A global perspective on Mesozoic marine amniotes

Proceedings of the 5th Triennial Mosasaur Meeting

Sven Sachs, Johan Lindgren & Benjamin P. Kear

The 5th Triennial Mosasaur Meeting took place 16–20 May 2016 at the Museum of Evolution in Uppsala (Sweden). Previous meetings were held in 2004 at Natuurhistorisch Museum Maastricht, Maastricht, The Netherlands, in 2007 at the Sternberg Museum of Natural History, Hays, Kansas, USA, in 2010 at the Muséum national d'Histoire naturelle, Paris, France, and in 2013 at the Shuler Museum of Paleontology, Southern Methodist University, Dallas, Texas, USA.

We, the organisers of the 5th meeting, want to thank all attendees and all contributors of the special volume for being part of this great conference.

Benjamin P. Kear, R. Ewan Fordyce, Norton Hiller & Mikael Siversson

A palaeobiogeographical synthesis of Australasian Mesozoic marine tetrapods

THE LAST 15 years has witnessed a blossoming of research on Australasian Mesozoic marine tetrapod fossils. Much of this work has focused on amniotes, particularly those from the prolific Lower Cretaceous (Aptian–Albian) Lagerstätten of the Eromanga Basin in central and eastern Australia, and Upper Cretaceous (Campanian–Maastrichtian) sequences of the North and South islands of New Zealand. However, rare and less popularized remains have also been found in Lower Triassic–mid-Cretaceous rocks from Australia, New Zealand and the Chatham Islands, and on the tectonically proximal landmasses of New Caledonia and Timor. Currently identified taxa include estuarine–paralic rhytidostean, brachyopid, capitosaurian and trematosaurian temnospondyls from the earliest Triassic (Induan–Olenekian), Middle–Late Triassic (Anisian–Norian) eosauropterygians, and mixosaurian, shastasaurian and euichthyosaurian ichthyosaurians, Early–Middle Jurassic (Sinemurian–Bajocian) ichthyosaurians, together with plesiosauroid and rhomaleosaurid-like plesiosaurians, and diverse Early (Aptian–Albian) through to Late Cretaceous (Campanian–Maastrichtian) elasmosaurid, leptocleidid, polycotylid, probable cryptoclidid and pliosaurid plesiosaurians, as well as ophthalmosaurid ichthyosaurians, sea turtles incorporating protostegids, and mosasaurid squamates. This faunal succession evidences almost continuous occupation of southern high-palaeolatitude seas, and repeated endemic diversifications (including nascent members of some key lineages) amongst emigrant cosmopolitan clades. The primary dispersal routes are likely to have been peri-Gondwanan, with coastal migrations along the western Tethys and polar margins of the Panthalassan Ocean. However, augmentation by increasing continental fragmentation and seaway corridor connectivity probably occurred from the Middle Jurassic to Late Cretaceous. Latest Cretaceous mosasaurid and elasmosaurid taxa also reveal regional affinities with the emergent western Pacific and Weddellian austral bioprovinces. The extreme rarity, or complete absence, of many major groups prevalent elsewhere in Gondwana (e.g., tanystropheids, Triassic sauropterygians, bothremydid marine turtles, thalattosuchians and dyrosaurid crocodylomorphs) is conspicuous, and might be related to stratigraphical/collecting biases, or the predominantly higher-palaeolatitude, cooler-water Mesozoic palaeogeography of the Australasian region. Although the burgeoning record is substantial, much still awaits discovery and adequate documentation; thus Australasia is still one of the most exciting prospects for future insights into the global history of Mesozoic marine tetrapods.

Sven Sachs & Benjamin P. Kear

A rare new Pliensbachian plesiosaurian from the Amaltheenton Formation of Bielefeld in northwestern Germany

We describe a new plesiosaurian from the upper Pliensbachian Amaltheenton Formation of Bielefeld in northwestern Germany. The taxon is based upon an incomplete associated skeleton comprising part of the right mandibular ramus, several teeth, a series of cervical, pectoral, dorsal and caudal vertebrae, as well as ribs, limb girdle elements including a nearly complete right scapula, and various distal limb bones. A unique character state combination serves to distinguish the Amaltheenton Formation remains from other previously documented Early Jurassic plesiosaurians. The most important features are the presence of a longitudinal notch incising the posterior rim of the glenoid fossa and retroarticular process, and a pronounced ventrolateral shelf on the scapula, both of which constitute derived states otherwise shared with Early Cretaceous leptocleidians. However, phylogenetic analysis using a ‘total group’ Plesiosauria data-set that specifically accommodates for Pliensbachian taxa unanimously placed the Amaltheenton Formation plesiosaurian among Early–Middle Jurassic pliosaurids. This discovery is significant because it reveals unexpected homoplasy, but also because it establishes what is only the third formally named plesiosaurian taxon thus far documented from Pliensbachian strata worldwide.

Sven Sachs, Jahn J. Hornung, Jens N. Lallensack & Benjamin P. Kear

First evidence of a large predatory plesiosaurian from the Lower Cretaceous non-marine ‘Wealden facies’ deposits of northwestern Germany

Here, we describe the incomplete mandible of a large-skulled ‘pliosauromorph’ plesiosaurian from the Lower Cretaceous (Berriasian) of northwestern Germany. The fossil derives from limnic–brackish ‘Wealden facies’ deposits of the Deister Formation (Bückeberg Group), and is preserved as a natural mould in fine-grained sandstone. Examination of the original remains, in conjunction with a three-dimensional photogrammetrically digitized ‘cast’, revealed a conspicuous rosette of symphyseal alveoli, which would otherwise typically characterize Early–Middle Jurassic macrophagous plesiosaurians including rhomaleosaurids and the pliosaurid Simolestes. The Deister Formation ‘pliosauromorph’ represents the first record of a large-bodied plesiosaurian macrocarnivore from the ‘Wealden-facies’ strata of Europe, and thus adds a previously unrecognized trophic level of aquatic apex predators to the Early Cretaceous non-marine ecosystems of Europe.

Nathan E. Van Vranken

An overview of ichthyosaurian remains from the Cretaceous of Texas, USA

The clade nominal Ophthalmosauridae encompasses all currently recognized ichthyosaurian taxa from the mid-Cretaceous (Albian–Cenomanian) time interval in North America. These would have inhabited what is today known as the Western Interior Seaway. The remains represent mid to large-sized (up to around 5 m in maximum body length) ichthyosaurians that are mainly based upon isolated teeth and vertebrae, although some partial skeletons have been recovered from both the USA and Canada. Most of these fossils have taxonomic affinities with platypterygiines, such as the ubiquitous ‘form genus’ Platypterygius. This taxon has historically been the default identification for most North American Cretaceous ichthyosaur specimens. However, in comparison with remains from elsewhere, the ichthyosaurian fossils from Texas have been relatively understudied. Here, I review the existing records and present a concise list of all Cretaceous ichthyosaurian material currently held in public museum collections across the state. I have recognized 11 Cretaceous ichthyosaurian occurrences from Texas, including an indeterminate tooth, numerous vertebral centra, and some fragmentary skeletal remains attributable to Platypterygius. Ichthyosaurians therefore demonstrably occupied the Cretaceous epeiric seas of Texas from at least the Albian to middle Cenomanian, a distribution that traverses both the western and eastern coastlines of the Western Interior Seaway during its early phase of southern epicontinental incursion.

Joshua R. Lively

Taxonomy and historical inertia: Clidastes (Squamata: Mosasauridae) as a case study of problematic paleobiological taxonomy

I review the taxonomic history of two problematic mosasaur species, Clidastes liodontus and Clidastes moorevillensis. The genus Clidastes is thought to represent an early radiation of a diverse clade known as the Mosasaurinae. However, most phylogenetic analyses recover the genus as paraphyletic with respect to more highly nested mosasaurines such as Mosasaurus, Prognathodon, and Globidens. The fragmentary holotype of Clidastes liodontus was never figured or fully described, and was destroyed in World War II. Over 20 years after destruction of the holotype, relatively complete specimens were referred to Clidastes liodontus based upon a single, variable character. Another taxon, originally designated Clidastes liodontus moorevillensis in a master’s thesis and then elevated to Clidastes moorevillensis in a separate dissertation, has never been formally described and lacks definitive diagnostic characters that differentiate it from the contemporary concept of Clidastes liodontus. Clidastes provides an excellent example of how historical inertia in taxonomic nomenclature can build over decades and skew our interpretations of the diversity, paleobiology, biogeography, and biostratigraphy of a taxon. Clidastes liodontus is a nomen dubium and Clidastes moorevillensis a nomen nudum. I recommend that both names be abandoned. Removal of those names frees us from a burdensome taxonomy and eliminates cognitive biases that hinder objective understanding and exploration of early-diverging mosasaurines, and is a necessary first step toward a taxonomic revision of the lineage(s) involved.

Nikolay G. Zverkov, Alexander O. Averianov & Evgeny V. Popov

Basicranium of an elasmosaurid plesiosaur from the Campanian of European Russia

The braincase of elasmosaurid plesiosaurs is poorly known. Here, we describe the exceptionally well-preserved elasmosaurid basicranium from the Rybushka Formation (lower Campanian) of Saratov Province, Russia. The material provides new anatomical information and peculiar features: single anterior foramen for the cerebral carotid arteries, anteroposteriorly elongated sella turcica and deep canal on the basioccipital process. This allow us to reconstruct a carotid circulation in plesiosaurs and propose new basicranial features (anteroposteriorly elongated sella turcica and single anterior foramen for the cerebral carotids), which could be potentially synapomorphic for a clade within the Elasmosauridae.

Jahn J. Hornung, Mike Reich & Udo Frerichs

A mosasaur fauna (Squamata: Mosasauridae) from the Campanian (Upper Cretaceous) of Hannover, northern Germany

Isolated teeth and a humerus from the Campanian of Hannover indicate a considerable local diversity of mosasaur taxa. The lower Misburg Formation (lower Campanian of the Lehrte West Syncline) yields Clidastes sp. (Mosasaurinae), Prognathodon sp. (Mosasaurinae), ?Hainosaurus sp. (Tylosaurinae) and an unidentified mosasaurid. It further confirms the presence of the genus Clidastes in northern central Europe and also proves the early Transatlantic distribution of a basal member of Prognathodon during the lower Campanian. ?Hainosaurus sp. is similar to roughly contemporaneous material of Hainosaurus sp. from southern Sweden. The upper Misburg Formation (upper upper Campanian) shows a different taxonomic composition with Tylosaurus sp. (Tylosaurinae), a second indeterminate species of Prognathodon and ?Platecarpus sp. The limited material of Prognathodon sp. shows closest affinities to P. lutugini (Yakovlev) from eastern Europe. The records of Tylosaurus and ?Platecarpus are among the stratigraphically youngest of these taxa. Facies and lithology of the Misburg Formation indicate that the mosasaurs lived in an open marine, mid-sublittoral environment with a water-depth around 70–100 m. In the contemporaneous chalk facies, deposited in somewhat deeper waters, as well as in shallow littoral deposits, mosasaurs are very rare and of lower diversity.

Sven Sachs, Johan Lindgren & Benjamin P. Kear

Reassessment of the Styxosaurus snowii (Williston, 1890) holotype specimen and its implications for elasmosaurid plesiosaurian interrelationships

The holotype (KUVP 1301) of Styxosaurus snowii—one of the earliest described elasmosaurid plesiosaurians—consists of a well-preserved cranium, mandible and articulated sequence of anterior–mid-series cervical vertebrae found in the lowermost Campanian strata of the Smoky Hill Chalk Member in the Niobrara Formation of Kansas, USA. This particular specimen has proven important for recent phylogenies of Elasmosauridae, and is integral to resulting definitions of the subfamily-level clade, Styxosaurinae. Despite this, KUVP 1301 has not been redescribed or figured in detail since its original taxonomic establishment. We, therefore, re-evaluated KUVP 1301 and assessed its phylogenetic implications. Several notable character states are pertinent for diagnosing S. snowii at genus and species level: (1) an anisodont functional dentition comprising enlarged premaxillary and dentary teeth with a pair of maxillary ‘fangs’, and elongate posterior-most dentary teeth that overlap the upper tooth row; (2) a prominent dorsomedian crest extending from the tip of the premaxillary rostrum, and expanding into a low ‘mound-like’ boss between the external bony nasal openings and orbits; (3) a pronounced convex projection on the posterolateral edge of the squamosals; and (4) platycoelous post-axial cervical vertebral centra that are substantially longer than high, and bear both lateral longitudinal ridges and ventral notches. Character state comparisons with the congeneric subfamily specifier Styxosaurus browni suggest that taxonomic distinction is possible, but equivocal. We, therefore, restrict our definition of Styxosaurus to morphologies observable in KUVP 1301. Furthermore, phylogenetic analysis of our first-hand data returns inconsistent elasmosaurid intra-clade relationships, especially with regard to Styxosaurinae. Consequently, we posit that a more targeted reassessment of Elasmosauridae is necessary to resolve both species-level topologies and higher taxonomy within the group.

José P. O’Gorman, Karen M. Panzeri, Marta S. Fernández, Sergio Santillana, Juan J. Moly & Marcelo Reguero

A new elasmosaurid from the upper Maastrichtian López de Bertodano Formation: new data on weddellonectian diversity

Elasmosaurids are one of the most frequently recorded marine reptiles from the Weddellian Province (Patagonia, Western Antarctica and New Zealand). Improvements in our knowledge of elasmosaurid diversity have been problematic because of their conservative postcranial morphology. However, recent studies have helped to improved our understanding of the diversity of this group. Here, a new elasmosaurid specimen from the upper Maatrichtian horizons of the López de Bertodano Formation, Antarctica, MLP 14-I-20-16, is described. MLP 14-I-20-16 is one of the youngest non-aristonectine weddellonectian elasmosaurids from Antarctica. We confirm the coexistence of aristonectine and non-aristonectine elasmosaurids in Antarctica until the end of the Cretaceous. MLP 14-I-20-16 shows distinctive short and broad posterior cervical vertebrae, a feature only shared among the weddellonectian elasmosaurids by the Maastrichtian Morenosaurus stocki, although the same vertebral proportions are also recorded for the giant Cenomanian elasmosaurids Thalassomedon haningtoni. Comparison between MLP 14-I-20-16 and other elasmosaurids from the Maastrichtian of Antarctica indicates that at least two different non-aristonectine elasmosaurids were present in Antarctica during the late Maastrichtian.

John W.M. Jagt, Elena A. Jagt-Yazykova, Hani F. Kaddumi & Johan Lindgren

Ammonite dating of latest Cretaceous mosasaurid reptiles (Squamata, Mosasauroidea) from Jordan—preliminary observations

Newly collected ammonoid material from the uppermost Cretaceous portion of the Muwaqqar Chalk Marl Formation exposed some 30 km southeast of the Qasr Al’Harrana area (east-central Jordan) includes medium-sized baculitids (Baculites ovatus auctorum, non Say), the sphenodiscid Libycoceras acutodorsatus (Noetling) and the pachydiscids Menuites fresvillensis (Seunes) and Pachydiscus (Pachydiscus) dossantosi (Maury). Of the two last named taxa, the former is a good marker species for the upper Maastrichtian, with records from Europe, central Chile, South India, Baluchistan (Pakistan), Australia, Madagascar and South Africa. The latter is known from the United Arab Emirates/Oman border area, from strata of (late) early to early late Maastrichtian age, as well as from more poorly constrained Maastrichtian levels in Brazil and Nigeria. A comparison with ammonoid assemblages from the Maastrichtian type area (southeast Netherlands/northeast Belgium) suggests correlation of the Muwaqqar Chalk Marl Formation with the middle/upper Maastricht Formation (Emael and Nekum members, ca 66.5–66.1 Ma) and the upper part of the coeval Kunrade Formation. However, associated ‘tegulated’ inoceramids of the Tenuipteria argentea group from the Muwaqqar Chalk Marl Formation favour equivalence with a higher level of the Maastrichtian type area, i.e., the Meerssen Member. From the upper Maastricht Formation and the equivalent upper part of the Kunrade Formation, the following mosasaur genera are currently known: Mosasaurus, Prognathodon, Plioplatecarpus and Carinodens. Interestingly, coeval strata of the Muwaqqar Chalk Marl Formation in east-central Jordan have yielded remains of a largely comparable suite comprising Prognathodon, Mosasaurus, Carinodens and an unnamed, highly derived plioplatecarpine.

Jesper Milàn, John W.M. Jagt, Johan Lindgren & Anne S. Schulp

First record of Carinodens (Squamata, Mosasauridae) from the uppermost Maastrichtian of Stevns Klint, Denmark

Here we report on an important addition to the Late Cretaceous fossil record of marine reptiles from Denmark: a tooth crown of the rare durophagous mosasaur Carinodens minalmamar found in the uppermost Maastrichtian strata at the UNESCO World Heritage Site Stevns Klint. The tooth was found within the uppermost few metres of the Maastrichtian chalk placing it within the latest 50.000 years prior to the K/Pg boundary. The new find is a shed crown probably representing a tooth from the 11th to 13th position in the jaw. The tooth represents the northernmost occurrence of the genus Carinodens. Previous mosasaur finds from Denmark have all been from the hypercarnivorous mosasaurids Mosasaurus hoffmannii and Plioplatecarpus sp., thus our specimen adds a new trophic niche exploited by marine tetrapods in the food web of the latest Maastrichtian of Denmark.