Why ichthyosaurs — marine Mesozoic reptiles — disappeared before the dinosaur extinction has remained a mystery. New research suggests they may have gone extinct stepwise, during one of the most extreme greenhouse periods in the history of complex life-forms.
A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction
Authors:
Jiang et al
Abstract:
Contrary to the fast radiation of most metazoans after the end-Permian mass extinction, it is believed that early marine reptiles evolved slowly during the same time interval. However, emerging discoveries of Early Triassic marine reptiles are questioning this traditional view. Here we present an aberrant basal ichthyosauriform with a hitherto unknown body design that suggests a fast radiation of early marine reptiles. The new species is larger than coeval marine reptiles and has an extremely small head and a long tail without a fluke. Its heavily-built body bears flattened and overlapping gastral elements reminiscent of hupehsuchians. A phylogenetic analysis places the new species at the base of ichthyosauriforms, as the sister taxon of Cartorhynchus with which it shares a short snout with rostrally extended nasals. It now appears that ichthyosauriforms evolved rapidly within the first one million years of their evolution, in the Spathian (Early Triassic), and their true diversity has yet to be fully uncovered. Early ichthyosauromorphs quickly became extinct near the Early-Middle Triassic boundary, during the last large environmental perturbation after the end-Permian extinction involving redox fluctuations, sea level changes and volcanism. Marine reptile faunas shifted from ichthyosauromorph-dominated to sauropterygian-dominated composition after the perturbation.
Ichthyosaurs - shark-like marine reptiles from the time of dinosaurs - were driven to extinction by intense climate change and their own failure to evolve quickly enough, according to new research by an international team of scientists.
The study provides an explanation for one of the longest-standing enigmas in palaeobiology: how and why ichthyosaurs died out. Unlike other marine reptile groups, ichthyosaurs disappeared tens of millions of years before the end-Cretaceous extinction (65 million years ago) that marked the end for dinosaurs and the beginning of the age of mammals.
The research is published in the journal Nature Communications.
First author Dr Valentin Fischer, of the University of Liège, Belgium, and the University of Oxford, UK, said: "We analysed the extinction of this crucial marine group thoroughly for the first time. We compared the diversity of ichthyosaurs with the geological record of global change, emphasising the dynamics of these datasets.
"Ichthyosaurs were actually well diversified during the last chapter of their reign, with several species, body shapes and ecological niches present. However, their evolution was much slower than earlier in their history. Additionally, they were seemingly negatively affected by the profound global changes going on during the Cretaceous, as their extinction rate correlates with environmental volatility."
Causes of extinctions - including the demise of the ichthyosaurs, or 'sea dragons' - have often remained elusive and conjectural, particularly when they cannot be linked to an obvious geological or geochemical event such as a large meteorite or massive volcanic eruption. Ichthyosaurs were regarded as undiversified for a prolonged period before their extinction, and their dying out has previously been linked to minor events including increased competition with other marine predators and a decline in their assumed principal source of food.
However, using a battery of cutting-edge techniques to quantify ancient biodiversity and its fluctuations, the team was able to reconstruct the evolution of the ichthyosaurs during the last 120 million years of their lifetime and assess the causes of their extinction. The researchers - comprising Belgian, British, French and Russian scientists - demonstrated that before their extinction, ichthyosaurs were in fact highly diverse, both in terms of body shape and ecological role.
A two-phase event then suppressed their ecological diversity and wiped out the group at the beginning of the Late Cretaceous period, about 100 million years ago. At that time, the Earth's poles were essentially ice-free, and sea levels were much higher than today. Analyses revealed that this two-phase extinction can be associated both with reduced evolutionary rates (a failure to evolve novel body plans for a prolonged period) and intense climate change (strong variations in sea surface temperatures and sea levels).
A new ophthalmosaurid ichthyosaur from the Early Cretaceous of Colombia
Authors:
Maxwell et al
Abstract:
Cretaceous ichthyosaurs were relatively diverse in temperate latitudes, but few species have been described from the palaeotropics. Here, we describe a new ophthalmosaurid ichthyosaur, Muiscasaurus catheti gen. et sp. nov., from the Barremian–Aptian aged Paja Formation of Colombia. This species is known only from a partial skull and differs from all other ichthyosaurs in the unusual configuration of the external narial opening, slender rostrum, narrow postorbital region, and gracile dentition. It is the second ichthyosaur described from the Paja Formation, suggesting moderate taxonomic and ecological ichthyosaur diversity in the region during the Early Cretaceous.
An Upper Jurassic ichthyosaur (Ichthyosauria: Ophthalmosauridae) from the Bowser Basin, British Columbia
Authors:
Sissons et al
Abstract:
Although the Jurassic was a period of high diversity in ichthyosaurs, only a small number of specimens have been recorded from Canada to date. We describe here a new occurrence of an ophthalmosaurid ichthyosaur from a shallow marine depositional environment within the Bowser Basin of northern British Columbia. Based on vertebral diameters and the size of the humerus, the ichthyosaur was relatively large compared to other contemporaneous forms, yet possessed teeth that were small for its body size. As well, the height to length ratio of the preserved vertebrae suggests it may have had a more elongate, less regionalized body shape. Although indeterminate at a generic level, the presence of Late Jurassic ichthyosaurs in nearshore waters of northwestern North America further demonstrates their cosmopolitan distribution.
Phylogeny of the Ichthyopterygia incorporating recent discoveries from South China
Authors:
Ji et al
Abstract:
During the last decade, abundant ichthyopterygian material has been found from the Triassic of South China as well as the Upper Jurassic and Lower Cretaceous of Europe and South America, significantly expanding our knowledge of ichthyopterygian diversity through the Mesozoic. Previous phylogenetic hypotheses of the group no longer account for these extensive additions, necessitating a new phylogenetic framework for the entire Ichthyopterygia to enable evolutionary studies of the group. We present here a comprehensive phylogenetic hypothesis for Ichthyopterygia based on cladistic analysis of 163 characters coded for 59 ingroup and five outgroup taxa. The monophyly of Ichthyopterygia is strongly supported by a Bremer index value of 7. Five major groups of Ichthyopterygia during the Triassic, viz., Grippioidea, Cymbospondylidae, Mixosauridae, Shastasauridae, and Toretocnemidae, are well supported by Bremer index values between 3 and 5. Major clades that evolved in the Triassic, including Merriamosauria, Euichthyosauria, and Parvipelvia, are also robustly supported, whereas most post-Triassic clades are very weakly supported with a Bremer index value of 1, with a few exceptions, such as Thunnosauria and Ophthalmosauridae. The traditional Shastasauridae is expanded to comprise six genera but excludes Callawayia, which is more closely related to Parvipelvia than to Shastasauridae. ‘C.’ wolonggangensis is a shastasaurid but does not form a monophyletic clade with Callawayia neoscapularis or Guizhouichthyosaurus tangae as previously asserted. The new phylogenetic hypothesis is generally consistent with the stratigraphic occurrences of each taxon especially for the Triassic taxa.
A new species of Ichthyosaurus from the Lower Jurassic of West Dorset, England, U.K.
Authors:
Lomax et al
Abstract:
We describe a new species of Lower Jurassic (Hettangian/Sinemurian–Pliensbachian) ichthyosaur, Ichthyosaurus anningae, sp. nov., from west Dorset, England, U.K. The holotype of I. anningae (DONMG:1983.98), at least a subadult, is from the lower Pliensbachian Stonebarrow Marl Member (Charmouth Mudstone Formation). It is the most complete ichthyosaur known from this time interval worldwide. The species is assigned to Ichthyosaurus on the basis of humerus, forefin, and pectoral girdle morphologies. Diagnostic features of the species include a short, robust humerus with prominent processes; a femur in which the proximal width is almost as large as the distal width; and a very small femur relative to the humerus (humerus/femur ratio >1.7). Four other specimens, at least three of which are juveniles, are referred to this species. The new species may display sexual dimorphism in humeral morphology, but this cannot be confirmed due to a lack of stratigraphic information. With the recognition of I. anningae, at least three and possibly as many as five ichthyosaur species, representing three genera, are known from the Pliensbachian.
Ichthyosaurs from the Jurassic of Skye, Scotland
Authors:
Brusatte et al
Abstract:
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.
Status of Chaohusaurus chaoxianensis (Chen, 1985)
Authors:
Motani et al
Abstract:
The Early Triassic ichthyopterygian Chaohusaurus chaoxianensis was previously known from juvenile specimens only. It was considered to represent immature forms of C. geishanensis, which was based on a single adult specimen, because allometric regression lines of the forelimb skeletons suggested that the two name groups fitted along a single growth trajectory. The sample size for the analysis, however, was small (n = 5). We collected new specimens of Chaohusaurus from Majiashan, Chaohu City, Anhui Province, China, through field excavations that started in 2010. New specimens included an immature individual whose forelimb resembled that of mature C. geishanensis, as well as a large adult individual whose forelimb retained the characteristics of immature C. chaoxianensis. Therefore, C. chaoxianensis is not the juvenile of C. geishanensis. With the addition of the new specimens, two growth trajectories are now clearly detected on allometric regression plots (n = 15). The two species are unambiguously distinguished from each other based on a suite of discrete characters that are diagnostic to each species, as well as statistical analyses of forelimb measurements. It is unlikely that the two forms represent the genders of a single species given that C. geishanensis is vastly outnumbered by C. chaoxianensis and completely lacking in major fossiliferous rock layers. Therefore, C. chaoxianensis should be resurrected as a valid species.
Ammonoid age control of the Early Triassic marine reptiles from Chaohu (South China)
Authors:
Ji et al
Abstract:
Here we described a series of ammonoid specimens from the Early Triassic of Chaohu, South China and recognized the occurrence of Procolumbites for the first time in this area. The Procolumbites layer is about one meter above the first appearance of Chaohusaurus, indicating that the oldest Chaohusaurus is within the Procolumbites Zone of middle Spathian age. This new age constrain is significantly older than the previously suggested Subcolumbites Zone assignment (early late Spathian). To date, Chaohusaurus is the oldest known ichthyopterygian.
An Upper Triassic (Norian) ichthyosaur (Reptilia, Ichthyopterygia) from northern Alaska and dietary insight based on gut contents
Authors:
Druckenmiller et al
Abstract:
In 1950, the partial skeleton of a large vertebrate was discovered in the western Brooks Range of Alaska by a team of geologists mapping the U.S. Naval Petroleum Reserve No. 4 (now National Petroleum Reserve—Alaska). Because the preserved portion of the specimen was large (approximately 4 m in length) and found in an extremely remote location (Fig. 1), it was not collected at the time of discovery, although the site was subsequently revisited at later dates by geologists familiar with the find. A brief note mentioning the skeleton and its probable age was published 23 years later (Tailleur et al., 1973); however, the identity of the skeleton as an ichthyosaur, though suspected, was equivocal. In 2002, the specimen was relocated and collected by a team from the University of Alaska Museum, where it is now housed.
A basal ichthyosauriform with a short snout from the Lower Triassic of China
Authors:
Motani et al
Abstract:
The incompleteness of the fossil record obscures the origin of many of the more derived clades of vertebrates. One such group is the Ichthyopterygia, a clade of obligatory marine reptiles that appeared in the Early Triassic epoch, without any known intermediates. Here we describe a basal ichthyosauriform from the upper Lower Triassic (about 248 million years ago) of China, whose primitive skeleton indicates possible amphibious habits. It is smaller than ichthyopterygians and had unusually large flippers that probably allowed limited terrestrial locomotion. It also retained characteristics of terrestrial diapsid reptiles, including a short snout and body trunk. Unlike more-derived ichthyosauriforms, it was probably a suction feeder. The new species supports the sister-group relationships between ichthyosauriforms and Hupehsuchia, the two forming the Ichthyosauromorpha. Basal ichthyosauromorphs are known exclusively from south China, suggesting that the clade originated in the region, which formed a warm and humid tropical archipelago in the Early Triassic. The oldest unequivocal record of a sauropterygian is also from the same stratigraphic unit of the region.
The Carnian/Norian boundary succession at Berlin-Ichthyosaur State Park (Upper Triassic, central Nevada, USA)
Authors:
Balini et al
Abstract:
The Upper Carnian-Lower Norian (Upper Triassic) Luning Formation at Berlin-Ichthyosaur State Park (BISP) in central NV (USA) has been sampled using for the first time the bed-by-bed approach for ammonoids, pelagic bivalves, and conodonts, more than 60 years after its first description by Silberling (U.S. Geological Survey Professional Paper 322: 1–63, 1959). BISP is historically important for the definition of the uppermost Carnian of the North American Triassic chronostratigraphic scale and is known worldwide as one of the most important ichthyosaur Fossil-Lagerstätte because of its extraordinary record of 37 articulated, large-sized specimens of Shonisaurus popularis. Nearly 190 ammonoids were collected from two stratigraphic sections, documenting all the latest Carnian to Early Norian ammonoid faunas previously described by Silberling. Halobiids were collected from five levels, and the first report of conodonts from BISP includes faunas from 13 levels. The ~340-m thick Brick Pile section, the most complete in the study area, includes the uppermost Carnian Macrolobatus Zone, which provides conodont faunas of the lower primitia zone and Halobia septentrionalis. The 200-m thick lowermost Norian Kerri Zone, which begins 52 m above the Macrolobatus Zone, yields conodonts of the upper primitia zone in its lower part, together with H. cf. beyrichi and H. cf. selwyni. The ichthyosaur-bearing interval, whose stratigraphic position has been interpreted quite differently by previous authors, is documented in the uppermost Carnian Macrolobatus Zone and is characterized by rich Tropites-dominated ammonoid faunas and by the onset of Halobia. All models proposed by various workers to explain the unusual ichthyosaur record are discussed and an additional explanation for the main ichthyosaur-bearing bed is proposed. The new hypothesis is that a harmful algal bloom (HAB) may have been the trigger for the mass mortality recorded in this level. Although the C/N boundary in the Brick Pile section lies within a 52 m interval that presently lacks paleontologic data, this succession is included in a small group of sections that are expected to contribute to the definition of the GSSP of the Norian stage. Correlation of the Brick Pile section with the best Carnian/Norian sections in northeastern British Columbia is discussed. Compared to the British Columbia Juvavites Cove and the GSSP candidate Black Bear Ridge sections, the Brick Pile section exhibits an ammonoid and Halobia record that is slightly more similar to that of the Tethyan sections. Correlation of the Brick Pile section with the second GSSP candidate Pizzo Mondello (Sicily, Italy) well demonstrates the significant problems encountered in calibration of the Tethyan and North American scales.
Ichthyosaurs from the French Rhaetian indicate a severe turnover across the Triassic–Jurassic boundary
Authors:
Fischer et al
Abstract:
Mesozoic marine reptiles went through a severe turnover near the end of the Triassic. Notably, an important extinction event affected ichthyosaurs, sweeping a large part of the group. This crisis is, however, obscured by an extremely poor fossil record and is regarded as protracted over the entire Norian–earliest Jurassic interval, for the lack of a more precise scenario. The iconic whale-sized shastasaurid ichthyosaurs are regarded as early victims of this turnover, disappearing by the middle Norian. Here we evaluate the pattern of this turnover among ichthyosaurs by analysing the faunal record of two Rhaetian localities. One locality is Autun, eastern France; we rediscovered in this material the holotypes or partial ‘type’ series of Rachitrema pellati, Actiosaurus gaudryi, Ichthyosaurus rheticus, Ichthyosaurus carinatus and Plesiosaurus bibractensis; a revised taxonomic scheme is proposed. The second assemblage comes from a new locality: Cuers, southeastern France. Both these assemblages provide several lines of evidence for the presence of shastasaurid-like ichthyosaurs in the Rhaetian of Europe. These occurrences suggest that both the demise of shastasaurids and the sudden radiation of neoichthyosaurians occurred within a short time window; this turnover appears not only more abrupt but also more complex than previously postulated and adds a new facet of the end-Triassic mass extinction.
A New Upper Jurassic Ophthalmosaurid Ichthyosaur from the Slottsmøya Member, Agardhfjellet Formation of Central Spitsbergen
Authors:
Roberts et al
Abstract:
Abundant new ichthyosaur material has recently been documented in the Slottsmøya Member of the Agardhfjellet Formation from the Svalbard archipelago of Norway. Here we describe a partial skeleton of a new taxon, Janusaurus lundi, that includes much of the skull and representative portions of the postcranium. The new taxon is diagnosed by a suite of cranial character states including a very gracile stapedial shaft, the presence of a dorsal process on the prearticular and autapomorphic postcranial features such as the presence of an interclavicular trough and a conspicuous anterodorsal process of the ilium. The peculiar morphology of the ilia indicates a previously unrecognized degree of morphological variation in the pelvic girdle of ophthalmosaurids. We also present a large species level phylogenetic analysis of ophthalmosaurids including new and undescribed ichthyosaur material from the Upper Jurassic of Svalbard. Our results recover all Svalbard taxa in a single unresolved polytomy nested within Ophthalmosaurinae, which considerably increases the taxonomic composition of this clade. The paleobiogeographical implications of this result suggest the presence of a single clade of Boreal ophthalmosaurid ichthyosaurs that existed during the latest Jurassic, a pattern also reflected in the high degree of endemicity among some Boreal invertebrates, particularly ammonoids. Recent and ongoing descriptions of marine reptiles from the Slottsmøya Member Lagerstätte provide important new data to test hypotheses of marine amniote faunal turnover at the Jurassic-Cretaceous boundary.
Osteohistology of the Early Triassic ichthyopterygian reptile Utatsusaurus hataii: Implications for early ichthyosaur biology
Authors:
Nakajima et al
Abstract:
Ichthyosaurs were highly adapted to a marine lifestyle, as shown by their fish-like body shape and their assumed active swimming abilities and high metabolic rates. However, the processes of adaptation to an aquatic life in the early stages of this lineage remain poorly understood. Here, we present the first osteohistological data concerning the most basal ichthyopterygian yet known, Utatsusaurus hataii, from the Lower Triassic of Japan. The cancellous bone structure suggests adaptation to active swimming in an open marine environment. Moreover, the possible occurrence of rapidly deposited bone tissue, in a fibrolamellar complex, suggests a higher metabolic rate than in modern poikilothermic reptiles, and therefore a trend toward homeothermy. This basal ichthyosaur, with its elongate body, was already more adapted to an aquatic lifestyle than expected from its morphology, and the process of adaptation to a marine lifestyle was already well advanced by the Early Triassic.
A mysterious giant ichthyosaur from the lowermost Jurassic of Wales
Authors:
Martin et al
Abstract:
Ichthyosaurs rapidly diversified and colonised a wide range of ecological niches during the Early and Middle Triassic period, but experienced a major decline in diversity near the end of the Triassic. Timing and causes of this demise and the subsequent rapid radiation of the diverse, but less disparate, parvipelvian ichthyosaurs are still unknown, notably because of inadequate sampling in strata of latest Triassic age. Here, we describe an exceptionally large radius from Lower Jurassic deposits at Penarth near Cardiff, South Wales (UK) the morphology of which places it within the giant Triassic shastasaurids. A tentative total body size estimate, based on a regression analysis of various complete ichthyosaur skeletons, yields a value of 12-15 m. The specimen is substantially younger than any previously reported last known occurrences of shastasaurids and implies a Lazarus range in the lowermost Jurassic for this ichthyosaur morphotype.
A Lower Cretaceous ichthyosaur graveyard in deep marine slope channel deposits at Torres del Paine National Park, southern Chile
Authors:
Stinnesbeck et al
Abstract:
Remnants of ophthalmosaurid ichthyosaurs recently discovered in the vicinity of the Tyndall Glacier in the Torres del Paine National Park of southern Chile are extremely abundant and well preserved. After three field campaigns to the area, a total of 46 articulated and virtually complete ichthyosaur specimens, both adults and juveniles, were tentatively assigned to four different species of Ophthalmosauridae. Preservation is excellent and occasionally includes soft tissue and embryos. The skeletons are associated with ammonites, belemnites, inoceramid bivalves, and fishes as well as numerous plant remains. The enormous concentration of ichthyosaurs is unique for Chile and South America and places the Tyndall locality among the prime fossil Lagerstätten for Early Cretaceous marine reptiles worldwide.
The deposit is Early Cretaceous (Valanginian–Hauterivian) in age and forms part of a monotonous bathyal to abyssal sequence of the Late Jurassic to late Early Cretaceous Rocas Verdes back-arc basin. In this region, the Tyndall ichthyosaur population may have profited from cold upwelling currents that caused abundant life at the shelf edge including masses of belemnites and small fish, the preferred diet of ichthyosaurs. The abundance of almost completely articulated ichthyosaur skeletons in the Tyndall area suggests that some animals fell victim to episodic mass-mortality events caused by turbidity currents traveling downslope through a submarine canyon. They lost orientation, drowned, and were dragged into the deep sea by these turbulent high-energy gravity flows. Their bodies ended up in an oxygen-deficient basin environment where they were immediately embedded by the fine turbidite suspension fallout. The Tyndall ichthyosaur locality thus combines characteristics of both concentration and conservation Lagerstätten.
Authors:DAL SASSO et alAbstract:Here we report for the first time on the presence of ichthyosaurs in Sicily, southern Italy. The deposit of origin (Mufara Formation) can be dated to the upper Carnian (Tuvalian substage) based on a typical association of ammonites, one of which (Shastites sp.) is embedded in the sediment still encrusting one of the bone specimens recently found. The latter consist of two isolated vertebral centra that are referred to the Ichthyosauria thanks to their disk-like shape (i.e. they are much taller than long) combined with the amphicelous condition, lack of transverse processes, and presence of rib articulations on the central sides. The largest specimen is more precisely an anterior dorsal vertebra from an adult individual, ascribed to Shastasauridae indet. By the presence of elongated reniform diapophyseal facets, cranially not truncated, and absence of parapophyses. The smaller specimen represents an anterior cervical element from an immature individual of a likely smaller-sized, indeterminate taxon. These finds indicate that the biodiversity of the Monte Scalpello Triassic fauna is higher than previously known, but above all represent the southernmost occurrence of Triassic ichthyosaurs in the western Tethys basin.
