages of the middle Miocene to middle Pliocene section of the Urumaco sequence (Socorro, Urumaco and Codore formations) are described. Based on new findings, we document at least 21 taxa of the Lamniformes, Carcharhiniformes, Myliobatiformes and Rajiformes, and describe a new carcharhiniform species (†Carcharhinus caquetius sp. nov.). Moreover, the Urumaco Formation has a high number of well-preserved fossil Pristis rostra, for which we provide a detailed taxonomic revision, and referral in the context of the global Miocene record of Pristis as well as extant species. Using the habitat preference of the living representatives, we hypothesize that the fossil chondrichthyan assemblages from the Urumaco sequence are evidence for marine shallow waters and estuarine habitats.

First Middle Jurassic record of Saurichthyidae (Actinopterygii)

Saurichthyidae, a speciose group of Early Mesozoic actinopterygian fishes, was previously documented as ranging from the Late Permian to the end of the Early Jurassic. Saurichthyids were most diverse in the Triassic: only two species persisted into the Early Jurassic, restricted to marine sediments in Europe and North America. Here, I report previously undescribed specimens from the Middle Jurassic of the District of Göppingen, Baden-Württemberg, Germany. The new material consists of a partial skull, and a second skull with associated lower jaw, both from lower Aalenian sediments of the marine Opalinuston Formation. Although poor preservation prevents referral to species, these specimens are consistent with the Early Jurassic taxon Saurorhynchus based on the elongate jaws anterior to the orbit, organization of the dentition, and structure and ornamentation of the lateral mandible. These finds extend the stratigraphic range of Saurichthyidae into the Middle Jurassic, eliminating any connection between the extinction of this family and the early Toarcian Oceanic Anoxic Event and supporting minimal faunal change across the Toarcian–Aalenian boundary.KurzfassungDie Saurichthyidae sind eine frühmesozoische Gruppe von Strahlenflossern, deren stratigraphische Reichweite bislang mit Oberperm bis Unterjura angegeben wurde. Ihren höchsten Artenreichtum hatten sie in der Trias, während aus dem Jura nur zwei Arten aus marinen Sedimenten von Europa und Nordamerika bekannt sind. Vorliegender Artikel befasst sich mit bislang unbeschriebenen Exemplaren aus dem Mitteljura des Landkreises Göppingen (Baden-Württemberg, Deutschland). Das neue Material besteht aus einem Teil des Schädels sowie einem zweiten Schädel mit Unterkiefer, beide aus Sedimenten des frühen Aaleniums der marinen Opalinuston-Formation. Auch wenn die schlechte Erhaltung keine Artbestimmung zulässt, entsprechen diese Exemplare dem frühjurassischen Taxonxa0Saurorhynchus. Die Zuordnung basiert auf dem verlängerten Kiefer im präorbitalen Bereich, der Organisation des Gebisses und der Struktur und Ornamentierung des lateralen Unterkieferbereichs. Diese Funde erweitern die stratigraphische Reichweite der Saurichthyidae in den Mitteljura, was einen Zusammenhang zwischen dem Aussterben dieser Familie und dem ozeanischen anoxischen Event des frühen Toarciums widerlegt und auf einen nur geringfügigen Faunenwechsel an der Toarcium–Aalenium-Grenze hindeutet.

Ontogenetic Tooth Reduction in Stenopterygius quadriscissus (Reptilia: Ichthyosauria): Negative Allometry, Changes in Growth Rate, and Early Senescence of the Dental Lamina.

We explore the functional, developmental, and evolutionary processes which are argued to produce tooth reduction in the extinct marine reptile Stenopterygius quadriscissus (Reptilia: Ichthyosauria). We analyze the relationship between mandible growth and tooth size, shape, and count, to establish an ontogenetic trend. The pattern in S. quadriscissus is consistent with hypotheses of tooth size reduction by neutral selection, and this unusual morphology (a functionally edentulous rostrum) was produced by a series of different evolutionary developmental changes that are known for other taxa showing tooth reduction and loss. Specifically, this species evolved functional edentulism by evolutionary changes in the growth allometry of the dentition and by altering growth rates through ontogeny. This observation supports previous hypotheses that S. quadriscissus underwent ontogenetic tooth reduction. Tooth reduction in S. quadriscissus may be caused by unique selective pressures resulting from prey choice and feeding behavior, expanding our current understanding of the mechanisms producing tooth reduction.

First Record of a True Mortichnium Produced by a Fish

ABSTRACT Among the numerous ichnofossils found in the Upper Jurassic Solnhofen-type Nusplingen Fossillagerstätte of southwestern Germany, there is a unique find of a fish associated with a trail, which must be interpreted as a mortichnium. Like similar cases of fatally injured crustaceans from the same locality, this fish mortichnium resulted from an unsuccessful attack on the fish by a predator and does not provide evidence for a hostile environment.

A new species of the deep-bodied actinopterygian Dapedium from the Middle Jurassic (Aalenian) of southwestern Germany

Dapedium is one of the most abundant and diverse genera of ganoid fishes from the Early Jurassic fossil lagerstätte of Europe. In spite of its abundance, however, its timing of extinction is poorly constrained, with the youngest described material being Early Jurassic in age. We describe new diagnostic and relatively complete material of a large species of Dapedium (standard length estimated at 50 cm) from the Middle Jurassic (earliest Aalenian) Opalinuston Formation of Baden-Württemberg, Germany. The Aalenian material represents a distinct species, D. ballei sp. nov., differing from Early Jurassic species in a unique combination of characters pertaining to the shape of the dermal skull elements, pectoral fin position, and scale shape and ornamentation. However, although D. ballei sp. nov. exhibits a unique combination of characters, there are no autapomorphies with which to distinguish it from the Toarcian species of Dapedium. Dapedium ballei represents the geologically youngest species of Dapedium, extending the range of this genus into the Middle Jurassic. The Opalinuston Formation fills an important gap in the marine vertebrate fossil record, and finds from this horizon have the potential to greatly improve our understanding of evolutionary dynamics over this period of faunal transition.

Redescription of The ‘Lost’ Holotype of Suevoleviathan Integer (Bronn, 1844) (Reptilia: Ichthyosauria)

ABSTRACT The genus Suevoleviathan is one of the rarest and most phylogenetically problematic members of the Early Jurassic Posidonienschiefer Formation ichthyosaur fauna. Two named species have been referred to this genus, both from the Toarcian of Baden-Württemberg, Germany. The holotype of the first-described of the two, Suevoleviathan integer, was never properly figured and was considered lost or destroyed by all recent authors. This created uncertainty in the literature regarding the validity of the larger of the two species, Suevoleviathan disinteger. Here, I redescribe and figure the relocated holotype of Suevoleviathan integer and reevaluate characters proposed to separate the two taxa. I find little evidence for two species of Suevoleviathan in the Posidonienschiefer Formation and suggest that Suevoleviathan disinteger (Huene, 1926) is a junior synonym of S. integer (Bronn, 1844). Specimens historically referred to S. disinteger represent ontogenetically older stages of S. integer, as supported by ossification of the proximal forelimb and negative cranial allometry across the size series.