Ichthyovenator

''Ichthyovenator'' is a genus of spinosaurid dinosaur that lived in what is now Laos, sometime between 125 and 113 million years ago, during the Aptian stage of the Early Cretaceous period. It is known from fossils collected from the Grès supérieurs Formation of the Savannakhet Basin, the first of which were found in 2010, consisting of a partial skeleton without the skull or limbs. This specimen became the holotype of the new genus and species ''Ichthyovenator laosensis'', and was described by palaeontologist Ronan Allain and colleagues in 2012. The generic name, meaning "fish hunter", refers to its assumed piscivorous lifestyle, while the specific name alludes to the country of Laos. In 2014, it was announced that more remains from the dig site had been recovered; these fossils included teeth, more vertebrae (backbones) and a pubic bone from the same individual.

The holotype specimen is estimated to have been between long and to have weighed . ''Ichthyovenator''s teeth were straight and conical, and its neck resembled that of the closely related genus ''Sigilmassasaurus''. Like others in its family, ''Ichthyovenator'' had tall neural spines that formed a sail on its back. Unlike other known spinosaurids, ''Ichthyovenator'' sail had a sinusoidal (wave-like) shape that curved downwards over the hips and divided into two separate sails. The pelvic girdle was reduced; the ilium—the uppermost body of the pelvis—was proportionately longer than both the pubis and ischium than in other known theropod dinosaurs. ''Ichthyovenator'' was initially thought to belong to the subfamily Baryonychinae but more recent analyses place it as a primitive member of the Spinosaurinae.

As a spinosaur, ''Ichthyovenator'' would have had a long, shallow snout and robust forelimbs. Its diet likely mainly consisted of aquatic prey, hence its etymology. Spinosaurids are also known to have eaten small dinosaurs and pterosaurs in addition to fish. ''Ichthyovenator''s conspicuous sail might have been used for sexual display or species recognition. Fossil evidence suggests spinosaurids, especially spinosaurines, were adapted for semiaquatic lifestyles. The vertebral spines of ''Ichthyovenator''s tail were unusually tall, suggesting—as in today's crocodilians—the tail may have aided in swimming. ''Ichthyovenator'' lived alongside sauropod and ornithopod dinosaurs, as well as bivalves, fish and turtles.

Discovery and naming

The first fossils of ''Ichthyovenator'' were found in 2010 at Ban Kalum in the Grès supérieurs Formation of the Savannakhet Basin in Savannakhet Province, Laos. These fossilized bones were recovered from a red sandstone layer within a surface area of less than . Designated under the specimen numbers MDS BK10-01 to 15, they consist of a partly articulated, well-preserved skeleton lacking the skull and limbs, and including the third-to-last dorsal (back) vertebra, the neural spine of the last dorsal vertebra, five partial sacral (hip) vertebrae, the first two caudal (tail) vertebrae, both ilia (main hip bones), a right pubis (pubic bone), both ischia (lower and rearmost hip bones) and a posterior dorsal rib. The twelfth dorsal spine is bent sideways when viewed front-to-back due to taphonomic distorsion. The centra (vertebral bodies) of the sacrals are largely incomplete due to erosion, but preserved all of their accompanying spines with their upper edges intact. At the time of ''Ichthyovenator''s description, excavations at the site were still ongoing.

After undergoing preparation in 2011, the skeleton was used as the basis, or holotype, for the type species ''Ichthyovenator laosensis'', which was named and described in 2012 by palaeontologists Ronan Allain, Tiengkham Xeisanavong, Philippe Richir, and Bounsou Khentavong. The generic name is derived from the Old Greek word ἰχθύς (''ichthys''), "fish", and the Latin word ''venator'', "hunter", in reference to its likely piscivorous (fish-eating) lifestyle. The specific name refers to its provenance from Laos. ''Ichthyovenator'' is the third named spinosaurid dinosaur from Asia after the Thai genus ''Siamosaurus'' in 1986 and the Chinese species "''Sinopliosaurus''" ''fusuiensis'' in 2009. The latter may represent the same animal as ''Siamosaurus''. In 2014, Allain published a conference paper on ''Ichthyovenator''; the abstract indicated additional remains from the original individual had been found after excavations continued in 2012. These remains include three teeth, the left pubis, and many vertebrae, including a nearly complete neck, the first dorsal vertebra, and seven more caudal vertebrae. Some of these additional vertebrae were compared with those of other spinosaurids in a 2015 paper by German palaeontologist Serjoscha Evers and colleagues, in which they noted similarities with the vertebrae of the African spinosaurid ''Sigilmassasaurus''.

Description

In 2016, Gregory S. Paul estimated ''Ichthyovenator'' to have been approximately long and to have weighed . The same year, Rubén Molina-Pérez and Asier Larramendi gave an estimate of in length, tall at the hips, and in weight.

''Ichthyovenator''s teeth were conical, straight, and bore no serrations. The front edges of the maxillary and dentary teeth were evident on the base of the tooth crown. The front articulating surfaces of ''Ichthyovenator''s rear cervical and front dorsal vertebrae were one-and-a-half times wider than they were high and wider than the length of their centra. They also bore robust front tubercles ( processes for skeletal muscle attachment) and lacked interzygapophyseal laminae (bony plates), which resulted in their spinopre- and spinopostzygapophyseal fossae (depressions) having open undersides. The first dorsal vertebra had extensive transverse processes (wing-like projections that articulate with the ribs), as well as deep excavations at the front and back of its base that were filled by air sacs in life. The parapophyses (processes that articulated with the capitulum of the ribs) increased in height from the rear cervicals to the first dorsal; its underside remained in contact with the front lower edge of the centrum. This is unlike the condition in most theropods, in which the parapophyses shifted towards the top of the vertebra during the transition from cervical to dorsal vertebrae. All of these features were also present in ''Sigilmassasaurus''. ''Ichthyovenator''s mid-cervical vertebrae had elongate, somewhat wider-than-tall centra that became progressively shorter towards the rear of the neck, as well as well-developed keels on their bottom surfaces, traits that were shared with the spinosaurids '' Baryonyx, Suchomimus'', ''Sigilmassasaurus'', and ''Vallibonavenatrix''. The cervical neural spines of ''Ichthyovenator'' were taller than in ''Sigilmassasaurus'' and ''Baryonyx'' but shared the blade-like shape with those two taxa at the mid-cervicals. The holotype dorsal rib, which was found near the twelfth dorsal vertebra, had a head typical of the ribs of other moderate-to-large-sized theropods. The rib shaft formed a half-circle. The rib's lower end was slightly expanded both sideways and to the front and back. This condition, which differs from the tapered, pointed tips seen in the ribs of other theropods, suggests the rearmost dorsal ribs articulated with the complex of the sternum (breast bone).

Like many other spinosaurids, ''Ichthyovenator'' had a sail on its back and hips that was formed by the elongated neural spines of its vertebrae. Uniquely among known members of the family, ''Ichthyovenator''s sail was divided in two over the hips and had a sinusoid (wave-like) curvature. This is in contrast to the related genera '' Spinosaurus'' and ''Suchomimus''—both of which had continuous sails that rose to a peak before sloping down again—and ''Baryonyx'', which showed a less-developed sail, having much lower dorsal spines. ''Ichthyovenator''s preserved dorsal and sacral spinal column, which is over long, shows a very high spine on the twelfth dorsal vertebra representing a crest that rose from the back and a lower rounded sail that extended from the sacral vertebrae of the hips; the latter sail's apex was located above the third and fourth sacrals. The high spine of the twelfth dorsal vertebra widened towards the top, giving it a trapezoidal shape—similar to the condition seen in a neural spine from ''Vallibonavenatrix''—whereas the spines of other known spinosaurids were roughly rectangular. It also lacked the forward or backward inclination of ''Spinosaurus''s neural spines. Its front corner formed a narrow-process, pointing upwards. The spine of the thirteenth dorsal vertebra has been only partly preserved; its upper and lower ends had broken off from taphonomic causes. From its general shape, the describers inferred however, it was about as long as the preceding spine. This would imply the back edge of the front sail would form a rectangular corner, because the spine of the much lower first sacral vertebra is about , creating a sudden hiatus in the sail's profile. The spine of the second sacral vertebra steeply curved upwards again, joining the and high, broad, fan-shaped spines of the third and fourth sacrals. The tall spine of the fifth sacral gradually descended. Unlike in the sails of most other theropods, including that of ''Suchomimus'', the sacral spines were not fused and had no extensive contacts. The second and third sacral centra were fused; the suture connecting them was still visible. Only the rearmost two dorsal vertebrae are preserved, so to what extent the dorsal sail continued towards the front of the animal's back is unknown.

The sacral sail was continued by the spines of the first two caudal vertebrae, which were and tall respectively. They leaned backwards at 30 degrees and had smooth front and rear edges. The centra of the first two caudals were much broader than long and amphicoelous (deeply concave on both ends). The caudal transverse processes were prominent and sturdy and were angled upwards towards the rear of the vertebrae. The transverse processes of the first caudal vertebra, when seen from above, had a sigmoid (or S-shaped) profile. The prespinal and postspinal fossae (bony depressions in front and behind the neural spines) were confined to a position above the base of the neural spines. The sides of the first caudal were also deeply hollowed out between the prezygapophyses (articular surfaces of the preceding vertebra) and the diapophyses—processes on the sides for rib articulation—a condition that is not observed in other theropods. Some of the caudal vertebrae also had unusually tall, forwardly-extended prezygapophyses. The long ilium of the pelvis was blade-like, and longer in proportion to the long pubic bone than that of any other known large theropod. The postacetabular ala (rear expansion) was much longer than the preacetabular ala (front expansion), which had a shelf on its central surface that formed the middle edge of the preacetabular fossa. Viewed distally (towards the centre of attachment), the lower end of the pubis had an L-shape resembling that of ''Baryonyx''. The pubic apron—the expanded lower end of the pubis—had a large pubic foramen. On the hind rim of the pubic bone, two openings—the obturator foramen and a lower fenestra (opening)—were open and notch-like. The ischium was long, making it shorter in relation to the pubis than in all other known tetanuran theropods. The ischium's main body was large and extensive—atypical of the Y-shaped upper ischia of other tetanurans—and bore an oval-shaped obturator foramen in its side. The shaft of the ischium was flattened sideways, and had an unexpanded ischiadic apron, as in ''Monolophosaurus'' and ''Sinraptor''. The ischium attached to the ilium via a peg-and-socket-like articulation, unlike the flat, concave condition seen in ''Baryonyx''. This feature was also present in ''Vallibonavenatrix''.

Classification

In 2012, ''Ichthyovenator'' describers established the unique derived traits of the genus: its dorsal and sacral sinusoidal sail; the thirteenth dorsal neural spine being 410% the length of the centrum, and its distinct, finger-shaped process on its front upper corner; the broad, expanded tips of the third and fourth sacral spines; the first caudal vertebra's deep prezygapophyseal and centrodiapophyseal fossae and S-shaped transverse processes in top view; and the higher ratio of length between the ilium and the accompanying pubis than in any other known theropod. Allain and colleagues also identified some anatomical features that are unique among other known tetanuran theropods, including the rearmost dorsal ribs articulating with the sternal complex, the pubis's main body having obturator and pubic openings, and the ischium having a foramen on its upper end and a shaft that was flattened sideways. The shrinkage of the pubis and ischium relative to the pelvis has been observed in basal coelurosaurs and allosauroids, which the describers attributed to mosaic evolution: the evolution of certain anatomical traits at different times in separate species.

Allain and his team considered ''Ichthyovenator'' as representing the first unequivocal spinosaurid from Asia. Though prior spinosaurids had been named from the continent—including ''Siamosaurus'' from Thailand's Barremian Sao Khua Formation and "''Sinopliosaurus''" ''fusuiensis'' from China's Aptian Xinlong Formation

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