Tylosaurus
Tylosaurus | |
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Mounted cast of the T. proriger "Bunker" specimen (KUVP 5033) | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Order: | Squamata |
Clade: | †Mosasauria |
Family: | †Mosasauridae |
Clade: | †Russellosaurina |
Subfamily: | †Tylosaurinae |
Genus: | †Tylosaurus Marsh, 1872 |
Type species | |
†Tylosaurus proriger Cope, 1869
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Other species | |
Disputed or unpublished
| |
Synonyms | |
List of synonyms
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Tylosaurus (from the ancient Greek τύλος (tylos) 'protuberance, knob' + Greek σαῦρος (sauros) 'lizard') is a genus of mosasaur, a large, predatory marine reptile closely related to modern monitor lizards and snakes, from the Late Cretaceous.
Discovery and naming
Tylosaurus was the third new genus of mosasaur to be described from North America behind Clidastes and Platecarpus and the first in Kansas.[9] The early history of the genus as a taxon was subject to complications spurred by the infamous rivalry between American paleontologists Edward Drinker Cope and Othniel Charles Marsh during the Bone Wars.[9][10] The type specimen was described by Cope in 1869 based on a fragmentary skull measuring nearly 5 feet (1.5 m) in length and thirteen vertebrae lent to him by Louis Agassiz of the Harvard Museum of Comparative Zoology.[11] The fossil, which remains in the same museum under the catalog number MCZ 4374, was recovered from a deposit of the Niobrara Formation located in the vicinity of Monument Rocks[12] near the Union Pacific Railroad at Fort Hays, Kansas.[13] Cope's first publication of the fossil was very brief and was named Macrosaurus proriger, the genus being a preexisting European mosasaur taxon.[9][11] The specific epithet proriger means "prow-bearing", which is in reference to the specimen's unique prow-like elongated rostrum[14][10] and is derived from the Latin word prōra (prow) and suffix -gero (I bear).[15] In 1870, Cope published a more thorough description of MCZ 4374. Without explanation, he moved the species into another European genus Liodon and declared his original Macrosaurus proriger a synonym.[9][13]
In 1872, Marsh argued that Liodon proriger is taxonomically distinct from the European genus and must be assigned a new one. For this, he erected the genus Rhinosaurus, which means "nose lizard" and is a
Description
Size
Tylosaurus was one of the largest known mosasaurs. The largest well-known specimen, a skeleton of T. proriger from the
Other Campanian-Maastrichtian species were similarly large. The most recent maximum estimate for T. bernardi is 12.2 meters (40 ft) by Lindgren (2005); historically the species was erroneously estimated at even larger sizes of 15–17 meters (49–56 ft).[32] A reconstruction of T. saskatchwanensis by the Royal Saskatchewan Museum estimated a total length of over 9.75 meters (32.0 ft).[33] A mounted skeleton of T. pembinensis, nicknamed "Bruce," at the Canadian Fossil Discovery Centre measures at 13.05 meters (42.8 ft) long and was awarded a Guinness World Records for "Largest mosasaur on display" in 2014.[34] However, the skeleton was assembled for display prior[35] to Bullard and Caldwell (2010)'s reassessment that found the species' number of vertebrae to be exaggerated.[36] T. 'borealis' is estimated at 6.5–8 meters (21–26 ft) in total length.[37]
Skull
The largest known skull of Tylosaurus is T. proriger KUVP 5033 (the "Bunker" specimen), estimated at 1.7 meters (5.6 ft) long.[38] Depending on age and individual variation,[38] Tylosaurus skulls were between 13 and 14% of the total skeleton length.[39] The head was strongly conical and the snout proportionally longer than most mosasaurs, with the exception of Ectenosaurus.[40]
Jaws and teeth
The upper jaws include the
Tylosaurine dentaries were elongate; the dentary is between 56 and 60% of total length of the entire lower jaw in adult T. nepaeolicus and T. proriger,[38] about 55% in T. pembinensis,[43] and 62% in T. saskatchwanensis.[44] The dentary is robust, though not as strongly built as it is in Mosasaurus, Prognathodon, or Plesiotylosaurus.[45] The ventral margin of the dentary ranges from straight[6] to slightly concave.[36][46] A small dorsal ridge appears anterior to the first dentary tooth in mature individuals of T. proriger.[38]
The marginal dentition of most species is adapted for cutting large marine vertebrates,
Bardet et al. (2006) classified Tylosaurus species into two morphological groups based on marginal dentition. The North American 'proriger group' includes T. proriger and T. nepaeolicus and is characterized by teeth with smooth or faint facets, less prominent carinae, and a vein-like network of primitive striations extending to near the tip.[52] The group was originally defined as having slender teeth,[52] but subsequent research has since recognized that slenderness is an ontogenetic trait in T. proriger with robust teeth appearing in adult forms.[50] Though not formally classified within a group, the marginal teeth of T. saskatchwanensis shares a comparable morphology with T. proriger.[44] The second is the Euro-American 'ivoensis group' and consists of T. ivoensis, T. gaudryi, and T. pembinensis. Their teeth are robust with prominent carinae with striations on the lingual and occasionally labial sides that do not reach the tooth's tip, and facets on the labial side.[52] The facets are gentle in T. pembinensis,[36] while in T. ivoensis they are slightly concave.[42] The latter feature is also known as fluting.[53] Marginal teeth in T. gaudryi are virtually indistinguishable from those in T. ivoensis.[42] T. iembeensis was not placed within either group; no further description is known of its teeth other than having striations and no facets.[52] The distinction of an 'ivoensis group' is contentious. Caldwell et al. (2008) argued that T. pembinensis cannot be compared with T. ivoensis as the former's teeth are not fluted, and that T. ivoensis is more allied with the distinctively fluted teeth of Taniwhasaurus.[53] Jiménez-Huidobro and Caldwell (2019) listed the absence of marginal fluting as a diagnostic (taxon-identifying) trait that differentiates Tylosaurus from Taniwhasaurus.[6]
The pterygoid teeth may have enabled ratchet feeding, in which the upper teeth held prey in place as the lower jaw slides back and forth via a streoptostylic jaw joint.[54] The bases of the pterygoid teeth are nearly circular, and each tooth is divided into front and back-facing sides of near-equal surface area via a pair of faint buccal and lingual carinae, except in T. gaudryi, in which the teeth are mediolaterally compressed.[42] Carinae are not serrated.[51][5] The anterior surface tends to be either smooth of faintly faceted, while the posterior surface is striated.
Cranium
The most recognizable characteristic of Tylosaurus is the elongated
The premaxilla, maxilla, and frontal bones border the external nares, or body nostril openings; unlike other mosasaurs, the prefrontal bones are excluded from the border of the nares by a long posterodorsal process of the maxilla.[6] The nares open above the fourth maxillary tooth anteriorly in T. proriger and T. pembinensis,[36][a] between the third and fourth tooth in T. nepaeolicus,[36] and posterior to the fourth tooth in T. bernardi.[51] Nare length relative to skull length varied between species: it is proportionally short in T. proriger (20-27% skull length[61][42]), T. bernardi (24% skull length[36]), and T. gaudryi (25-27% skull length),[42] and long in T. pembinensis (28-31% skull length).[36] The nasal bones were either free-floating or lightly articulated to the internarial bar,[46] did not contact the frontal,[61] and were not fused to each other as they are in extant varanid lizards. The nasals' loose association with the rest of the skull in Tylosaurus and other mosasaurs may be why the bones are frequently lost and therefore exceedingly rare;[61] Tylosaurus is one of the only mosasaurs in which the nasal bones are clearly documented;[46] the other is the holotype of Plotosaurus, although one of the bones is missing.[62]
The external nares lead to the
The frontal bone in Tylosaurus usually, but not always, possesses a low midline crest. It is most prominent in T. proriger,
The quadrate bones (homologous to the incus in mammals) are located at the back of the skull, articulating the lower jaw to the cranium[66] and holding the eardrums.[67] The complex anatomy of the bone[68] renders it extremely diagnostic, even to the species level.[6] In lateral view, the quadrate resembles a hook in immature T. nepaeolicus and T. proriger individuals, but in adult forms for both species[38] and in T. bernardi,[6][51] T. pembinensis,[36] and T. saskatchweanensis it takes on a robust oval-like shape.[44] The eardrum (tympanum) attached to the lateral sufrace of the bone within a bowl-like depression called the alar conch.[67] The conch is shallow in T. nepaeolicus,[68] T. proriger, and T. bernardi,[6] and deep in T. pembinensis[68] and T. saskatchewanensis.[6] The alar rim is thin in T. nepaeolicus, T. proriger,[38] and T. bernardi,[51][38] and thick in T. bernardi, T. pembinensis,[51] and T. saskatchewanensis.[44] The suprastapedial process is a hook-like extension of bone that curves posteroventrally from the apex of the shaft into an incomplete loop, and it likely served as the attachment point for the depressor mandibulae muscles that opened the lower jaw.[36][69][68] The process is slender and proportionally long in immature T. nepaeolicus and T. proriger, and thickened as the animals matured.[38] The process is of similar length to T. proriger in T. saskatchwanensis[44] and shorter in T. bernardi.[51] In T. pembinensis, it abruptly turns medially at a 45° downward angle.[36] A similar deflection appears in some juvenile T. nepaeolicus quadrates.[48] Emerging from the posteroventral margin of the alar conch is the infrastapedial process. Its shape appears to changes ontogenetically in T. nepaeolicus and T. proriger; in the former, the process is absent in juveniles but appears as a small bump in adults, while in T. proriger, it is present as a subtle point in juveniles of and becomes a distinctively broad semicircle in adults.[38] The process is small in T. bernardi,[51] and in T. pembinensis[36] and T. saskatchewanensis,[44] it is rounded. In T. saskatchewanensis, the suprastapedial process almost touches the infrastapedial process.[44] At the bottom of the shaft is the mandibular condyle, which forms the joint between the quadrate and the lower jaw. It is rounded in shape in adults.[68][51][44][38] On the medial surface of the bone, a thick, pillar-like vertical ridge often protrudes beyond the dorsal margin of the quadrate so that it is visible in lateral view.[68]
Postcranial skeleton
Both pectoral and pelvic girdles are unfused in adult Tylosaurus, in contrast to other taxa (e.g.,
Tylosaurus limbs are primitive relative to other mosasaurs; their
Tylosaurus had 29 to 30 presacral
Soft tissue
Skin and coloration
Fossil evidence of the skin of Tylosaurus in the form of scales has been described since the late 1870s. These scales were small and diamond-shaped and were arranged in oblique rows, comparable to that found in modern rattlesnakes and other related reptiles. However, the scales in the mosasaur were much smaller in proportion to the whole body.[75][76] An individual measuring 5 meters (16 ft) in total body length had dermal scales measuring 3.3 by 2.5 millimeters (0.130 in × 0.098 in),[77] and in each square inch (2.54 cm) of the mosasaur's underside an average of ninety scales were present.[75] Each scale was keeled in a form resembling that of a shark's denticles.[76] This probably helped reduce underwater drag[76] and reflection on the skin.[78]
Microscopic analysis of scales in a T. nepaeolicus specimen by Lindgren et al. (2014) detected high traces of the pigment
Respiratory system
AMNH FR 221 preserves parts of the cartilaginous respiratory system. This includes parts of the larynx (voice box), trachea (windpipe), and bronchi (lung airways). They were however only briefly described in the preserved position by Osborn (1899). The larynx is poorly preserved; a piece of its cartilage first appears below just between the pterygoid and quadrate and extends to behind the latter. This connects to the trachea, which appears below the atlas vertebra but is not preserved afterwards. The respiratory tract reappears below the fifth rib as a pair of bronchi and extends to just behind the as-preserved coracoids where preservation is lost.[73] The pairing is suggestive of two functional lungs like modern limbed lizards but unlike snakes.[79] Similar branching is also found in Platecarpus[79] and putatively Mosasaurus, the only two other derived mosasaurs with their respiratory systems documented.[80] The bifurcation point for the Tylosaurus specimen is anywhere between the first and sixth cervical vertebrae.[c][73] In Platecarpus, the bronchi probably diverged below the sixth cervical into near-parallel pairs,[81] while in Mosasaurus the organ is dislocated.[80] A bifurcation point's position ahead of the forelimbs would be unlike terrestrial lizards, whose point is within the chest region, but similar to the short trachea and parallel bronchi of whales.[79]
Classification
Taxonomy
Tylosaurus is classified within the family
Tylosaurus was among the earliest derived mosasaurs. The oldest fossil attributable to the genus is a premaxilla (
Phylogeny and evolution
In 2020, Madzia and Cau performed a
In the Western Interior Seaway, two species—T. nepaeolicus and T. proriger—may represent a
The following cladogram is modified from a phylogenetic analysis by Jiménez-Huidobro & Caldwell (2019) using Tylosaurus species with sufficiently known material to model accurate relationships; T. gaudryi, T. ivoensis, and T. iembeensis were excluded from the analysis due to extensive missing data (i.e., lack of material with scoreable phylogenetic characters).[6]
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Paleobiology
Growth
Konishi and colleagues in 2018 assigned specimen FHSM VP-14845, a small juvenile with an estimated skull length of 30 centimeters (12 in), to Tylosaurus based on the shape of the premaxilla, the proportions of the basisphenoid, and the arrangement of the teeth on the pterygoid. However, the specimen lacks the characteristically long premaxillary rostrum of other Tylosaurus, which is present in juveniles of T. nepaeolicus and T. proriger with skull lengths of 40–60 cm (16–24 in). This suggests that Tylosaurus rostrum grew rapidly at an early stage in life, and also suggests that it did not develop due to sexual selection. Konishi and colleagues suggested a function in ramming prey, as employed by the modern orca.[56]
Metabolism
Nearly all squamates are characterized by their cold-blooded
Mobility
Scientists previously interpreted Tylosaurus as an anguilliform swimmer that moved by undulating its entire body like a snake due to its close relationship with the animal. However, it is now understood that Tylosaurus actually used carangiform locomotion, meaning that the upper body was less flexible and movement was largely concentrated at the tail like in mackerels. A BS thesis by Jesse Carpenter published in 2017 examined the vertebral mobility of T. proriger spinal columns and found that the dorsal vertebrae were relatively rigid but the cervical, pygal, and caudal vertebrae were more liberal in movement, indicating flexibility in the neck, hip, and tail regions. This contrasted with more derived mosasaurs like Plotosaurus, whose vertebral column was stiff up to the hip. Interestingly, an examination of a juvenile T. proriger found that its cervical and dorsal vertebrae were much stiffer than those in adult specimens. This may have been an evolutionary adaptation among young individuals; a more rigid tail-based locomotion is associated with faster speed, and this would allow vulnerable juveniles to better escape predators or catch prey. Older individuals would see their spine grow in flexibility as predator evasion becomes less important for survival.[89]
Tylosaurus likely specialized as an
A 1988 study by Judith Massare attempted to calculate the sustained swimming speed, the speed at which the animal moves without tiring, of Tylosaurus through a series of mathematical models incorporating hydrodynamic characteristics and estimations of locomotive efficiency and metabolic costs. Using two T. proriger specimens, one 6.46 meters (21.2 ft) long and the other 6.32 meters (20.7 ft), she calculated a consistent average maximum sustained swimming speed of 2.32 m/s (5.2 mph). However, when testing whether the models represented an accurate framework, they were found to exaggerated. This was primarily because the variables accounting for drag may have been underestimated; estimation of drag coefficients for an extinct species can be difficult as it requires a hypothetical reconstruction of the morphological dimensions of the animal. Massare predicted that the actual sustained swimming speed of Tylosaurus was somewhere near half the calculated speed.[90]
Feeding
One of the largest marine carnivores of its time, Tylosaurus was an apex predator that exploited the wide variety of marine fauna in its ecosystem. Stomach contents are well documented in the genus, which includes other mosasaurs, plesiosaurs, turtles, birds, bony fish, and sharks.[91] Additional evidence from bite marks suggests the animal also preyed on giant squid[92] and ammonites.[93]
The enormous and varied appetite of Tylosaurus can be demonstrated in a 1987 find that identified fossils of a mosasaur measuring 2 meters (6.6 ft) or longer, the diving bird Hesperornis, a Bananogmius fish, and possibly a shark all within the stomach of a single T. proriger skeleton (SDSM 10439) recovered from the Pierre Shale of South Dakota.[e][46][91][96] Other records of stomach contents include a sea turtle in a T. bernardi-like species,[f][91] a 2.5 meters (8.2 ft) long Dolichorhynchops in another (8.8 metres (29 ft) long) T. proriger,[29] partially digested bones and scales of a Cimolichthys in a third T. proriger,[94] partially digested vertebrae of a Clidastes in a fourth T. proriger, remains of three Platecarpus individuals in a T. nepaeolicus,[28] and Plioplatecarpus bones in a T. saskatchewanensis.[33][97] Puncture marks on fossils of ammonites,[93] the carapace of a Protostega,[98] and the gladius of an Enchoteuthis have been attributed to Tylosaurus.[92]
Pasch and May (2001) reported bite marks from a dinosaur skeleton known as the
Social behavior
The behavior of Tylosaurus towards each other may have been mostly aggressive, evidenced by fossils with injuries inflicted by another of their own kind. Such remains were frequently reported by fossil hunters during the late 19th and early 20th centuries, but few examples reside as specimens in scientific collections. Many of these fossils consist of healed bite marks and wounds that are concentrated around or near the head region, implying that there were the result of non-lethal interaction, but the motives of such contact remain speculative. In 1993, Rothschild and Martin noted that some modern lizards affectionately bite their mate's head during courtship, which can sometimes result in injuries. Alternatively, they also observed that some males lizards also employ head-biting as territorial behavior against rivals in a show of dominance by grappling the head to turn over the other on its back. It is possible that Tylosaurus behaved in similar ways.[28]
Lingham-Soliar (1992) noted suggestions that use of the combat-oriented elongated rostrum of Tylosaurus was not exclusive to hunting and that it may have also been applied in sexual behavior through battles over female mates between males.[46] However, he observed the elongated rostrum was invariably present in all individuals regardless of sex,[46] and subsequent studies by Konishi et al. (2018) and Zietlow (2020) confirmed this pattern.[56][38] This would imply that sexual selection was not a driver in its evolution and instead refined through sex-independent means.[56]
At least one fatal instance of intraspecific combat among Tylosaurus is documented in the T. kansasensis holotype FHSM VP-2295, representing a 5 meters (16 ft) long animal, which possesses numerous injuries that indicate it was killed by a larger Tylosaurus. The skull roof and surrounding areas exhibit signs of trauma in the form of four massive gouges, and the dentary contains at least seven puncture wounds and gouges. These pathologies are characteristic of bite marks from a larger Tylosaurus that measured around 7 meters (23 ft) in length. The largest of the marks are about 4 centimeters (1.6 in) in length, matching the size of large mosasaur teeth, and they are positioned along two lines that converge close to 30°, matching the angle that each jaw converges towards in a mosasaur skull. In addition, FHSM VP-2295 suffered damage to its neck: the cervical vertebrae were found articulated at an unnatural angle of 40° relative to the long axis of the skull. The pattern of preservation makes it unlikely that the condition of the vertebrae was a result of disturbances by scavengers and instead indicates damage caused by a violently twisted neck during life. In a reconstructed scenario, the larger Tylosaurus would have first attacked at an angle slightly below the left side of the victim's head. This impact would cause the victim's skull to roll to the right side, allowing the aggressor to sink its teeth into the skull roof and right lower jaw, crushing the jaw and causing further breaks of nearby bones, such as the pterygoid, and the twisting of the jaw outwards, which would cause the quadrate to detach from its position and for the spinal cord to twist and sever at the skull's base, leading to a swift death.[28]
Paleopathology
Examining 12 North American Tylosaurus skeletons and one T. bernardi skeleton, Rothschild and Martin (2005) identified evidence of avascular necrosis in every individual. For aquatic animals, this condition is often a result of decompression illness, which is caused when bone-damaging nitrogen bubbles build up in inhaled air that is decompressed either by frequent deep-diving trips or by intervals of repetitive diving and short breathing. The studied mosasaurs likely gained avascular necrosis through such behaviors, and given its invariable presence in Tylosaurus it is likely that deep or repetitive diving was a general behavioral trait of the genus. The study observed that between 3-15% of vertebrae in the spinal column of North American Tylosaurus and 16% of vertebrae in T. bernardi were affected by avascular necrosis.[101] Carlsen (2017) posited that Tylosaurus gained avascular necrosis because it lacked the necessary adaptations for deep or repetitive diving, although noted that the genus had well-developed eardrums that could protect themselves from rapid changes in pressure[102]
Unnatural fusion of some vertebrae in the tail has been reported in some Tylosaurus skeletons. A variation of these fusions may concentrate near the end of the tail to form a single mass of multiple fused vertebrae called a "club tail." Rothschild and Everhart (2015) surveyed 23 North American Tylosaurus skeletons and one T. bernardi skeleton and found that five of the North American skeletons exhibited fused tail vertebrae. The condition was not found in T. bernardi, but this does not rule out its presence due to the low sample size. Vertebral fusion occurs when the bones remodel themselves after damage from trauma or disease. However, the cause of such events can vary between individuals and/or remain hypothetical. One juvenile specimen with the club tail condition was found with a shark tooth embedded in the fusion, which confirms that at least some cases were caused by infections inflicted by predator attacks. The majority of vertebral fusion cases in Tylosaurus were caused by bone infections, but some cases may have alternatively been caused by any type of joint disease such as arthritis. However, evidence of joint disease was rare in Tylosaurus when compared to mosasaurs such as Plioplatecarpus and Clidastes.[103] Similar amassing of remodeled bone is also documented in bone fractures in other body parts. One T. kansasensis specimen possesses two fractured ribs that fully healed. Another T. proriger skull shows a fractured snout, probably caused by ramming into a hard object such as a rock. Presence of some healing indicates that the individual survived for some extended time before death. The injury in a snout region containing many nerve endings would have inflicted extreme pain.[104]
Notes
- ^ Behind the fifth tooth in the holotype.[60]
- ^ In one juvenile T. proriger specimen, it appears at the bottom of the vertical ramus instead.[48]
- ^ Latter corresponds to the fifth rib in Osborn (1899).[73]
- ^ The 2018 MS thesis of Cyrus Green disputes the notion that Clidastes was an endotherm based on the skeletochronology of the genus, finding that its growth rates were too low to be endothermic and instead similar to ectotherms. The dissertation argued that the high body temperatures calculated by Harrell et al. (2016) were a result of gigantothermy. However, only four specimens were studied, and Clidastes is considered a basal mosasaur.[88]
- ^ Identification of the mosasaur and shark vary. Scientists have identified the mosasaur as either a Platecarpus,[28] Clidastes,[94] or Latoplatecarpus.[91] The shark is either interpreted as a Cretalamna,[94] a sand shark,[95] or of uncertain identity.[91]
- ^ Usually identified as Hainosaurus sp.;[91] Lingham-Soliar (1992) identifies the species as T. bernardi.[46]
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Cited books
- Ellis, R. (2003). Sea Dragons: Predators of the Prehistoric Oceans. University Press of Kansas. ISBN 978-0-7006-1394-6.
- Everhart, M.J. (2017). Oceans of Kansas, Second Edition: A Natural History of the Western Interior Sea (Life of the Past). Indiana University Press. ISBN 978-0-253-02632-3.
- Leidy, J. (1873). Contributions to the extinct vertebrate fauna of the western interior territories: Report of the United States Geological Survey of the Territories. U.S. Government Printing Office. .
- Mann, J.; Conner, R.C.; Tyack, P.L.; Whitehead, H. (2000). Cetacean Societies: Field Studies of Dolphins and Whales. University of Chicago Press. ISBN 978-0-226-50341-7.
- Russell, D.A. (1967). Systematics and Morphology of American Mosasaurs (PDF). Vol. 23. Bulletin of the Peabody Museum of Natural History. ISBN 978-1-933789-44-6. Archived from the original(PDF) on June 24, 2016. Retrieved December 24, 2020.