Jeff Liston

100-million-year dynasty of giant planktivorous bony fishes in the Mesozoic seas.

From Big Fish to Big Whales Whales are the largest animals today, and many feed on the abundant plankton, particularly diatoms, in the oceans. Whales arose and diversified in the Cenozoic, about 30 to 40 million years ago (see the Perspective by Cavin). Marx and Uhen (p. 993) show that their diversity parallels the diversity of diatoms and changes in ocean temperature. Whether there were large predators of plankton before whales has been enigmatic, because the fossil record during the Mesozoic (245 to 65 million years ago) is sparse. Friedman et al. (p. 990) now show that a group of large fish filled this role for nearly 100 million years in the Mesozoic. Although not as large as whales, these globally distributed fish were still several meters long. Their extinction at the Cretaceous-Paleogene boundary 65.5 million years ago may have cleared the seas for the evolution of whales. The extinction of widespread large plankton-eating fish led to the emergence of whales in the Cenozoic. Large-bodied suspension feeders (planktivores), which include the most massive animals to have ever lived, are conspicuously absent from Mesozoic marine environments. The only clear representatives of this trophic guild in the Mesozoic have been an enigmatic and apparently short-lived Jurassic group of extinct pachycormid fishes. Here, we report several new examples of these giant bony fishes from Asia, Europe, and North America. These fossils provide the first detailed anatomical information on this poorly understood clade and extend its range from the lower Middle Jurassic to the end of the Cretaceous, showing that this group persisted for more than 100 million years. Modern large-bodied, planktivorous vertebrates diversified after the extinction of pachycormids at the Cretaceous-Paleogene boundary, which is consistent with an opportunistic refilling of vacated ecospace.

New Ophthalmosaurid Ichthyosaurs from the European Lower Cretaceous Demonstrate Extensive Ichthyosaur Survival across the Jurassic–Cretaceous Boundary

Background Ichthyosauria is a diverse clade of marine amniotes that spanned most of the Mesozoic. Until recently, most authors interpreted the fossil record as showing that three major extinction events affected this group during its history: one during the latest Triassic, one at the Jurassic–Cretaceous boundary (JCB), and one (resulting in total extinction) at the Cenomanian-Turonian boundary. The JCB was believed to eradicate most of the peculiar morphotypes found in the Late Jurassic, in favor of apparently less specialized forms in the Cretaceous. However, the record of ichthyosaurs from the Berriasian–Barremian interval is extremely limited, and the effects of the end-Jurassic extinction event on ichthyosaurs remains poorly understood. Methodology/Principal Findings Based on new material from the Hauterivian of England and Germany and on abundant material from the Cambridge Greensand Formation, we name a new ophthalmosaurid, Acamptonectes densus gen. et sp. nov. This taxon shares numerous features with Ophthalmosaurus, a genus now restricted to the Callovian–Berriasian interval. Our phylogenetic analysis indicates that Ophthalmosauridae diverged early in its history into two markedly distinct clades, Ophthalmosaurinae and Platypterygiinae, both of which cross the JCB and persist to the late Albian at least. To evaluate the effect of the JCB extinction event on ichthyosaurs, we calculated cladogenesis, extinction, and survival rates for each stage of the Oxfordian–Barremian interval, under different scenarios. The extinction rate during the JCB never surpasses the background extinction rate for the Oxfordian–Barremian interval and the JCB records one of the highest survival rates of the interval. Conclusions/Significance There is currently no evidence that ichthyosaurs were affected by the JCB extinction event, in contrast to many other marine groups. Ophthalmosaurid ichthyosaurs remained diverse from their rapid radiation in the Middle Jurassic to their total extinction at the beginning of the Late Cretaceous.

The oldest known metriorhynchid super-predator: a new genus and species from the Middle Jurassic of England, with implications for serration and mandibular evolution in predacious clades

The Oxford Clay Formation of England has yielded numerous sympatric species of metriorhynchid crocodylomorphs, although disagreement has persisted regarding the number of valid species. For over 140 years teeth reminiscent of the genus Dakosaurus have been known from the Oxford Clay Formation but these have never been properly described and their taxonomy and systematic affinity remain contentious. Furthermore, an enigmatic mandible and associated postcranial skeleton discovered by Alfred Leeds in the Fletton brick pits near Peterborough also remains undescribed. We show that this specimen, and several isolated teeth, represents the oldest known remains of a large-bodied predatory metriorhynchid. This material is described herein and referred to Tyrannoneustes lythrodectikos gen. et sp. nov. This species has a unique occlusal pattern: the dentition was arranged so that the posterior maxillodentary teeth interlock in the same plane and occlude mesiodistally. It is the first described crocodylomorph with microscopic denticles that are not contiguous along the carinae (forming short series of up to 10 denticles) and do not noticeably alter the height of the keel. Additionally, the dorsally expanded and curved posterior region of the mandible ventrally displaced the dentary tooth row relative to the jaw joint facilitating the enlargement of the dentition and increasing optimum gape. Therefore, Tyrannoneustes would have been a large-bodied marine predator that was well-suited to feed on larger prey than other contemporaneous metriorhynchids. A new phylogenetic analysis finds Tyrannoneustes to be the sister taxon to the subclade Geosaurini. An isolated tooth, humerus, and well-preserved mandible suggest a second species of metriorhynchid super-predator may also have lived in the Oxford Clay sea. Finally, we revise the diagnoses and descriptions of the other Oxford Clay metriorhynchid species, providing a guide for differentiating the many contemporaneous taxa from this exceptional fossil assemblage. http://zoobank.org/urn:lsid:zoobank.org:pub:C6222858-EC36-430D-8E28-1F636A6697CA

A basal thunnosaurian from Iraq reveals disparate phylogenetic origins for Cretaceous ichthyosaurs

Cretaceous ichthyosaurs have typically been considered a small, homogeneous assemblage sharing a common Late Jurassic ancestor. Their low diversity and disparity have been interpreted as indicative of a decline leading to their Cenomanian extinction. We describe the first post-Triassic ichthyosaur from the Middle East, Malawania anachronus gen. et sp. nov. from the Early Cretaceous of Iraq, and re-evaluate the evolutionary history of parvipelvian ichthyosaurs via phylogenetic and cladogenesis rate analyses. Malawania represents a basal grade in thunnosaurian evolution that arose during a major Late Triassic radiation event and was previously thought to have gone extinct during the Early Jurassic. Its pectoral morphology appears surprisingly archaic, retaining a forefin architecture similar to that of its Early Jurassic relatives. After the initial latest Triassic radiation of early thunnosaurians, two subsequent large radiations produced lineages with Cretaceous representatives, but the radiation events themselves are pre-Cretaceous. Cretaceous ichthyosaurs therefore include distantly related lineages, with contrasting evolutionary histories, and appear more diverse and disparate than previously supposed.

The first relatively complete exoccipital-opisthotic from the braincase of the Callovian pliosaur, Liopleurodon

A newly recognized left exoccipital-opisthotic of a Callovian pliosaur, derived from the Peterborough or lower Stewarby Members of the Oxford Clay Formation of Peterborough, is described and figured. This isolated bone is tentatively identified as belonging to an ‘adult’ individual of Liopleurodon ferox that is inferred to have had a skull length of 1.26 metres and an overall body length of 6.39 metres.

A re-examination of a Middle Jurassic sauropod limb bone from the Bathonian of the Isle of Skye

Synopsis A limb bone from the Bathonian of the Isle of Skye, previously identified as a possible femur of the sauropod dinosaur Cetiosaurus, is reassessed in light of comparisons with other sauropod material, including the lectotype of Cetiosaurus oxoniensis. Although the closest match for the Skye bone is found to be a left humerus identified as Cetiosaurus oxoniensis by J. Phillips in 1871, the identification of the Skye bone as cetiosaurid is unsafe, given the poor diagnostic power of long bones in most sauropod taxa.

Phyllocarid crustaceans from the Upper Devonian Gogo Formation, Western Australia

Phyllocarids (Crustacea: Malacostraca) from the Upper Devonian (Givetian-Frasnian) Gogo Formation of Western Australia are described and two new species are reconstructed. The mineralized exoskeleton is well preserved in concretions, but the appendages, apart from the mandible, are unknown. Montecaris gogoensis sp. nov. (Echinocarididae) is represented by >600 specimens; a possible second species of Montecaris is represented only by ∼14 specimens of antero-dorsal fragments of the valves, which are highly tuberculate. Schugurocaris wami sp. nov. (Ceratiocarididae) is represented by >300 specimens; a possible second species with a very elongate telson is represented by four specimens. An undetermined species of Dithyrocaris (Dithyrocarididae) is represented by >80 specimens.

Ichthyosaurs from the Jurassic of Skye, Scotland

Fossils of Mesozoic vertebrates are rare in Scotland, particularly specimens of marine reptiles such as plesiosaurs and ichthyosaurs. We describe a suite of ichthyosaur fossils from the Early to Middle Jurassic of Skye, which to our knowledge are the first ichthyosaurs from Scotland to be described and figured in detail. These fossils span approximately 30 million years, from the Sinemurian to the Bathonian, and indicate that ichthyosaurs were a major component of Scottish marine faunas during this time. The specimens include isolated teeth that could represent the most northerly known occurrences of the widespread Sinemurian species Ichthyosaurus communis, a characteristic component of the famous Lyme Regis faunas of England, suggesting that such faunas were also present in Scotland during the Early Jurassic. An associated humerus and vertebrae from Toarcian–Bajocian-aged deposits are named as a new genus and species of basal neoichthyosaurian, Dearcmhara shawcrossi. The taxonomic affinities of this taxon, which comes from a critical but poorly sampled interval in the fossil record, suggest that non-ophthalmosaurid neoichthyosaurians dominated European assemblages around the Early–Middle Jurassic boundary, and were later replaced by ophthalmosaurids, whose radiation likely took place outside Europe. Many of these specimens were collected by amateurs and donated to museum collections, a co-operative relationship essential to the preservation of Scotland’s fossil heritage.

Highly specialized suspension-feeding bony fish Rhinconichthys (Actinopterygii: Pachycormiformes) from the mid-Cretaceous of the United States, England, and Japan

We re-define the Cretaceous bony fish genus Rhinconichthys by re-describing the type species, R. taylori, and defining two new species; R. purgatorensis sp. nov. from the lowermost Carlile Shale (middle Turonian), southeastern Colorado, United States, and R. uyenoi sp. nov. from the Mikasa Formation (Cenomanian), Middle Yezo Group, Hokkaido, Japan. Rhinconichthys purgatoirensis sp. nov. is designated on a newly discovered specimen consisting of a nearly complete skull with pectoral elements. Only known previously by two Cenomanian age specimens from England and Japan, the North American specimen significantly extends the geographic and stratigraphic range of Rhinconichthys. The skull of Rhinconichthys is elongate, including an expansive gill basket, and estimated maximum body length ranges between 2.0 and 2.7 m. Rhinconichthys was likely an obligate suspension-feeder due to its derived cranial morphology, characterized by a remarkably large and elongate hyomandibula. The hyomandibula mechanically acts as a lever to thrust the jaw articulation and hyoid arch both ventrally and anterolaterally during protraction, thus creating a massive buccal space to maximize filtering of planktonic prey items. Cladistic analysis supports a monophyly of suspension-feeding pachycormids including Rhinconichthys, but further resolution within this clade will require more information through additional fossil specimens.

‘Old bones, dry subject’: the dinosaurs and pterosaur collected by Alfred Nicholson Leeds of Peterborough, England

Abstract Alfred Nicholson Leeds, F.G.S., amassed one of the largest collections of fossil vertebrates from a single geological horizon anywhere in the world. The Leeds Collection is world famous for its large marine reptiles, but also includes the remains of a fine range of dinosaurs and a fragmentary pterosaur. The Leeds Collection ornithodirans were almost exclusively recovered from the Peterborough Member of the Oxford Clay Formation, with a single specimen of a sauropod derived from the underlying Kellaways Formation. The Leeds Collection includes the remains of at least 12 individual dinosaurs representing at least eight taxa (with other remains currently generically indeterminate) and a single fragmentary rhamphorhynchid pterosaur. Perhaps most intriguingly of all, in 1898 Alfred Leeds discovered a probable reptile egg, later attributed to a dinosaur. Each dinosaur and the pterosaur from the Leeds Collection is discussed, and, where known, details of the provenance, a brief history of research and pertinent archive material are included to provide the most comprehensive and up-to-date survey of the Leeds Collection ornithodirans to date.