Mosasaur
Mosasaurs | |
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Mounted skeleton of a Plesioplatecarpus planifrons )
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Order: | Squamata |
Clade: | †Mosasauria |
Superfamily: | †Mosasauroidea |
Family: | †Mosasauridae Gervais, 1853 |
Subgroups | |
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Mosasaurs (from Latin Mosa meaning the 'Meuse', and Greek σαύρος sauros meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. They belong to the order Squamata, which includes lizards and snakes.
During the last 20 million years of the Cretaceous period (
Description
Mosasaurs breathed air, were powerful swimmers, and were well-adapted to living in the warm, shallow
The smallest-known mosasaur was Dallasaurus turneri, which was less than 1 m (3.3 ft) long. Larger mosasaurs were more typical, with many species growing longer than 4 m (13 ft). Mosasaurus hoffmannii, the largest known species reached up to 17 m (56 ft),[3] but it has been considered to be probably overestimated by Cleary et al. (2018).[4] Currently, the largest publicly exhibited mosasaur skeleton in the world is on display at the Canadian Fossil Discovery Centre in Morden, Manitoba. The specimen, nicknamed "Bruce", is just over 15 m (49 ft) long,[5] but this might be an overestimate as the skeleton was assembled for display prior to a 2010 reassessment of the species that found its original number of vertebrae to be exaggerated, implying that the actual size of the animal was likely smaller.[6][7]
Mosasaurs had a body shape similar to that of modern-day monitor lizards (varanids), but were more elongated and streamlined for swimming. Their limb bones were reduced in length and their paddles were formed by webbing between their long finger and toe bones. Their tails were broad, and supplied their locomotive power.
Until recently, mosasaurs were assumed to have swum in a method similar to the one used today by
Early reconstructions showed mosasaurs with dorsal crests running the length of their bodies, which were based on misidentified remains of tracheal cartilage. By the time this error was discovered, depicting mosasaurs with such crests in artwork had already become a trend.[11][12]
Paleobiology
Mosasaurs had double-hinged jaws and flexible skulls (much like those of
One of the food items of mosasaurs were
Virtually all forms were active predators of fish and ammonites; a few, such as Globidens, had blunt, spherical teeth, specialized for crushing mollusk shells. The smaller genera, such as Platecarpus and Dallasaurus, which were about 1–6 m (3.3–19.7 ft) long, probably fed on fish and other small prey. The smaller mosasaurs may have spent some time in fresh water, hunting for food. The largest mosasaur Mosasaurus hoffmannii was the apex predator of the Late Cretaceous oceans, reaching more than 11 metres (36 ft) in length and weighing up to 10 metric tons (11 short tons) in body mass.[13]
Soft tissue
Despite the many mosasaur remains collected worldwide, knowledge of the nature of their skin coverings remains in its early stages. Few mosasaurid specimens collected from around the world retain fossilized scale imprints. This lack may be due to the delicate nature of the scales, which nearly eliminates the possibility of preservation, in addition to the preservation sediment types and the marine conditions under which the preservation occurred. Until the discovery of several mosasaur specimens with remarkably well-preserved scale imprints from late Maastrichtian deposits of the Muwaqqar Chalk Marl Formation of Harrana[14] in Jordan, knowledge of the nature of mosasaur integument was mainly based on very few accounts describing early mosasaur fossils dating back to the upper Santonian–lower Campanian, such as the famous Tylosaurus specimen (KUVP-1075) from Gove County, Kansas.[15]
Material from Jordan has shown that the bodies of mosasaurs, as well as the membranes between their fingers and toes, were covered with small, overlapping, diamond-shaped scales resembling those of snakes. Much like those of modern reptiles, mosasaur scales varied across the body in type and size. In Harrana specimens, two types of scales were observed on a single specimen: keeled scales covering the upper regions of the body and smooth scales covering the lower.[14] As ambush predators, lurking and quickly capturing prey using stealth tactics,[16] they may have benefited from the nonreflective, keeled scales.[14] Additionally, mosasaurs had large pectoral girdles, and such genera as Plotosaurus may have used their front flippers in a breaststroke motion to gain added bursts of speed during an attack on prey.[17]
More recently, a fossil of
In 2011, collagen protein was recovered from a Prognathodon humerus dated to the Cretaceous.[18]
In 2005, a
Metabolism
A study published in 2016 by T. Lyn Harrell, Alberto Pérez-Huerta and Celina Suarez showed that mosasaurs were endothermic. The study contradicted findings published in 2010 indicating mosasaurs were ectothermic. The 2010 study did not use warm-blooded animals for comparison but analogous groups of common marine animals. Based on comparisons with modern warm-blooded animals and fossils of known cold-blooded animals from the same time period, the 2016 study found mosasaurs likely had body temperatures similar to those of contemporary seabirds and were able to internally regulate their temperatures to remain warmer than the surrounding water.[20]
Coloration
The coloration of mosasaurs was unknown until 2014, when the findings of Johan Lindgren of
Teeth
Mosasaurs possessed a thecodont dentiton, meaning that the roots were cemented deeply into the jaw bone. Mosasaurs did not use permanent teeth but instead constantly shed them. Replacement teeth developed within a pit inside the roots of the original tooth called the resorption pit. This is done through a distinctively unique eight-stage process. The first stage was characterized by the mineralization of a small tooth crown developed elsewhere that descended into the resorption pit by the second stage. In the third stage, the developing crown firmly cemented itself within the resorption pit and grew in size; by the fourth stage, it would be of the same size as the crown in the original tooth. Stages five and six were characterized by the development of the replacement tooth's root: in stage five the root developed vertically, and in stage six the root expanded in all directions to the point that the replacement tooth became exposed and actively pushed on the original tooth. In the seventh stage, the original tooth was shed and the now-independent replacement tooth began to anchor itself into the vacancy. In the eighth and final stage, the replacement tooth has grown to firmly anchor itself.[22]
Ontogeny and growth
Mosasaur growth is not well understood, as specimens of juveniles are rare, and many were mistaken for hesperornithine birds when discovered 100 years ago. However, the discovery of several specimens of juvenile and neonate-sized mosasaurs unearthed more than a century ago indicate that mosasaurs gave birth to live young, and that they spent their early years of life out in the open ocean, not in sheltered nurseries or areas such as shallow water as previously believed. Whether mosasaurs provided parental care, like other marine reptiles such as plesiosaurs, is currently unknown. The discovery of young mosasaurs was published in the journal Palaeontology.[23]
Possible eggs
A 2020 study published in
Environment
Paleontologists compared 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.[27]
Sea levels were high during the Cretaceous period, causing marine transgressions in many parts of the world, and a great inland seaway in what is now North America. Mosasaur fossils have been found in the Netherlands, Belgium, Denmark, Portugal, Sweden, South Africa, Spain, France, Germany, Poland, the Czech Republic, Italy[28] Bulgaria, the United Kingdom,[29][30] Russia, Ukraine, Kazakhstan, Azerbaijan,[31] Japan,[32] Egypt, Israel, Jordan, Syria,[33] Turkey,[34] Niger,[35][36] Angola, Morocco, Australia, New Zealand, and on Vega Island off the coast of Antarctica. Tooth taxon Globidens timorensis is known from the island of Timor; however, the phylogenetic placement of this species is uncertain and it might not even be a mosasaur.[37]
Mosasaurs have been found in Canada in Manitoba and Saskatchewan[38] and in much of the contiguous United States. Complete or partial specimens have been found in Alabama, Mississippi, New Jersey, Tennessee, and Georgia, as well as in states covered by the Cretaceous seaway: Texas, southwest Arkansas, New Mexico, Kansas,[39] Colorado, Nebraska, South Dakota, Montana, Wyoming, and the Pierre Shale/Fox Hills formations of North Dakota.[40] Lastly, mosasaur bones and teeth are also known from California, Mexico, Colombia,[41] Brazil,[33] Peru, and Chile.[42]
Many of the so-called 'dinosaur' remains found on New Zealand are actually mosasaurs and plesiosaurs[citation needed], both being Mesozoic predatory marine reptiles.
The largest mosasaur currently on public display is Bruce, a 65-70%-complete specimen of Tylosaurus pembinensis dating from the late Cretaceous Period, approximately 80 million years ago, and measuring 13.05 m (42.815 ft) from nose tip to tail tip. Bruce was discovered in 1974 north of Thornhill, Manitoba, Canada, and resides at the nearby Canadian Fossil Discovery Centre in Morden, Manitoba. Bruce was awarded the Guinness Record for the largest mosasaur on public display in 2014.[43]
Discovery
The first publicized discovery of a partial fossil mosasaur skull in 1764 by quarry workers in a subterranean gallery of a limestone quarry in Mount Saint Peter, near the Dutch city of Maastricht, preceded any major dinosaur fossil discoveries, but remained little known. However, a second find of a partial skull drew the Age of Enlightenment's attention to the existence of fossilized animals that were different from any known living creatures. When the specimen was discovered between 1770 and 1774, Johann Leonard Hoffmann, a surgeon and fossil collector, corresponded about it with the most influential scientists of his day, making the fossil famous. The original owner, though, was Godding, a canon of Maastricht cathedral.
When the French
The Maastricht limestone beds were rendered so famous by the mosasaur discovery, they have given their name to the final six-million-year epoch of the Cretaceous, the Maastrichtian.
Classification
Relationship with modern squamates
Lower classifications
The traditional view of mosasaur evolution held that all paddle-limbed (hydropedal) mosasaurs originated from a single common ancestor with functional legs (plesiopedal). However, this was shaken with the discovery of Dallasaurus, a plesiopedal mosasauroid more closely related to the Mosasaurinae than other mosasaurs. Bell and Polycn (2005) grouped these outside mosasaurs into two clades: the Russellosaurina, whose basal members include plesiopedal genera (Tethysaurinae) of their own and derived members consisting of the Plioplatecarpinae and Tylosaurinae; and the Halisauromorpha, containing the Halisaurinae. The placement of Dallasaurus suggested that the Russellosaurina and Halisauromorpha may have evolved a hydropedal form independently, the former through the tethysaurines, meaning that their placement within the Mosasauridae creates an unnatural polyphyly and thus potentially invalid.[44][45] Caldwell informally proposed in a 2012 publication that the definition of a mosasaur must thus be redefined into one that does not consider russellosaurines and halisauromorphs as true mosasaurs, but as an independent group of marine lizards.[45]
However, phylogenetic studies of mosasaurs can be fickle, especially when wild card taxa like Dallasaurus remain poorly understood. For example, some studies such as a 2009 analysis by Dutchak and Caldwell instead found that Dallasaurus was ancestral to both russellosaurines and mosasaurines,[46] although results were inconsistent in later studies.[47] A 2017 study by Simoes et al. noted that utilization of different methods of phylogenetic analyses can yield different findings and ultimately found an indication that tethysaurines were a case of hydropedal mosasaurs reversing back to a plesiopedal condition rather than an independent ancestral feature.[47]
The following cladograms illustrate the two views of mosasaur evolution. Topology A follows an ancestral state reconstruction from an implied weighted maximum parsimony tree by Simoes et al. (2017), which contextualizes a single marine origin with tethysaurine reversal.[47] Topologies B and C illustrate the multiple-origins hypothesis of hydropedality; the former follows Makádi et al. (2012),[48] while the latter follows a PhD dissertation by Mekarski (2017) that experimentally includes dolichosaur and poorly-represented aigialosaur taxa.[49] Placement of major group names follow definitions by Madzia and Cau (2017).[50]
Ancestral state reconstruction by Simoes et al. (2017) | |||||||||||||||||||||
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Free-standing iliac crest
Ambiguous state |
Strict consensus of maximum parsimony by Makádi et al. (2012) | |||||||||||||||||||||||||||||||||||||||||||||
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Plesiopedal and plesiopelvic taxa
Plesiopedal and hydropelvic taxa
Hydropedal and hydropelvic taxa
Taxa with unknown limb structure |
Maximum clade credibility tree by Mekarski (2017) | |||||||||||||||||||||||||||||||||||||||||||||||||||
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'Dolichosaur' taxa
'Aigialosaur' taxa
Mosasaur taxa |
Phylogeny
The following diagram illustrates simplified phylogenies of the three major mosasaur groups as recovered by Strong et al. (2020), Longrich et al. (2021), and Longrich et al. (2022).
Implied weighting maximum parsimony by Strong et al. (2020)[51] | |||
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Strict consensus of maximum parsimony by Longrich et al., (2021)[52] |
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Maximum parsimony by Longrich et al. (2022)[53] |
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Distribution
Though no individual genus or subfamily is found worldwide, the Mosasauridae as a whole achieved global distribution during the Late Cretaceous with many locations typically having complex mosasaur faunas with multiple different genera and species in different ecological niches.
Two African countries are particularly rich in mosasaurs: Morocco[54] and Angola.[55][56]
References
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External links
This article's use of external links may not follow Wikipedia's policies or guidelines. (June 2021) |
- Palaeos: Vertebrates: Mosasaurs
- BBC Science and Nature: Mosasaurs
- Mike Everhart and David Lewis, "Mesozoic marine monsters of the Mangahouanga": New Zealand fossil fauna
- Mike Everhart, "A day in the life of a Mosasaur": life in the Sea of Kansas, illus. by Carl Buell
- Mike Everhart, "Mosasaurus hoffmani" until 1829.
- "The Mosasaur of Maastricht" by Hennie Reuvers in Crossroads web magazine
- "Mosasaurs terrorized Cretaceous rivers" Planet Earth online
- Kansas Geological Survey Vol IV (1899), containing the famous summary of American mosasaurs by Samuel Williston.
- William R. Wahl * MOSASAUR BITE MARKS ON AN AMMONITE. PRESERVATION OF AN ABORTED ATTACK?
- Mosasaur diet