海巨龍屬

海巨龍屬 化石時期：晚馬斯垂克期， ~
殘暴海巨龍的頭骨和下頜的合模標本（MNHM.KH.231），產自摩洛哥奧勒德阿卜杜恩盆地
科學分類
界：	動物界 Animalia
門：	脊索動物門 Chordata
綱：	爬蟲綱 Reptilia
目：	有鱗目 Squamata
總科：	†滄龍總科 Mosasauroidea
科：	†滄龍科 Mosasauridae
亞科：	†滄龍亞科 Mosasaurinae
屬：	†海巨龍屬 Thalassotitan Longrich et al., 2022
模式種
†殘暴海巨龍 Thalassotitan atrox Longrich et al., 2022

海巨龍屬（學名：Thalassotitan，意為「海洋巨人」）是一屬已滅絕的大型滄龍，生活於約6600萬年前晚白堊紀馬斯垂克期的摩洛哥。該屬下有且僅有殘暴海巨龍（T. atrox）一個物種，該物種描述自一組發現於奧勒德阿卜杜恩盆地（Ouled Abdoun Basin）的化石。海巨龍屬於傾齒龍族（學名：Prognathodontini），該類群包括傾齒龍等其他滄龍。

該屬的發現表明滄龍類在白堊紀晚期已經演化出了佔據頂級掠食者生態位的物種。嚴重磨損的海巨龍牙齒和發現於正模標本附近的、酸蝕過的化石表明，海巨龍可能會捕食小型滄龍科、蛇頸龍、大型魚類和海龜。

語源學

海巨龍的屬名Thalassotitan組合自古希臘語「θìλασσα」（thálassa，意為「海」）和「τιτìν」（tītā́n，「巨人」），指代海巨龍的巨大體型。種加詞atrox則來自拉丁語，意為「殘忍的」或「無情的」，指代該物種的頂級捕食者地位和化石上因種內搏鬥產生的大量咬痕。

描述

復原後的海巨龍與成年人的大小對比

海巨龍是最大的滄龍之一，其頭骨可達1.3米，身長 可達9-10米。

頭骨

滄龍頭骨結構示意圖

像所有的傾齒龍一樣，海巨龍的頭骨又鈍又結實，前頜骨是承載頭骨頂端的骨頭，從側面看非常短，但從背面看則寬而凸。前頜骨的身體包含許多被稱為神經血管孔的凹坑，這些凹坑被認為容納了對觸摸非常敏感的觸覺神經。內齶杆是前頜骨的一個很長的延伸部分，一直延伸到額骨，它在上頜骨(主要承載牙齒的上頜骨)和外鼻孔(容納鼻孔的開口)之間穿過時很寬，但當它與額骨接觸時變窄成一個細長的杆。在上頜骨 之間，內鼻骨形成一個明顯的低而短的龍骨。上頜骨短而結實，而且很深。它的表面是平的，除了在牙齒上方有一個低而寬的脊襯。

Neurovascular foramina line this margin, increasing in size as they progress towards the back of the skull. The texture of the maxilla's surface is rough, which is especially apparent in larger individual, caused by a network of veined grooves to house blood vessels. The external nares extend from and to above the fourth and twelfth maxillary teeth. The jugal bone, which is located just below the eye, is broad and robust. The frontal bone is short and broad, shaped almost like an isosceles triangle, with large neurovascular foramina at the center. The pineal foramen, which contains the parietal eye, is small and long. The supratemporal fenestrae, large openings between the eyes and the back end of the skull, take up nearly a quarter of the entire skull length and are somewhat triangular. The dentary, the tooth-bearing bone of the lower jaw, is short, wide, robust, and curved concave towards the upper jaws. Many bones of the upper jaw are tightly sutured together; its two tooth-bearing bones the premaxilla and maxilla were connected through interlocking joints containing an unusual series of flanges and grooves while an interdigitating set of tongue-and-groove joints secure the maxilla and prefrontal bone.

牙齒

Skull with teeth shown

Thalassotitan teeth are roughly conical in shape, lightly curved, large in size, and robust in build. They are most similar to the teeth of P. saturator except in being slightly shorter and stockier. Tooth crowns are slightly swollen around its base next to the root, but they do not form a round circumference. The surfaces of the crown are generally smooth but may sometimes have faint ridges depending on individual or ontogenetic variation. The enamel at the tip contain veinous ridges and coarse bumps. Cutting edges are well-developed and finely serrated. Each tooth has two cutting edges, but their positions differ depending on the tooth's position in the jaw. Towards the front of the jaw, the front-facing cutting edges are more pronounced than the diminished back-facing edges. In the middle and near the end of the jaw, both edges are of equal development and located diametrically opposite to each other. At the end of the jaw, the back-facing edges become more pronounced. The tooth roots are massive and barrel-shaped. Deep pits occur within the roots, from which new replacement teeth are formed.

Like all mosasaurs, Thalassotitan had four types of teeth, corresponding to the jaw bones they are located on. On the upper jaw were the premaxillary teeth, maxillary teeth, and pterygoid teeth (located separate from the main jawline near the rear of the skull); while on the lower jaw only the dentary teeth were present. Thalassotitan had in each jaw row from front to back: two premaxillary teeth, twelve maxillary teeth, at least six pterygoid teeth (the pterygoids were not fully preserved), and fourteen dentary teeth. The dentary teeth are generally flatter by the side than the maxillary teeth. Heterodonty is present, meaning that tooth shape changes down the jawline. The first four or five teeth are tall, narrow, and slightly curved, which become stockier, erect, and more robust around the middle of the jawline, then become shorter (as broad as they are tall), hooked, and flatter by the side. The pterygoid teeth are strongly hooked but are also large and robust, nearly approaching the size of the teeth on the main jawlines.

顱後骨骼

The postcranial skeleton is not fully known, only fossils representing a little more than the front half of the body have been found.

The general shape of the vertebrae are typical for mosasaurines. They are procoelous, meaning that the front side is deeply cupped concavely and the back side is bulged convexly. The cervical (neck) vertebrae are slightly wider than long. Its atlas holds rectangular or triangular neural arches; another single tall neural arch is also present at the top of the vertebra. The articular surfaces, which atach to the cartilage that connect vertebrae together, is initially heart-shaped but becomes rounded at the rearmost cervicals. The dorsal (back) vertebrae are slightly longer than wide with tall neural arches, rounded articular surfaces, and large rectangular transverse processes. The ribs are short and robust.

The pectoral girdle is robust and most similar with that in P. overtoni and Mosasaurus conodon, albeit more square-shaped than the latter. The two bones making up the girdle, the scapula and coracoid, are similar in sizes. They loosely contact with each other, but their contact point is nevertheless wider than the glenoid fossa. The scapula is shaped like a square, being as long as it is wide. It lacks a defined scapular neck but expands from front to back, forming a fan-like convex blade. The coracoid is also somewhat squarish and lacks a well-defined neck. Its margins are weakly concave in the front and back but very convex at the bottom.

The forelimbs formed long paddles that resembled mosasaurin mosasaurs like Mosasaurus and Plotosaurus but more primitive in possessing longer but fewer phalanges. The humerus is very stocky and resemble that in P. overtoni except in the expansion of the glenoid condyle beyond the postglenoid process. The radius is unusually shaped for a mosasaur. It is as large as the humerus and much larger than the ulna and takes on a crescent-like or subrectangular form, unlike smaller hourglass-shaped radii in typical mosasaurs.

分類

T. atrox is most closely related to Prognathodon saturator (left) and P. currii (right), and the three species may be congeneric.

Thalassotitan is a member of the Prognathodontini tribe of the Mosasaurinae subfamily, with other members including Prognathodon and Gnathomortis. Morphologically, it is most similar to the giant mosasaurs P. currii and P. saturator, and a phylogenetic analysis by Longrich et al. (2022) recovered Thalassotitan in a clade between the two. This creates an unnatural paraphyletic relationship that reflects a wider issue with the genus Prognathodon as a whole. Several studies over the past decade found that Prognathodon is in general not monophyletic and in need of revision. Longrich et al. (2022) suggested such a revision may include an expansion of the Thalassotitan genus to include P. currii and P. saturator. However, due to a high degree of convergent evolution in the relationship-determining traits among many mosasaurs (especially among prognathodontines), phylogenetic results between each study are seldom consistent, mystifying exactly which species must be revised to stabilize the Prognathodontini. For example, some studies recovered P. currii and P. saturator as phylogenetically unrelated species with either falling outside a monophyletic Prognathodon, while other studies yield variable placements for the type species P. solvayi as either outside a monophyletic P. currii-P. saturator clade in support of Longrich et al. (2022) or within it, which would in theory invalidate Thalassotitan as a junior synonym under the principle of priority.

The following cladogram is modified from Longrich et al. (2022).

Mosasaurinae

Kourisodon       Clidastes       Globidensini     Globidens simplex     Globidens schumani     Globidens phosphaticus     Prognathodon rapax (=Ancylocentrum hungerfordi)         Globidens alambamensis     Globidens dakotensis       Prognathodontini   Gnathomortis         Prognathodon overtoni         Prognathodon saturator       Thalassotitan atrox     Prognathodon currii         Prognathodon giganteus           Prognathodon lutugini     Prognathodon solvayi           Mosasaurini     Moanasaurus     Mosasaurus mokoroa               Mosasaurus conodon     Plesiotylosaurus     Plotosaurus         Mosasaurus missouriensis       Mosasaurus lemonnieri         Mosasaurus hoffmannii     Mosasaurus beaugei       Mosasaurus maximus         Liodon     Mosasaurus sp. (MGGC 21876)             "Magahouanga mosasaurine"         Carinodens     Xenodens

古環境學

Life restoration of Thalassotitan (left) and other animals of the Ouled Abdoun Basin

The phosphate deposits of Morocco have revealed an extremely diverse environment of late maastrichtian age. The oceans of the area were full of an abundance of fish, from bony fish like Enchodus and Stratodus to cartilaginous fish like Cretalamna, Squalicorax and Rhombodus. There was also an abundance of marine reptiles, most notably the mosasaurs, with more than 10 genera alone known from this single site. This possibly suggests that niche partitioning took place here, in which predators take on different niches to avoid competition with one another (for example, mosasaurs like Carniodens and Globidens had blunt teeth for crushing shellfish while Thalassotitan and Mosasaurus hunted much larger food). Other marine reptiles include the elasmosaurid plesiosaur Zarafasaura, the sea turtle Alienochelys and the gavialoid crocodilian Ocepesuchus .

It seems that Thalassotitan was an apex predator in its ecosystem, with evidence being digestive damage found on some of the fossils in the nearby vicinity including those of plesiosaurs, turtles, and large fish. In the skies flew multiple species of pterosaurs including the azhdarchid Phosphatodraco, the nyctosauromorph Alcione and Simurghia, the nyctosaurid Barbaridactylus, and the possible pteranodontid Tethydraco. On land three species of dinosaurs are known, these being the abelisaur Chenanisaurus, the small lambeosaurine hadrosaur Ajnabia and a so far unnamed titanosaur. Thalassotitan lived alongside other giant mosasaurs like Prognathodon, Mosasaurus and Gavialimimus, as well as smaller mosasaurs like Xenodens, Halisaurus and Pluridens.