Were Mosasaurs Warm Blooded?

Endothermic mosasaurs? Possible thermoregulation of Late Cretaceous mosasaurs (Reptilia, Squamata) indicated by stable oxygen isotopes in fossil bioapatite in comparison with coeval marine fish and pelagic seabirds

Authors:

Lynn Harrell et al

Abstract:

The thermoregulatory style of Late Cretaceous mosasaurs has become a highly controversial subject in vertebrate palaeontology. These extinct marine reptiles have previously been described as poikilothermic, endothermic or gigantothermic. Here we analyse three genera of mosasaurs from the Mooreville Chalk in Alabama (USA) of differing body mass, and compare their δ18OPO4 derived body temperatures (Tb) with those of coeval poikilothermic fish (Enchodus) and endothermic pelagic seabirds (Ichthyornis). Results show that all mosasaurs, Clidastes (Tb = 33.1°C), Platecarpus (Tb = 36.3°C), and Tylosaurus (Tb = 34.3°C), had elevated average body temperatures in relation to those of the fish (Tb = 28.3°C) and were closer to those of Ichthyornis (Tb = 38.6°C). The temperatures calculated for Enchodus compare well with previously reported temperature estimates for the Mooreville Chalk and the Tb of Ichthyornis compares well with temperatures that have been reported for modern seabirds, suggesting that this method provides accurate results. Finally, although there are small differences of body temperature among mosasaur genera, these are independent of size, and thus inferred body mass, suggesting that mosasaurs were not gigantotherms, but rather endotherms.

Phosphorosaurus ponpetelegans: a Halisaurine Mosasaur With Binocular Vision From Cretaceous Japan

A new halisaurine mosasaur (Squamata: Halisaurinae) from Japan: the first record in the western Pacific realm and the first documented insights into binocular vision in mosasaurs

Authors:

Konishi et al

Abstract:

A specimen of a halisaurine mosasaur is reported from Japan for the first time, closing the pre-existing biogeographical gap between the Middle East and the eastern Pacific. Phosphorosaurus ponpetelegans sp. nov., from the lowermost Maastrichtian of Hokkaido, has been assigned to the genus Phosphorosaurus for sharing the following suite of major cranial characters with P. ortliebi, the type species from Belgium: apex of posterodorsal triangular plateau on frontal reaching level of interorbital constriction; frontal lateral border forming a step-like junction between interorbital and preorbital segments of frontal; preorbital segment of frontal sloping anteroventrally; and stapedial meatus parallel-sided in latero-medial view. Potential autapomorphies that distinguish P. ponpetelegans from other members of Halisaurinae include: postorbitofrontal jugal process elongate and stalk-like, projecting laterally; this process distally bearing a ventrally facing depression for jugal articulation; and lateral surangular–articular suture angular rather than round. The long and laterally projecting jugal processes, when combined with a depressed as well as narrow snout, provide compelling evidence for well-developed binocular vision for the new mosasaur, with an estimated binocular field of view (BFoV) of 35°. This value is unusually high for non-ophidian squamates that typically exhibit a BFoV of 10—20°, and is higher than those of other measured mosasaur taxa by at least 5°. Among colubrid snakes, nocturnal species exhibit greater BFoV than diurnal ones in both arboreal and terrestrial taxa. Known also from the Maastrichtian of Hokkaido are fossils of lantern fish (myctophids) and 10-armed cephalopods (coleoids), both of which are typically bioluminescent today. It is hence proposed that the exceptionally large, forward-facing eyes of P. ponpetelegans may well have been a special adaptation for a nocturnal lifestyle, where they hunted small, bioluminescent prey at night while avoiding direct competition with larger, more piscine mosasaurine taxa such as Mosasaurus hobetsuensis that co-existed with P. ponpetelegans.

Mosasaurs Didn't lay Eggs on the Beach, Were Viviparous

Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs

Authors:

Field et al

Abstract:

Mosasaurs were large marine squamates that inhabited all of the world's oceans during the Late Cretaceous. Their success as apex predators has been attributed to their rapid acquisition of aquatic adaptations, which allowed them to become fully pelagic. However, little is known about the breeding biology of derived, flipper-bearing mosasaurs, as the record of neonatal mosasaur fossils is extremely sparse. Here, we report on the fragmentary cranial remains of two neonatal mosasaurs from the Niobrara Formation, referred to Clidastes sp. Comparison with other preliminary reports of neonatal mosasaurs reveals that these specimens are among the smallest individuals ever found and certainly represent the smallest known Clidastes specimens. The recovery of these extremely young specimens from a pelagic setting indicates that even neonatal mosasaurs occupied open oceanic habitats and were likely born in this setting. These data shed new light on the ecology of neonatal mosasaurs and illustrate the degree to which size-related taphonomic and collection biases have influenced our understanding of the early life history of these iconic marine reptiles.

Mosasaurs From Campanian/Maastrichtian Cretaceous Germany

Tylosaurine mosasaurs (Squamata) from the Late Cretaceous of northern Germany.

Authors:

Hornung et al

Abstract:

Two genera of tylosaurine mosasaurs, Tylosaurus and Hainosaurus, are recorded for the first time from Germany. Tylosaurus sp. is represented by two isolated tooth crowns, originally described as Mosasaurus? alseni (here considered a nomen dubium) from the latest Santonian–Early Campanian, which are very similar to T. ivoensis and T. gaudryi. The material of Hainosaurus sp. comprises a maxillary with associated postorbitofrontal, two pterygoid teeth and several indeterminate cranial fragments. The specimen from the Late Campanian is slightly less derived than H. bernardi from the Maastrichtian in retaining labiolingually less compressed anterior maxillary teeth and unserrated pterygoid teeth with only very weak carinae. Despite only minor skeletal differences, the genus Hainosaurus is considered to be distinct from Tylosaurus because of its significant modification of the dental apparatus compared to the plesiomorphic condition in the latter. This dental morphology suggests a phylogenetic trend from a generalised-piercing marginal dentition in Tylosaurus towards the increasingly labiolingually compressed, symmetrical, strongly bicarinate cutting marginal teeth in Hainosaurus spp. from the Early through Late Campanian and Maastrichtian. A similar trend is also present in pterygoid teeth with very indistinct unserrated carinae in the Campanian Hainosaurus sp. towards serrated ones in the Maastrichtian H. bernardi. A short review indicates the presence of Hainosaurus in northern, central and western Europe (Sweden to Spain) since the Early Campanian, and the occurrence of Tylosaurus spp. in the same area until the Late Campanian. Hainosaurus persisted until the end of the Maastrichtian; outside Europe it may have been present in the Late Campanian of the USA and the Maastrichtian of the Democratic Republic of Congo. Judging from a simple, uni- to bicarinate, stoutly conical tooth morphology in aigialosaurs and very basal mosasaurs as well as phylogenetic patterns, the development of blade-like cutting tooth crowns appears to have been convergent in several clades of large-bodied Campanian–Maastrichtian mosasaurids. These include both mosasaurines ('Leiodon' mosasauroides, Prognathodon? sectorius, Prognathodon? kianda, Eremiasaurus heterodontus) and tylosaurines (Hainosaurus spp.).

Habitat Partitioning by Upper Cretaceous Mosasaurs

Habitat preference of mosasaurs indicated by rare earth element (REE) content of fossils from the Upper Cretaceous marine deposits of Alabama, New Jersey, and South Dakota (USA)

Authors:

Harrell et al

Abstract:

Knowledge of habitat segregation of mosasaurs has been based on lithology and faunal assemblages associated with fossil remains of mosasaurs and stable isotopes (δ13C). These approaches have sometimes provided equivocal or insufficient information and, therefore, the preference of habitat by different mosasaur taxa is still suboptimally constrained. The present study is focused on the analysis of rare earth element (REE) ratios of mosasaur fossils from the Upper Cretaceous formations of western Alabama, USA. Results of the REE analysis are used to infer the relative paleobathymetry associated with the mosasaur specimens and then to determine if certain taxonomic groups showed a preference for a particular water depth. Comparisons are then made with mosasaur specimens reported in the literature from other regions of North America from different depositional environments. Results indicate that Mosasaurus, Platecarpus and Plioplatecarpus may have preferred more restricted habitats based on water depth whereas Tylosaurus and Clidastes favoured a wider range of environments. Results also suggest that Plioplatecarpus lived in a shallower environment than its Platecarpus predecessor. Although the results of this study are encouraging, caution must be exercised before drawing any final conclusions due to the small sample size of most of the taxa analysed.

The Maastrichtian Cretaceous Mosasaurs of Morocco

Mosasaurids (Squamata) from the Maastrichtian Phosphates of Morocco: Biodiversity, palaeobiogeography and palaeoecology based on tooth morphoguilds

Authors:

Bardet et al

Abstract:

Mosasaurid squamates are the most numerically abundant, and taxonomically/ecologically diverse clade of marine amniotes represented in the Maastrichtian Phosphates of Morocco. With few exceptions, they are faunally typical of the Southern Mediterranean Tethys Margin (around palaeolatitude 25°N) and range from the base to the top of the stage. The Moroccan assemblages include at least 7 genera and 10 species representing a broad spectrum of sizes and morphologies that illustrate several ecological trends. Noteworthy is the predominance of Mosasaurinae which are widespread in contemporaneous outcrops worldwide and constitute 80% and 70% of the total genus/species number respectively. In contrast, Halisauromorpha and Russellosaurina (plioplatecarpines) are scarce and tylosaurines are presently unknown. All of the Moroccan mosasaurids exhibit characteristic tooth morphologies and can be placed into resource partitioning morphoguilds indicative of adaptations for piercing, crushing or cutting. Medium to large predators are found to distribute along the ‘Crush’-‘Cut’ axis of the morphoguild projection, and a new ‘Crush-Cut’ guild, previously unrecognised amongst Mesozoic marine amniotes, accommodates several Prognathodon species. Also of importance is the lack of mosasaurids along the ‘Pierce’-‘Crush’ axis, potentially inferring that these ecological niches were occupied by other marine vertebrates such as selachians and plesiosaurs. In addition, the relative abundance of mosasaurids throughout the Maastrichtian series of the Gantour Basin evidences direct ecological competition or predation phenomena.

Reassessment of Mosasaurus gracilis From Turonian Cretaceous England

Reassessment of Turonian mosasaur material from the ‘Middle Chalk’ (England, U.K.), and the status of Mosasaurus gracilis Owen, 1849

Authors:

Street et al

Abstract:

The first described genus of mosasaur, Mosasaurus hoffmannii, has been coarsely diagnosed and defined since its creation, with numerous specimens and new species being assigned to the genus with little or no reference to, and thus few real similarities with, the generic type specimen. One of the earliest examples of a weakly defined and diagnosed species assigned to the genus is Mosasaurus gracilis. Location and examination of the various assigned specimens and the holotype indicate that M. gracilis shares more characters, such as a short rostrum on the dentary anterior to the first teeth, with russellosaurine mosasaurs. In addition, M. gracilis is known from Turonian deposits, whereas other species belonging to Mosasaurus are upper Campanian–Maastrichtian in age. Based on the evidence of shared characters and contemporaneity, we suggest that M. gracilis be removed from Mosasaurus because it shares more affinities with members of the Russellosaurina. This represents the first step in the very necessary process of untangling the alpha taxonomy, and subsequently the complete systematics, of the genus Mosasaurus hoffmannii.

Eonatator coellensis: a new, Seemingly Ovoviviparous Campanian Cretaceous Mosasaur From Colombia

Eonatator coellensis nov. sp. (Squamata: Mosasauridae), a new species from the Upper Cretaceous of Colombia.

Author:

Páramo-Fonseca

Abstract:

In this study a new mosasaur found in Colombia is given out. The fossil was extracted from Campanian rocks, North of the town of Coello, Tolima Department. It is a nearly complete articulated skeleton with soft tissue remains preserved in its cavities. It was
determined as a new species of the genus Eonatator, E. coellensis, based on the systematics proposed in 2005 by Bardet and others. The anatomy of the anterior part of the skull, as well the morphology and the inter-relationships of the bones of the pelvic girdle and limbs, constitute a new contribution to the genus definition. The Colombian specimen represents the most complete halisaurine mosasaur known so far in the world, and provides new evidence of ovoviviparity in mosasaurs.

More Material From Globidensine Mosasaur Prognathodon From Maastrichtian Cretaceous Netherlands

New material of Prognathodon (Squamata, Mosasauridae) from the type Maastrichtian of the Netherlands

Authors:

Schulp et al

Abstract:

A partial quadrate, here assigned to the globidensine mosasaur Prognathodon cf. saturator, is recorded from the basal Valkenburg Member (Maastricht Formation) in the type area of the Maastrichtian Stage (southeast Netherlands, northeast Belgium). Occurrences of Prognathodon saturator in this area are extremely rare, which might well be linked to the offshore habitat preferred by this species.

Giant Mosasaurus hoffmanni From Maastrichtian Cretaceous Russia

Giant Mosasaurus hoffmanni (Squamata, Mosasauridae) from the Late Cretaceous (Maastrichtian) of Penza, Russia.

Author:

Grigoriev

Abstract:

This study provides a morphological description of the fragmentary skull of a mosasaur discovered in 1927 in the Upper Cretaceous (Maastrichtian) deposits in the city of Penza (Russia). Some bones from the original material had been lost since their discovery; their
description is based on plaster casts. The Penza mosasaur displays characteristic features of Mosasaurus hoffmanni such as the posterior carina that shifts from a somewhat lateral position in the anterior teeth to a posterior position further along the tooth row, a frontal
with convex lateral margins, and a powerfully built dentary. This is the first unequivocal record of this taxon from Russia. M. hoffmanni from the Penza is one of the largest mosasaurs ever known with an overall length of the body about 17 m.

An Amazing Mosasaurus missouriensis Specimen From Campanian Cretaceous Canada

A small, exquisitely preserved specimen of Mosasaurus missouriensis (Squamata, Mosasauridae) from the upper Campanian of the Bearpaw Formation, western Canada, and the first stomach contents for the genus

Authors:

Konishi et al

Abstract:

A new, exquisitely preserved specimen of a small mosasaur, referable to Mosasaurus missouriensis, is reported from the Bearpaw Formation (ca. 75 Ma, upper Campanian) of southern Alberta, Canada. Many calcified cartilaginous elements, including tracheal rings and the sternum, are preserved. The sternum most closely resembles that of Clidastes propython, bearing five shallow sternal rib facets on each side. Our comparative study of the new material with the holotype, referred material, and the genotype M. hoffmannii is congruent with the preexisting hypothesis that M. missouriensis and M. hoffmannii are phylogenetically more closely related to each other than to the other congeners, in spite of a temporal gap of nearly 10 million years between them. Also preserved with the mosasaur, inside its ribcage and around the specimen, are well-preserved aulopiform fish bones, including a skull. The fish skull is punctured and its centra truncated, suggesting macrophagy was employed by M. missouriensis despite the apparent lack of tooth wear. A sympatric specimen of Prognathodon overtoni is known to have consumed a sea turtle as well as fishes, and consistently exhibits apical wear across marginal teeth. We hypothesize that coexistence of these apex predators in the Bearpaw Sea was possible because of niche partitioning. Finally, the mosasaur carcass was likely scavenged by at least three lamniform sharks, based on their shed teeth and a series of truncated transverse processes on the mosasaur tail.

The Drivers for Mosasaur Evolution

Physical drivers of mosasaur evolution

Authors:

Polcyn et al

Abstract:

Mosasaurs are marine squamates with a 32.5 million-year history from their appearance at 98 Ma to their extinction at the K–Pg boundary (65.5 Ma). Using a database of 43 generic and 94 species-level taxa, we compare the taxonomic diversity and patterns of morphological disparity in mosasaurs with sea level, sea surface temperature, and stable carbon isotope curves for the Upper Cretaceous to explore factors that may have influenced their evolution. No single factor unambiguously accounts for all radiations, diversification, and extinctions; however, the broader patterns of taxonomic diversification and morphological disparity point to niche differentiation in a “fishing up” scenario under the influence of “bottom-up” selective pressures. The most likely driving force in mosasaur evolution was high productivity in the Late Cretaceous, driven by tectonically controlled sea levels and climatically controlled ocean stratification and nutrient delivery. When productivity collapsed at the end of the Cretaceous, coincident with bolide impact, mosasaurs became extinct.

A Giant Campanian Cretaceous Mosasaur With Twisted Teeth From Northern Italy

A giant mosasaur (Reptilia, Squamata) with an unusually twisted dentition from the Argille Scagliose Complex (late Campanian) of Northern Italy

Authors:

Fanti et al

Abstract:

A snout of a large-sized mosasaur from the Upper Cretaceous pelagic-turbiditic deposits of the Argille Scagliose Complex of Northern Italy is described. Nannofossil assemblages from the immediately overlying strata belong to the late but not latest Campanian calcareous nannofossil standard zone CC22, based on the presence of Uniplanarius trifidus and Eiffellithus eximius. The specimen includes a broken premaxilla, the anterior part of the maxillae and dentaries in articulation: preserved teeth show distinctive characters previously unreported in other mosasaurs. Although the marginal teeth show a posterior migration of the labial carina along the jaw length – diagnostic of derived mosasaurines – they are unusual in the combination of features, including anteriormost teeth with asteroid cross section followed by teeth with crowns twisted labioposteriorly from one-third to one-half of their height toward the apices. A comparison between the new specimen and Mosasaurus hoffmanni skulls suggests an estimated skull length comparable with some of the largest known mosasaurids. Consequently, the new specimen represents the largest mosasaur and the largest fossil reptile found in Italy to date. Several lines of dental evidence also support the interpretation of the Italian mosasaur as a macrophagous generalist predator. Paleogeographic reconstruction for the Argille Scagliose Complex as well as the occurrence of typical low- to mid-latitude nannofossils support a southern Tethyan margin affinity of the taxon.

Santonian/Campanian Cretaceous Puskwaskau Formation Shows Diverse Marine Reptile Fauna Near Paleo Arctic Circle

Marine Reptiles (Plesiosauria and Mosasauridae) from the Puskwaskau Formation (Santonian–Campanian), West-Central Alberta

Authors:

Bell et al

Abstract:

Plesiosaurs and mosasaurs are identified from the Puskwaskau Formation of west-central Alberta, Canada. These deposits record the final stages during which the Western Interior Seaway remained open to the Boreal Sea to the North and therefore are important for determining the ranges of high-latitude marine reptiles. Polycotylid and elasmosaurid plesiosaurs shared these waters with russellosaurine (including plioplatecarpine) mosasaurs suggesting a diverse ecology of large-bodied marine predators occupied these high-latitude waters in the early Campanian. This locality, situated at 65°N paleolatitude, helps link the poorly known faunas from northern Canada with the better-known faunas from central and southern North America. Rare articulated material from the Puskwaskau Formation urges further investigation of this poorly explored unit.

How Dark is Your Santonian Cretaceous Mosasaur? Your Sinemurian Jurassic Ichthyosaur?

Unique finds of original pigment in fossilised skin from three multi-million-year old marine reptiles attract considerable attention from the scientific community. The pigment reveals that these animals were, at least partially, dark-coloured in life, which is likely to have contributed to more efficient thermoregulation, as well as providing means for camouflage and UV protection. Researchers at Lund University are among the scientists that made the spectacular discovery.

During the Age of the dinosaurs, huge reptiles, such as mosasaurs and ichthyosaurs, ruled the seas. Previously, scientists could only guess what colours these spectacular animals had; however, pigment preserved in fossilised skin has now been analysed at SP Technical Research Institute of Sweden and MAX IV Laboratory, Lund University, Sweden. The unique soft tissue remains were obtained from a 55 million-year-old leatherback turtle, an 85 million-year-old mosasaur and a 196 million-year-old ichthyosaur. This is the first time that the colour scheme of any extinct marine animal has been revealed.

"This is fantastic! When I started studying at Lund University in 1993, the film Jurassic Park had just been released, and that was one of the main reasons why I got interested in biology and palaeontology. Then, 20 years ago, it was unthinkable that we would ever find biological remains from animals that have been extinct for many millions of years, but now we are there and I am proud to be a part of it", said Johan Lindgren about the discovery of the ancient pigment molecules.

link.

A Tooth Wear Pattern Study of the Durophagus Maastrichtian Cretaceous Mosasaur Carinodens belgicus

Dental macro- and microwear in Carinodens belgicus, a small mosasaur from the type Maastrichtian

Authors:

Holwerda et al

Abstract:

Teeth of the small durophagous mosasaur Carinodens belgicus are known from Maastrichtian Atlantic-Tethyan deposits worldwide. The peculiar dentition of Carinodens inspired debate and speculation on its dietary niche ever since its first description. In this contribution, we describe the macro- and microwear pattern in five well-preserved isolated teeth, allowing further and independent evaluation of aspects of feeding behavior and diet. Macroscopically, wear is concentrated on the apex and mesiodistal sides. Microwear was mapped using Scanning Electron Microscopy at several magnifications and can be characterised as scratches and pits. Coarse scratches were found to be the most common and pits were found to be the least common feature. Scratch orientation is primarily along the mesiodistal plane or in the labiolingual plane with an angle of ~130°. These microwear features can be explained either by oral processing or passive abrasion by sediments or food. As scratch width only indicates the minimum width of the abrading particle, the material causing the wear here could have ranged from silica-based silts to larger abrasives. However, in this case, abrasion by sediments might not explain this wear because of the biocalcarenitic nature of the type Maastrichtian sediments; siliciclastics are virtually absent. Therefore it is more likely that hard food particles, such as benthic organisms with hard exoskeletons, caused the wear on the enamel of Carinodens, or Carinodens ventured out to more sandy areas to forage as well. The mesiodistal and labiolingual direction of the microwear scratches might suggest that Carinodens showed more complexity in the use of its teeth than simple grasping, and that a gripping and pulling motion during feeding similar to that employed by modern varanids may have been the cause.

It was a Sea Monster Eat Sea Monster World

The mosasaur, a fearsome marine reptile that stalked the Cretaceous seas, scavenged its own kin, a new fossil find reveals.

A fossilized mosasaur found in Angola contains the partial remains of three other mosasaurs in its stomach, researchers reported here Tuesday (Oct. 29) at the annual meeting of the Geological Society of America.

"These are three different species of mosasaur inside the belly of a fourth species of mosasaur," said study researcher Louis Jacobs, a vertebrate paleontologist at Southern Methodist University in Texas.

link.

A Turonian Cretaceous Baby Tethysaurus (mosasaur) Found in Morocco

A baby mosasauroid (Reptilia, Squamata) from the Turonian of Morocco – Tethysaurus ‘junior’ discovered?

Authors:

Alexandra Houssaye and Nathalie Bardet

Abstract:

New Middle Turonian mosasauroid remains were discovered in the same large-sized nodule that yielded a specimen referred to Tethysaurus nopcsai Bardet et al., 2003, from the Goulmima region (southern Morocco). They comprise isolated, fragmentary cranial elements (skull and mandible) and some vertebrae. Their very small size suggests a juvenile condition, an observation supported also by anatomical (spongious nature of numerous bone parts), micro-anatomical (loose inner spongiosa) and histological (numerous remains of calcified cartilage deep into the centrum; radial vascular canals) data. These bones belonged to a basal mosasauroid that cannot be distinguished from Tethysaurus nopcsai to which taxon we tentatively assign the material.

First Evidence of Mosasaurs in Maastrichtian Cretaceous Spain

First finding of Mosasauridae (Squamata) in the Maastrichtian (Cretaceous final) Alicante (Comunidad de Valencia, Levante Spanish)

Authors:

Bardet et al.

Abstract:

Although the genre is known worldwide Prognathodon mosasaur for the end Cretaceous (Campanian-Maastrichtian), we present the first occurence in the Spanish Levante. Previously it was known in Basque contemporary standards. The Ibi specimen corresponds to a tooth of the pterygoid. Although no specific level can identify the tooth belongs to a species of toothed Prognathodon 'thin' as P. compressidens, P. sectorius and P. mosasauroides, the Campanian-Maastrichtian of Europe, or P. Kianda Maastrichtian of Angola.

This was google translated.  Link in spanish.

Bone Histology Hints at Mosasaur Paleobiology and Evolution

Microanatomical and Histological Features in the Long Bones of Mosasaurine Mosasaurs (Reptilia, Squamata) – Implications for Aquatic Adaptation and Growth Rates

Authors:

Alexandra Houssaye, Johan Lindgren, Rodrigo Pellegrini, Andrew H. Lee, Damien Germain and Michael J. Polcyn

Abstract:
Background

During their evolution in the Late Cretaceous, mosasauroids attained a worldwide distribution, accompanied by a marked increase in body size and open ocean adaptations. This transition from land-dwellers to highly marine-adapted forms is readily apparent not only at the gross anatomic level but also in their inner bone architecture, which underwent profound modifications.

Methodology/Principal Findings

The present contribution describes, both qualitatively and quantitatively, the internal organization (microanatomy) and tissue types and characteristics (histology) of propodial and epipodial bones in one lineage of mosasauroids; i.e., the subfamily Mosasaurinae. By using microanatomical and histological data from limb bones in combination with recently acquired knowledge on the inner structure of ribs and vertebrae, and through comparisons with extant squamates and semi-aquatic to fully marine amniotes, we infer possible implications on mosasaurine evolution, aquatic adaptation, growth rates, and basal metabolic rates. Notably, we observe the occurrence of an unusual type of parallel-fibered bone, with large and randomly shaped osteocyte lacunae (otherwise typical of fibrous bone) and particular microanatomical features in Dallasaurus, which displays, rather than a spongious inner organization, bone mass increase in its humeri and a tubular organization in its femora and ribs.

Conclusions/Significance

The dominance of an unusual type of parallel-fibered bone suggests growth rates and, by extension, basal metabolic rates intermediate between that of the extant leatherback turtle, Dermochelys, and those suggested for plesiosaur and ichthyosaur reptiles. Moreover, the microanatomical features of the relatively primitive genus Dallasaurus differ from those of more derived mosasaurines, indicating an intermediate stage of adaptation for a marine existence. The more complete image of the various microanatomical trends observed in mosasaurine skeletal elements supports the evolutionary convergence between this lineage of secondarily aquatically adapted squamates and cetaceans in the ecological transition from a coastal to a pelagic lifestyle.