Body size estimation and evolution in metriorhynchid crocodylomorphs: implications for species diversification and niche partitioning

Metriorhynchids were a peculiar group of fully marine Mesozoic crocodylomorphs, some of which reached large body size and were probably apex predators. The estimation of their total body length in the past has proven problematic. Rigorous size estimation was provided using five complete metriorhynchid specimens, by means of regression equations derived from basicranial and femoral length against total body length. The use of the Alligator femoral regression equation as a proxy to estimate metriorhynchid total body length led to a slight underestimation, whereas cranial regression equations of extant genera resulted in an overestimation of body length. Therefore, the scaling of crania and femora to total body length of metriorhynchids is noticeably different from that of extant crocodylians, indicating that extant crocodylians are not ideal proxies for size reconstruction of extinct taxa that deviate from their semi‐aquatic morphotype. The lack of a correlation between maximum, minimum, or the range of generic body lengths with species richness demonstrates that species diversification is driven by factors other than just variation in body size. Maximum likelihood modelling also found no evidence for directionality in body size evolution. However, niche partitioning in Metriorhynchidae is mediated not only by craniodental differentiation, as shown by previous studies, but also by body size variation. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163 , 1199–1216.     

On a longirostrine crocodylomorph (Thalattosuchia) from the Middle Jurassic of Tunisia

The geographic origins and distributional patterns of Gondwanan teleosauroids during the Jurassic have been fiercely debated over many years. Unlike the rich thalattosuchian fossil record from Laurasia, teleosauroids described from Gondwanan ecosystems are relatively scarce. Most of the known occurrences consist of isolated and fragmentary bones collected in Madagascar, Morocco, Tunisia, India, and Ethiopia. Nevertheless, these specimens, although fragmentary, have provided substantial information for assessing the evolutionary scenarios of multiple teleosauroid lineages and have shown that certain teleosauroid taxa were widespread rather than endemic to Western Europe. Here, a partial skeleton of a teleosauroid crocodylomorph is described. It was found in the late Middle Jurassic (Callovian) deposits of southeastern Tunisia by a team of French and Tunisian paleontologists; however, it has not been thoroughly studied at both macro- and microscopic scale until now. The new specimen is composed of an incomplete symphyseal portion of a lower jaw in addition to isolated teeth, osteoderms (both dorsal and ventral), thoracic and caudal vertebrae, and several thoracic ribs. The specimen has several morphological characters that are reminiscent of longirostrine teleosauroids. Due to the total absence of other cranial bones, as well as the pectoral and pelvic girdles, the specimen is not diagnostic to the generic level. However, these new remains represent the youngest ascertained occurrence of a definitive non-machimosaurin teleosauroid in Africa, provide additional insights into the geographic distribution of Thalattosuchia, and raise once again the question whether the origins of this clade were Gondwanan or Laurasian.     

Re-description of a putative Early Cretaceous “teleosaurid” from France,with implications for the survival of metriorhynchids and teleosaurids across the Jurassic-Cretaceous Boundary

Thalattosuchia was a diverse clade of marine crocodylomorphs known from the Early Jurassic to the Early Cretaceous. Recent studies have hypothesized that their extinction was two-phased: (1) habitat loss near/at the Jurassic-Cretaceous boundary heavily reduced their morphofunctional diversity, particularly in Europe, while (2) climate change and a shift in marine fauna during the Early Cretaceous (either at the Valanginian-Hauterivian boundary or during the early Hauterivian) finished off the already stressed clade. Unfortunately, the Cretaceous fossil record of thalattosuchians is poor, with only one putative “teleosaurid” specimen and approximately ten metriorhynchid specimens. Here we re-describe the youngest known teleosaurid from the Cretaceous (Valanginian of south-eastern France). Originally considered to be a teleosaurid (possibly Steneosaurus), we demonstrate that it belongs to Metriorhynchidae, and a newly discovered subclade, Plesiosuchina. It differs from Plesiosuchus in the pattern of tooth enamel ornamentation and the variation in dentary alveoli size. Referring this specimen to Metriorhynchidae means there are no definitive Cretaceous teleosaurid specimens. Furthermore, it suggests that both durophagous and piscivorous teleosaurids became extinct at the end of the Jurassic. Interestingly, this is the fourth metriorhynchid lineage known to cross the Jurassic-Cretaceous boundary. As such, it would appear that the two thalattosuchian families responded very differently to the lowering sea levels at the end of the Jurassic: teleosaurids possibly became extinct, while metriorhynchids were seemingly unaffected.     

The oldest durophagous teleosauroid (Crocodylomorpha,Thalattosuchia) from the lower Bathonian of central High Atlas,Morocco

Marine crocodylomorphs were particularly abundant in Europe during the Middle Jurassic, but were very scarce in Africa. New finds of thalattosuchian cranial remains in Morocco suggest that this scarcity is probably related to poor sampling rather than original diversity. These remains pertain to the coastal thalattosuchians, the teleosauroids, and particularly to the clade grouping the blunt‐toothed ‘Steneosaurus’ obtusidens and the genus Machimosaurus. A new tribe is erected grouping these two taxa: Machimosaurini. Until now the machimosaurins have been known from the middle Callovian. The new material extends the presence of this group further back to the lower Bathonian, nearly 5 myr earlier. The machimosaurins are the only teleosauroid group that has been recently reviewed, and the difference between the revised diversity provided herein and that previously reported is large. A review of other teleosauroids and clear establishment of their phylogenetic relationships are also likely to have considerable impact on their observed diversity. So, until a complete review of the teleosauroids is carried out, the results of crocodylomorph diversity analyses should be treated with caution.     

A NEW METRIORHYNCHID CROCODILIAN (MESOEUCROCODYLIA: THALATTOSUCHIA) FROM THE KIMMERIDGIAN (UPPER JURASSIC) OF WILTSHIRE,UK

Abstract: Recent revision of the marine metriorhynchid crocodilians indicates that a partial skull previously assigned to the species Metriorhynchus superciliosus and newly discovered postcranial elements from the Kimmeridge Clay of Westbury, Wiltshire belong to a new species of metriorhynchid. This material is herein described and referred to a new species of the genus Dakosaurus, characterised by four apomorphies: the size and shape of the enlarged supratemporal fossae; relatively large teeth, and half the number in relatives; the robust and unornamented cranium; and the angle that the prefrontal makes with the long axis of the skull. In a new phylogenetic analysis, Dakosaurus carpenteri sp. nov. is the basal member of a clade containing also D. maximus and D. andiniensis: it is not so short‐snouted and its teeth are not so few and large as in the other two species, but the new form illustrates the ecological transition among metriorhynchids from a piscivorous diet to high‐order carnivory.     

Marine reptiles (Thalattosuchia) from the Early Jurassic of Lombardy (northern Italy)

The fossil remains of two small reptiles recently discovered in the Sogno Formation (Lower Toarcian) near Cesana Brianza (Lecco Province), represent the first mesoeucrocodylians reported for Lombardy and some of the few Jurassic reptiles from Italy. Due to the absence of diagnostic skeletal elements (the skulls are lacking), it is not possible to refer the new specimens at genus level with confidence. Although the well developed dermal armour would characterise Toarcian thalattosuchians of the genera Steneosaurus (Teleosauridae) and Pelagosaurus (Metriorhynchidae), the peculiar morphology of the osteoderms allow to tentatively refer the remains to the latter taxon (cf. Pelagosaurus sp.). The small size, along with the opening of the neurocentral vertebral sutures and, possibly, the non sutured caudal pleurapophyses, indicate that the specimens were morphologically immature at death. These “marine crocodiles” confirm the affinities between the fauna of the Calcare di Sogno Formation and coeval outcrops of central Europe that also share the presence of similar fishes and crustaceans.     

First evidence of the Jurassic thalattosuchian (both teleosaurid and metriorhynchid) crocodylomorphs from Slovakia (Western Carpathians)

Abstract

We report here finds of thalattosuchians from Western Carpathians. The material consists of two isolated teeth from two different localities of the Pieniny Klippen Belt. The material represents the first evidence of the occurrence of both clades Teleosauridae and Metriorhynchidae in Slovakia. The first, a small tooth comes from the locality Beňatina and is dated early Middle Jurassic (Aalenian). Identification of the specimen shows it belongs to a longirostrine teleosaurid, possibly to Steneosaurus. Teleosaurids were largely restricted to lagoonal/near-shore environments. Their limbs allow them to support and walk on land. Regarding that fact, the presence of this taxon in Beňatina might be an indication of terrestrial land in the nearby area and shows a location of epicontinental seaways. A second larger tooth was found in the Upper Jurassic (Tithonian) sediments of the locality Kyjov, and is tentatively allocated in the genus Plesiosuchus. The members of Plesiosuchina are known in England, France, Portugal and Sicily. Our findings indicate a much wider spatial distribution of this clade in Europe and it fits well with the previous interpretation of sediments of the locality Kyjov as deep-water pelagic deposits.     

Sexual dimorphism and polymorphism in a Callovian Phlycticeras (Ammonoidea) assemblage of Kutch, India

An assemblage of Phlycticeras Hyatt from a precisely dated zone of Middle Callovian in Kutch, India has been analyzed. Systematic study reveals that this sample can be divided into two size groups. The larger set shows different adult modifications leading to ornamental polymorphism. Polymorphs are very similar, if not identical, to different chronospecies of Europe, which range between Middle to Upper Callovian. They are grouped under well-known Phlycticeras polygonium (Zieten). The group of smaller specimens on the other hand, strongly resembles a species, which has been previously described as Phlycticeras schaumburgi (Waagen) from the younger Upper Callovian horizons in Kutch. It has a peculiar ‘rooster’-like septicarinate keel in the venter near adult aperture. Phlycticeras-Oecoptychius have been considered as a possible dimorphic pair since long, but here shown to have many inconsistencies to support these views. Instead, sexual dimorphism is explored within Phlycticeras and P. polygonium-P. schaumburgi is proposed as possible antidimorphs. A similar association is found in different stratigraphic assemblages of Europe. Interestingly these two morphs show parallel evolutionary changes within the Phlycticeras genus.     

Morphospace occupation in thalattosuchian crocodylomorphs: skull shape variation, species delineation and temporal patterns

Abstract: Skull shape variation in thalattosuchians is examined using geometric morphometric techniques in order to delineate species, especially with respect to the classification of Callovian species, and to explore patterns of disparity during their evolutionary history. The pattern of morphological diversity in thalattosuchian skulls was found to be very similar to modern crocodilians: the main sources of variation are the length and the width of the snout, but these broad changes are correlated with size of supratemporal fenestra and frontal bone, length of the nasal bone, size of the orbit and premaxilla and position of the narial opening. Patterns of shape variation, in combination with discrete‐state morphology and stratigraphic and geographic range data were used to distinguish nine species of teleosaurid and 14 species of metriorhynchid, with the four currently recognized Callovian species being split into eight. Metriorhynchids were found to be more disparate from the average shape of morphospace than teleosaurids. However, short‐snouted metriorhynchids and long‐snouted teleosaurids showed the greatest amount of disparity with respect to snout morphotypes, indicating that each group tended to explore opposite areas of morphospace. Phylogeny was found to have a moderate influence on the pattern of morphospace occupation in metriorhynchids, but little effect in teleosaurids suggesting that other factors or constraints control the pattern of skull shape variation in thalattosuchians. A comparison of thalattosuchians with dyrosaur/pholidosaurids shows that thalattosuchians have a unique skull morphology, implying that there are multiple ways to construct a ‘long snout’. Moreover, the skull geometry of the problematic species Pelagosaurus typus was found to converge on the teleosaurid area of morphospace. Finally, the temporal distribution of thalattosuchian species and morphotypes demonstrate a clear and highly correlated relationship with sea level curves and mass extinction events through the Jurassic and the Early Cretaceous.     

西藏北部安多地区中侏罗统(巴通阶-卡洛夫阶)菊石

最近从藏北安多地区中侏罗统发现较丰富的菊石类化石,巴通期的菊石有OxyceritesoppeliElmi,Ho moeoplanulitescf. homoeomorphus(Buckman),H. cf. acuticosta(Roemer), Choffatiacf. vicentiMangold, Siemiradzkiacf. matisconensis(Lissajous), Proceritessp., Neuquenicerascf. yokoyamaiKobayashiandFukada和Indosphinctessp.。卡洛夫期的菊石有Oxyceritescf. subcotarius(Oppel), Macrocephalites? sp., Homoeoplanulitescf. furculus(Neumayr),Indospinctes(Elatmites) cf. reveliMangold和Reineckeitessp.。其中Indosphinctes,Indospinctes(Elatmites),Neu queniceras,Siemiradzkia和Procerites等属(或亚属)在西藏北部地区为首次报道。安多地区中—晚巴通期和早卡洛夫期菊石动物群凸现较强的地方性色彩; 晚巴通期ORBIS菊石带和早卡洛夫期KOENIGI菊石带是藏北地区菊石动物群扩散的重要时期。     

A stegosaur vertebra (Dinosauria: Ornithischia) from the Callovian (Middle Jurassic) of Sarthe, western France

A dinosaur vertebra found in the course of road works in the “Chevain Marls” (marine Callovian) of the Vermont outlier, near the village of Béthon (Sarthe, Pays-de-la-Loire, western France) is described and referred to a stegosaur (cf. Lexovisaurus). It is the first record of a dinosaur in the Middle Jurassic of that area. The specimen was probably derived from the floating carcass of an animal that had lived on the emerged areas of the Armorican Massif, near which the Chevain Marls were deposited. This fossil is an addition to the short list of stegosaur remains reported from France.     

The snout of Cricosaurus araucanensis: a case study in novel anatomy of the nasal region of metriorhynchids

Metriorhynchids are the only crocodyliforms adapted to pelagic marine life. Snout natural endocasts of the Tithonian (Late Jurassic) metriorhynchid Cricosaurus araucanensis indicated that skeletal changes defining the peculiar metriorhynchid body plan were coupled with changes of the soft cephalic anatomy such as the enlarged salt glands and restructuring of the paranasal sinus system. Seven new natural endocasts of the snout and a 3‐D reconstruction of C. araucanensis are described. Data from these casts and the reconstruction are congruent, and they are combined into an accurate reconstruction that improves our knowledge of the pre‐orbital anatomy. The olfactory tract, bulbs, olfactory nasal region and the anterior extension of the antorbital sinus within the maxilla are recognized. Osteological correlates of the salt gland body are also proposed. Palaeobiological inferences are erected based on the integration of natural endocasts and 3‐D reconstruction data. It is proposed that C. araucanensis nasal salt glands were highly vascularized with a blood supply comparable with those of extant marine birds. Reduced olfactory bulbs and olfactory nasal region indicate that the aerial olfaction, differing from extant crocodilians, was not well developed.     

Sunosuchus (Archosauria, Crocodyliformes) from the Toutunhe Formation (Middle Jurassic) of the Southern Junggar Basin (Xinjiang, NW-China)

A new specimen of the widespread Middle Jurassic to Lower Cretaceous Asian gonipholidid crocodilian genus Sunosuchus is described on the basis of a partial skeleton from the Upper Toutunhe Formation (Middle Jurassic, ?Bathonian-Callovian) of Liuhonggou, SW of Urumqi, Xinjiang Uygur Autonomous Region, People’s Republic of China. The specimen is represented by a partial mandible, teeth, vertebrae, limb and girdle bones and osteoderms. It can be distinguished from other nominal species of the genus by a unique combination of characters: slightly heterodontous dentition, strongly sculptured posteroventral part of the mandible, short fenestra mandibularis, convex dorsal surface of the retroarticular process, keeled cervical vertebral centra and ventral osteoderms with a distinctive sculpture of wide pits and narrow ridges. The heterodontous dentition is a potential autapomorphy of this form. The new specimen is closest in morphology to material described recently from the Callovian of Kirghisia as Sunosuchus sp. It represents the second Middle Jurassic record of the genus, the first crocodile from the Toutunhe Formation, the first substantial crocodile find from the Mesozoic of the Southern Junggar Basin, and the first Middle Jurassic record of Sunosuchus from China. This extends both the paleobiogeographical distribution of the genus in Asia and its stratigraphic distribution in China considerably.     

Largest known specimen of the genus Dakosaurus (Metriorhynchidae: Geosaurini) from the Kimmeridge Clay Formation (Late Jurassic) of England,and an overview of Dakosaurus specimens discovered from this formation (including reworked specimens from the Woburn Sands Formation)

We report the largest known British specimen of the metriorhynchid crocodylomorph genus Dakosaurus discovered offshore from Chesil Beach, Dorset, England (Kimmeridge Clay Formation). This specimen is large toothed, which has extreme enamel spalling on the labial surface, enlarged carinae (‘carinal flanges’), carinal wear, macroziphodont denticles, and the crown retains much of its labiolingual width along most of its apicobasal length. This suite of morphologies is unique to Dakosaurus. All known Kimmeridge Clay Formation Dakosaurus specimens are isolated tooth crowns. A skull previously referred to Dakosaurus lacks all the cranial apomorphies of D. maximus and D. andiniensis, and cannot be referred to this genus. Furthermore, the vast majority of putative Dakosaurus tooth crowns from the ‘Potton Sands’ ( = Woburn Sands Formation) do indeed represent Dakosaurus (as well as two Plesiosuchus specimens), and they most likely originate from the Kimmeridge Clay Formation.     

The callovian (Middle Jurassic) crinoids from northern Lithuania

Callovian strata in northern Lithuania (Papilė area) have yielded some crinoid taxa[Chariocrinus andreae (Desor),Balanocrinus berchteni Hess —Pugin,B. subteres (Münster inGoldfuss) andIsocrinus nicoleti (Desor)], which have not been described from this area. The only form known previously wasBalanocrinus pentagonalis (Goldfuss).Palaeocomaster jaegeri n. sp. is the first free-living crinoid recorded from the Callovian of eastern Europe. It is characterized by possessing a very low, strongly flattened and narrow radial cavity. A very similar crinoid assemblage is known from the Callovian glacially-derived clays exposed in the Łuków region (eastern Poland), which were originally located near the Baltic Sea during the Middle Jurassic. It is considered that the allochthonous Callovian deposits from Poland are facies equivalents of the deposits known from Lithuania. The sole significant exception is the domination of free-moving comatulids (Comatulida) in the sediments known from the Papilė region, which may suggest that the Lithuanian sea basin was a little shallower than that located farther west in the Baltic area.      

Thyreophoran vertebrae from the Callovian (Middle Jurassic) of the Vaches Noires cliffs (Normandy,France), with remarks on the dinosaur assemblage from the Vaches Noires

Two associated incomplete thyreophoran dorsal vertebrae from the Callovian Marnes de Dives Formation of the Vaches Noires cliffs, on the Normandy coast, are referred to an indeterminate stegosaur that appears to be different from Lexovisaurus, previously reported from the Callovian of western France. These vertebrae are the first evidence of thyreophorans from the Vaches Noires and complement the dinosaur assemblage from this locality, which hitherto consisted of several theropod taxa and an indeterminate sauropod. The dinosaur record from the Vaches Noires is heavily dominated by theropods and this imbalance is difficult to explain. A possible explanation may be that the dinosaur sample from the Vaches Noires is too small to be statistically significant and representative of the original faunal assemblage from which it is derived.     

A new Patagonian species of Cricosaurus (Crocodyliformes,Thalattosuchia): first evidence of Cricosaurus in Middle–Upper Tithonian lithographic limestone from Gondwana

Abstract:  Upper Jurassic (Tithonian) deposits in north‐western Patagonia, Argentina, have yielded rich and taxonomically diverse assemblages of marine reptiles. These assemblages are also remarkable by their quality of preservation and are represented by ichthyosaurs, plesiosaurs, turtles and crocodyliforms. Despite the abundant crocodyliform record, only two metriorhynchid taxa have been identified: Cricosaurus araucanensis and Dakosaurus andiniensis. Here we described a new species of Cricosaurus, which represents the second species of Cricosaurus in the Tithonian of the Neuquén Basin, and the first metriorhynchid found in lithographic limestone from Gondwana. Furthermore, this specimen has the most complete postcranial remains of any metriorhynchid from South America. The new species is characterized by a short distance between the premaxilla and the nasal, a relatively narrow interorbital width, 23–25 mandibular teeth, bicarinated teeth with fine apicobasally aligned ridges, interalveolar spaces between the first seven teeth approximately 1.5 times longer than the anteroposterior diameter of the respective alveoli. To test the assignment of the new species to Cricosaurus, we carried out two phylogenetic analyses. In both analyses, Cricosaurus lithographicus sp. nov. is nested with other species referred to this genus. This new species has peculiar enamel ornamentation, characterized by numerous, fine apicobasally aligned ridges, when compared to other species of the genus.     

Neoselachian sharks from the Callovian–Oxfordian (Jurassic) of Ogrodzieniec,Zawiercie Region,southern Poland

Abstract: Callovian and Oxfordian strata in Ogrodzieniec near Zawiercie, southern Poland, have yielded two shark tooth assemblages that collectively include 14 neoselachian taxa. A previously unrecognised member of the Orectolobiformes, Akaimia altucuspis gen. et sp. nov., is described and characterised by a dentition remarkably similar to modern wobbegong sharks (Orectolobidae) by convergence. The assemblages also include the first anterior teeth ever found of the palaeospinacid ‘Synechodus’prorogatus Kriwet, in addition to teeth from two other palaeospinacids, Sphenodus spp., four different orectolobiforms, two hexanchids and Protospinax spp. These shark tooth assemblages contribute to the poorly known Callovian and Oxfordian neoselachian faunas and indicate that the diversity was higher than previously appreciated, particularly within the Orectolobiformes.