Tylosaurus, a mosasaur genus from the Late Cretaceous, is a big, predatory marine reptile that is closely related to contemporary monitor lizards and snakes. The name Tylosaurus comes from the ancient Greek τύλoς (tylos) 'protuberance, knob' + Greek σαῦρος (sauros) 'lizard'.
Discovery and Naming
After Clidastes and Platecarpus, Tylosaurus was the third new genus of mosasaur reported from North America, with the first genus being found in Kansas. The notorious conflict between American paleontologists Edward Drinker Cope and Othniel Charles Marsh during the Bone Wars caused issues for the genus throughout its early career as a taxon. Based on a fragmented skull that was provided to Cope by Louis Agassiz of the Harvard Museum of Comparative Zoology, which was about five feet (1.5 meters) in length, and thirteen vertebrae, the type specimen was described by Cope in 1869. The fossil was found in a Niobrara Formation deposit close to Monument Rocks and the Union Pacific Railroad at Fort Hays, Kansas. It is still on display at the same museum under the catalog number MCZ 4374. With Macrosaurus proriger—a previous taxon of European mosasaurs—Cope published the fossil in a fairly succinct initial publication. The specific name proriger, which derives from the Latin word prōra (prow) and suffix -gero (I bear), means "prow-bearing" and refers to the specimen's distinct prow-like extended rostrum. Cope gave a more detailed description of MCZ 4374 in 1870. He shifted the species into the European genus Liodon and proclaimed his initial Macrosaurus proriger to be a synonym without providing any justification. Marsh claimed in 1872 that Liodon proriger should be placed in a different genus since it is taxonomically different from the European genus. Creating the genus Rhinosaurus—a combination of the Ancient Greek terms ῥίς (rhī́s, meaning "nose") and σαῦρoς (saûros, meaning "lizard")—he did this. The name Rhinosaurus means "nose lizard." Based on a fragmentary skeleton he found at the southern section of the Smoky Hill River, which is currently housed in the Yale Peabody Museum under the number YPM 1268, Marsh also classified a third species, which he named Rhinosaurus micromus.
Size
Among the biggest known mosasaurs was Tylosaurus. The biggest known specimen is a T. proriger skeleton known as "Bunker" (KUVP 5033) from the University of Kansas Natural History Museum. Its estimated length ranges from 12 to 15.8 meters (39 to 52 feet). The skeletal remains of a different T. proriger, housed in the Sternberg Museum of Natural History (FHSM VP-2496), might have been considerably larger; Everhart calculated that the specimen originated from an individual that measured 14 meters (46 feet) in height, as opposed to Bunker's estimate of 12 meters (39 feet). Cope's rule, which states that a species' body size should normally grow throughout geologic time, is exhibited by this genus. Early T. nepaeolicus and its progenitors were among the first known members of the Tylosaurus genus in North America between the Turonian and Coniacian (90–86 mya) periods. They were generally 5–7 meters (16–23 feet) long and 200–500 kg (440–1,100 lb) in weight. T. nepaeolicus and the recently discovered T. proriger were 8–9 meters (26–30 feet) long and weighed around 1,100 kilograms (2,400 lb) during the Santonian (86–83 mya). T. proriger reached lengths of 13–14 meters (43–46 feet) during the Early Campanian. Everhart speculated that some very elderly Tylosaurus individuals could have reached an absolute maximum length of 20 meters (66 feet) since mosasaurs continued to grow during their existence. He did, however, emphasize the rarity of any fossil evidence supporting such sizes and the likelihood that none would survive.
Skull
Estimated to be 1.7 meters (5.6 feet) long, Tylosaurus proriger KUVP 5033, also known as the "Bunker" specimen, has the biggest known head. The length of a Tylosaurus head ranged from 13 to 14% of the entire skeleton, depending on the age and individual variance. With the exception of Ectenosaurus, the skull was sharply conical and the snout was proportionately longer than those of other mosasaurs.
Jaws and Teeth
Premaxilla and maxilla are part of the upper jaws; dentary, splenial, coronoid, angular, surangular, and prearticluar-articular are part of the lower jaws (prearticular is fused to the articular, like other squamates). The palatal dentition is housed by the pterygoids, while the marginal dentition is housed by the premaxilla, maxilla, and dentary. Tylosaurus possessed two premaxillary teeth, ten to eleven pterygoid teeth, twelve to thirteen maxillary teeth, and thirteen dentary teeth on either side of the head. With the exception of the pterygoid teeth, which are smaller and more recurved than the marginal teeth, the dentition is homodont, meaning that all teeth are almost the same size and form. Tylosaurine dentaries were long; in mature T. nepaeolicus and T. proriger, the dentary makes up 56–60% of the complete lower jaw's length; in T. pembinensis, it makes up around 55%, while in T. saskatchwanensis, it makes up 62%. The dentary is powerful, however not as much as in Plesiotylosaurus, Prognathodon, or Mosasaurus. Dentary ventral margins vary in shape from straight to slightly concave. In adult T. proriger individuals, a tiny dorsal ridge is seen prior to the first dentary tooth.
Cranium
The extended edentulous rostrum that extends from Tylosaurus's snout—for which the genus is named—is its most distinctive feature. The premaxilla and dentary's elongated front ends combine to make this. At birth, the rostrum was short and sharply pointed, but it quickly grew into an extended, blunt "knob." The large, thick internarial bar, which is made up of the nasals, anterior process of the frontal, and posterodorsal process of the premaxilla, supports the powerfully constructed snout and effectively absorbs shock and transfers stress. Because of this, it has been suggested that the tylosaurine rostrum was elongated for use in ramming opponents or prey. However, new research on Taniwhasaurus suggests that the rostrum was highly sensitive and that using it as a ramming weapon is unlikely. Taniwhasaurus's snout contains a complex neurovascular system. Through sporadic foramina on the rostrum and along the ventral border of the maxilla, above the gum line, the snout contains the terminal branches of the trigeminal nerves.
Postcranial Skeleton
Unlike other taxa (e.g., Prognathodon overtoni), adult Tylosaurus has unfused pelvic and pectoral girdles. A well-developed pubic tubercle, the lack of the anterior emargination of the coracoid, and a scapula that is noticeably smaller than the coracoid are further characteristics that set Tylosaurus apart from other mosasaurs. In comparison to other mosasaurs, tylosaurus limbs are primitive because their stylopodia (humeri and femora) lack the significant proximodistal shortening and sophisticated muscle attachment sites found in other evolved species. While other mosasaurs typically had three to five carpals and tarsals, adult Tylosaurus never had more than two ossified carpal bones (usually only the ulnare, sometimes the ulnare and distal carpal four) and two ossified tarsal bones (usually the astragalus, sometimes the astragalus and distal tarsal four). Tylosaurines are primarily unossified in both carpals and tarsals. Both the forelimbs and the hindlimbs exhibit hyperphalangy, or an increase of phalanges compared to the ancestral state. The phalanges are spindle-shaped as opposed to the short, blocky hourglass-shaped phalanges found in mosasaurines.
Soft Tissue
Since the late 1870s, scale-like fossil evidence of Tylosaurus skin has been described. These scales resembled those of current rattlesnakes and other similar reptiles, with their tiny, diamond-shaped form and oblique row arrangement. On the other hand, the mosasaur's scales were substantially smaller in relation to its entire body. The skin scales of a 5-meter (16-foot)-long specimen measured 3.3 by 2.5 millimeters (0.130 in × 0.098 in). The average number of scales on the underside of the mosasaur was 90 in per square inch (2.54 cm). Every scale has a keeled shape akin to a shark's denticles. This most likely assisted in lowering skin reflection and underwater drag.
Phylogeny
A phylogenetic analysis by Jiménez-Huidobro & Caldwell (2019) that used Tylosaurus species with enough known material to model accurate relationships is used to create the modified cladogram below. T. gaudryi, T. ivoensis, and T. iembeensis were not included in the analysis because of a large amount of missing data, or a lack of material with scoreable phylogenetic characters.
PaleobiologyGrowthIn 2018, Konishi et al. classified specimen FHSM VP-14845, a juvenile with an approximate skull length of 30 centimeters (12 in), to Tylosaurus on the basis of the premaxilla's morphology, the basisphenoid's proportions, and the arrangement of the pterygoid's teeth. The specimen does not, however, have the long premaxillary rostrum that is characteristic of other Tylosaurus. Juveniles of T. nepaeolicus and T. proriger, whose skull lengths range from 16 to 24 inches, have this rostrum. This implies that Tylosaurus rostrum did not arise as a result of sexual selection and that it instead developed quickly at an early period of life. Konishi and associates proposed a role for ramming prey, as does the contemporary orca.
Social Behavior
The majority of Tylosaurus's behavior toward one another appears to have been hostile, as shown by remains showing wounds from fellow dinosaurs. While fossil hunters reported finding these remnants on a regular basis in the late 19th and early 20th centuries, there aren't many instances of them in scholarly collections as specimens. Numerous of these fossils have healed bite marks and wounds that are centered in or close to the head area, suggesting that non-lethal interaction occurred, albeit the reasons for this interaction are still unknown. Certain contemporary lizards may occasionally bite their mate's head during courting, according to research done in 1993 by Rothschild and Martin. On the other hand, they also noticed that certain male lizards use head-biting as a territorial activity to subdue competitors by slugging the head to flip the other over onto its back. It's probable that Tylosaurus exhibited comparable behaviors.
Paleopathology
After analyzing twelve Tylosaurus bones from North America and one from Trinidad and Tobago, Rothschild and Martin (2005) found signs of avascular necrosis in each specimen. For aquatic animals, this disease is frequently a consequence of decompression sickness, which is brought on by the accumulation of bone-damaging nitrogen bubbles in breathed air that is decompressed either by repeated deep diving periods or by brief breathing intervals between dives. Such actions probably resulted in avascular necrosis in the investigated mosasaurs, and as Tylosaurus consistently exhibited them, deep or repetitive diving was probably a common behavioral feature of the species. According to the study, avascular necrosis afflicted 3–15% of the vertebrae in the spinal column of North American Tylosaurus and 16% of the vertebrae in T. bernardi. Although Carlsen (2017) pointed out that the genus had well-developed eardrums that could defend itself from abrupt fluctuations in pressure, he hypothesized that Tylosaurus had avascular necrosis because it lacked the characteristics required for deep or repetitive dives.
Reconstruction of T. pembinensis by Dmitry Bogdanov.
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