| Ichthyovenator | |
|---|---|
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| Mountedholotype skeleton at theNational Museum of Nature and Science, Tokyo | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Spinosauridae |
| Subfamily: | †Spinosaurinae |
| Genus: | †Ichthyovenator Allainet al.,2012 |
| Type species | |
| †Ichthyovenator laosensis Allainet al., 2012 | |
Ichthyovenator is agenus ofspinosauriddinosaur that lived in what is nowLaos, sometime between 120 and 113 million years ago, during theAptianstage of theEarly Cretaceousperiod. It is known fromfossils collected from theGrès supérieurs Formation of theSavannakhet Basin, the first of which were found in 2010, consisting of a partial skeleton without theskull or limbs. This specimen became theholotype of the new genus andspeciesIchthyovenator laosensis, and was described bypalaeontologist Ronan Allain and colleagues in2012. The generic name, meaning "fish hunter", refers to its assumedpiscivorous 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, morevertebrae (backbones) and apubic bone from the same individual.
The holotype specimen is estimated to have been between 8.5 to 10.5 metres (28 to 34 feet) long and to have weighed 2.4tonnes (2.6short tons; 2.4long tons).Ichthyovenator's teeth were straight and conical, and its neck resembled that of the closely related genusSigilmassasaurus (which may be synonymous withSpinosaurus). Like others in its family,Ichthyovenator had tallneural spines that formed asail on its back. Unlike other known spinosaurids,Ichthyovenator's sail had asinusoidal (wave-like) shape that curved downwards over the hips and divided into two separate sails. Thepelvic girdle was reduced; theilium—the uppermost body of thepelvis—was proportionately longer than both the pubis and ischium than in other knowntheropod dinosaurs.Ichthyovenator was initially thought to belong to thesubfamilyBaryonychinae but more recent analyses place it as aprimitive member of theSpinosaurinae.
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 andpterosaurs in addition to fish.Ichthyovenator's conspicuous sail might have been used forsexual display orspecies recognition. Fossil evidence suggests spinosaurids, especially spinosaurines, were adapted forsemiaquatic lifestyles. The vertebral spines ofIchthyovenator's tail were unusually tall, suggesting—as in today'scrocodilians—the tail may have aided in swimming.Ichthyovenator lived alongsidesauropod andornithopod dinosaurs, as well asbivalves, fish and turtles.
The firstfossils ofIchthyovenator were found in 2010 atBan Kalum in theGrès supérieurs Formation of theSavannakhet Basin inSavannakhet Province, Laos. These fossilized bones were recovered from ared sandstone layer within a surface area of less than 2 square metres (22 sq ft). Designated under the specimen numbers MDS BK10-01 to 15, they consist of a partlyarticulated, well-preserved skeleton lacking the skull and limbs, and including the third-to-lastdorsal (back) vertebra, theneural spine of the last dorsal vertebra, five partialsacral (hip) vertebrae, the first twocaudal (tail) vertebrae, bothilia (main hip bones), a rightpubis (pubic bone), bothischia (lower and rearmost hip bones) and a posterior dorsalrib. The twelfth dorsal spine is bent sideways when viewed front-to-back due totaphonomic distortion. Thecentra (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 ofIchthyovenator's description, excavations at the site were still ongoing.[1]
After undergoing preparation in 2011, the skeleton was used as the basis, orholotype, for thetype speciesIchthyovenator laosensis, which was named and described in2012 bypalaeontologists Ronan Allain, Tiengkham Xeisanavong, Philippe Richir, and Bounsou Khentavong. Thegeneric name is derived from theOld Greek word ἰχθύς (ichthys), "fish", and the Latin wordvenator, "hunter", in reference to its likelypiscivorous (fish-eating) lifestyle. Thespecific name refers to its provenance from Laos.[1]Ichthyovenator is the third namedspinosauriddinosaur from Asia after theThai genusSiamosaurus in1986 and the Chinese species "Sinopliosaurus"fusuiensis in2009.[1][2][3] The latter may represent the same animal asSiamosaurus.[3][4] In 2014, Allain published a conference paper onIchthyovenator; 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.[5] 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 spinosauridSigilmassasaurus.[6]
In 2016,Gregory S. Paul estimatedIchthyovenator to have been approximately 8.5 metres (28 ft) long and to have weighed 2tonnes (2.2short tons; 2.0long tons).[7] The same year, Rubén Molina-Pérez and Asier Larramendi gave an estimate of 10.5 m (34 ft) in length, 2.95 m (9 ft 8 in) tall at the hips, and 2.4 t (2.6 short tons; 2.4 long tons) in weight.[8]
Ichthyovenator's teeth were conical, straight, and bore no serrations.[5] The front edges of themaxillary anddentary teeth were evident on the base of the tooth crown.[6] The front articulating surfaces ofIchthyovenator'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 fronttubercles (processes forskeletal muscle attachment) and lackedinterzygapophyseal laminae (bony plates), which resulted in their spinopre- and spinopostzygapophysealfossae (depressions) having open undersides. The first dorsal vertebra had extensivetransverse 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 byair sacs in life. Theparapophyses (processes that articulated with thecapitulum 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 mosttheropods, 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 inSigilmassasaurus.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 spinosauridsBaryonyx,Suchomimus,Sigilmassasaurus,[6] andVallibonavenatrix.[9] The cervical neural spines ofIchthyovenator were taller than inSigilmassasaurus andBaryonyx but shared the blade-like shape with those two taxa at the mid-cervicals.[6] 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 thesternum (breast bone).[1]
Like many other spinosaurids,Ichthyovenator had asail on its back and hips that was formed by the elongatedneural spines of its vertebrae. Uniquely among known members of the family,Ichthyovenator's sail was divided in two over the hips and had asinusoid (wave-like) curvature.[1] This is in contrast to the related generaSpinosaurus andSuchomimus—both of which had continuous sails that rose to a peak before sloping down again—andBaryonyx, which showed a less-developed sail, having much lower dorsal spines.Ichthyovenator's preserved dorsal and sacralspinal column, which is over 1 m (3 ft 3 in) 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 54.6-centimetre (21.5 in) high spine of the twelfth dorsal vertebra widened towards the top, giving it atrapezoidal shape[1]—similar to the condition seen in a neural spine fromVallibonavenatrix[9]—whereas the spines of other known spinosaurids were roughly rectangular. It also lacked the forward or backward inclination ofSpinosaurus's neural spines. Its front corner formed a 3-centimetre (1.2 in) narrow-process, pointing upwards. The spine of the thirteenth dorsal vertebra has been only partly preserved; its upper and lower ends had broken off fromtaphonomic 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 21 cm (8.3 in), creating a sudden hiatus in the sail's profile. The spine of the second sacral vertebra steeply curved upwards again, joining the 39 cm (15 in) and 48 cm (19 in) high, broad, fan-shaped spines of the third and fourth sacrals. The 40 cm (16 in) tall spine of the fifth sacral gradually descended. Unlike in the sails of most other theropods, including that ofSuchomimus, the sacral spines were not fused and had no extensive contacts. The second and third sacral centra were fused; thesuture 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.[1]
The sacral sail was continued by the spines of the first two caudal vertebrae, which were 28.4 cm (11.2 in) and 25.7 cm (10.1 in) 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 andamphicoelous (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 asigmoid (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 theprezygapophyses (articular surfaces of the preceding vertebra) and thediapophyses—processes on the sides for rib articulation—a condition that is not observed in other theropods.[1] Some of the caudal vertebrae also had unusually tall, forwardly-extended prezygapophyses.[6]
The 92-centimetre (36 in) long ilium of thepelvis was blade-like, and longer in proportion to the 65-centimetre (26 in) long pubic bone than that of any other known large theropod. Thepostacetabular ala (rear expansion) was much longer than thepreacetabular 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 ofBaryonyx. Thepubic apron—the expanded lower end of the pubis—had a largepubic foramen. On the hind rim of the pubic bone, two openings—theobturator foramen and a lowerfenestra (opening)—were open and notch-like. The ischium was 49.6 cm (19.5 in) long, making it shorter in relation to the pubis than in all other knowntetanuran 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 unexpandedischiadic apron, as inMonolophosaurus andSinraptor.[1] The ischium attached to the ilium via a peg-and-socket-like articulation, unlike the flat, concave condition seen inBaryonyx. This feature was also present inVallibonavenatrix.[9]
In 2012,Ichthyovenator's describers established theunique 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 basalcoelurosaurs andallosauroids, which the describers attributed tomosaic evolution: the evolution of certain anatomical traits at different times in separate species.[1]
Allain and his team consideredIchthyovenator as representing the first unequivocal spinosaurid from Asia.[1] Though prior spinosaurids had been named from the continent—includingSiamosaurus from Thailand'sBarremianSao Khua Formation and "Sinopliosaurus"fusuiensis from China'sAptianXinlong Formation—the authors noted that palaeontologists have debated the validity of these taxa because they are only confidently known from isolated teeth.[1][10] Brazilian palaeontologists Marcos Sales and Caesar Schultz have suggested these teeth may eventually be attributed to spinosaurids similar toIchthyovenator.[11] In addition to tooth fossils, a spinosaurid skeleton that possibly belongs toSiamosaurus was excavated from the ThaiKhok Kruat Formation in 2004[12] and was identified as a definite spinosaur in a 2007 conference abstract byAngela Milner and colleagues, nine years prior toIchthyovenator's description.[13]
In 2012, Allain and colleagues assignedIchthyovenator to theSpinosauridae; more precisely to the subfamilyBaryonychinae in abasal position as thesister taxon of aclade formed byBaryonyx andSuchomimus.[1] In Allain's 2014 abstract, he foundIchthyovenator instead as belonging to theSpinosaurinae, due to the lack of serrations on its teeth and the similarities of its vertebrae to those ofSigilmassasaurus.[5] In a 2015phylogenetic analysis by Evers and colleagues, they suggested the apparent presence of both baryonychine and spinosaurine characteristics inIchthyovenator means the distinction between the two subfamilies may not be as clear as previously thought.[6] In 2017, American palaeontologist Mickey Mortimer informally hypothesizedIchthyovenator may have been a sail-backedcarcharodontosaurid dinosaur closely related toConcavenator, rather than a spinosaurid. Mortimer consideredIchthyovenator asincertae sedis (of uncertain taxonomic affinity) within the cladeOrionides, pending description of the new material, which he states will likely confirmIchthyovenator's spinosaurid identity; however, he no longer has this identification on his website.[14] A 2017 analysis by Sales and Schultz questioned Baryonychinae's validity, citing themorphology of Brazilian spinosauridsIrritator andAngaturama, and suggesting they may have beentransitionary forms between the earlier baryonychines and the later spinosaurines. The authors said with further research, Baryonychinae may be found to be aparaphyletic (unnatural) grouping.[11]Ichthyovenator's spinosaurine classification was supported by Thomas Arden and colleagues in 2019, who resolved it as a basal member of the group due to its tall dorsal sail. Theircladogram can be seen below:[15]
| Spinosauridae |
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Though no skull remains have been found forIchthyovenator, all known spinosaurids had elongated, low, narrow snouts that allowed them to reach far for food and to quickly close their jaws in a manner similar to moderncrocodilians. The tips of spinosaurids' upper and lower jaws fanned out into arosette-like shape that bore long teeth, behind which there was a notch in the upper jaw; this formed a natural trap for prey.[10] Like those of other spinosaurids,Ichthyovenator's straight, unserrated teeth[5] would have been suitable for impaling and capturing small animals and aquatic prey. This type of jaw and tooth morphology, which is also observed in today'sgharials and other fish-eating predators, has led many palaeontologists to believe spinosaurids were largely piscivorous (as implied byIchthyovenator's name).[10][16] This is also evidenced by the discovery ofScheenstia fish scales in the stomach cavity of oneBaryonyx skeleton[10][17] and aSpinosaurus snout that was found with a vertebra from thesclerorynchid fishOnchopristis embedded in it.[18] A moregeneralist diet has also been proposed for spinosaurs, based on fossils such as the bones of a juvenileiguanodontid that was also found in the sameBaryonyx specimen, an "Irritator" tooth embedded in the vertebrae of apterosaur, and tooth crowns fromSiamosaurus that were found in association withsauropod dinosaur bones. It is thus likely that spinosaurids were also scavengers or hunters of larger prey.[10][19][20][21] Though no limb bones are known fromIchthyovenator, all known spinosaurids had well-built arms with enlarged thumb claws, which they likely used to hunt and process prey.[10]
Many possible functions, includingthermoregulation and energy storage, have been proposed for spinosaurid sails.[10][22] In 2012, Allain and colleagues suggested considering the high diversity in neural spine elongation observed in theropod dinosaurs, as well ashistological research done on the sails ofsynapsids (stem mammals),Ichthyovenator's sinusoidal sail may have been used forcourtship display or forrecognising members of its own species.[1] In a 2013 blog post,Darren Naish considered the latter function unlikely, favouring the hypothesis ofsexual selection forIchthyovenator's sail because it appears to have evolved on its own, without very close relatives. Naish also notes that it is possible similar relatives have not yet been discovered.[23]
Spinosaurids appear to have hadsemiaquatic lifestyles, spending much of their time near or in water, which has been inferred by the high density of their limb bones that would have made them less buoyant, and the oxygenisotope ratios of their teeth being closer to those of remains from aquatic animals like turtles, crocodilians, and hippopotamuses than those of other, more terrestrial theropods.[10] Semiaquatic adaptations seem to have been more developed in spinosaurines than baryonychines.[15][24][25] Arden and colleagues in 2019 suggested the shortness ofIchthyovenator's pubis and ischium relative to its ilium, coupled with the elongation of the neural spines in the tails of early spinosaurines, are indications that spinosaurids may have progressively made more use of their tails to propel themselves underwater as they grew more adapted to an aquatic lifestyle.[15] A similar, though more extreme, shrinkage of the pelvic girdle and elongation of the tail's neural spines, creating a paddle-like structure, was observed inSpinosaurus, which appears to have been more aquatic than any other known non-avian (or non-bird) dinosaur.[26]
Ichthyovenator is known from the Barremian toCenomanian Grès supérieurs Formation, and was found in a layer probably dating to the Aptianstage of theEarly Cretaceousperiod between 120 and 113 million years ago.[1] It coexisted with other dinosaurs such as the sauropodTangvayosaurus, and an indeterminate sauropod,iguanodontian, andneoceratopsian.[27][28]Tracks of theropod, sauropod andornithopod dinosaurs, as well as plant remains, are also known from the formation.[1][28] Fossils of non-dinosaurian fauna are represented byray-finned fish likeLanxangichthys[27] andLepidotes, as well as turtles includingShachemys,Xinjiangchelys, and an indeterminatecarettochelyid andtrionychid.[29] ThetrigoniidbivalvesTrigonioides andPlicatounio have also been recovered from the formation.[27] The Grès supérieurs Formation is the lateral equivalent of the neighbouring Khok Kruat andPhu Phan Formations in Thailand; from the Khok Kruat formation, fossils of theropods (including spinosaurids), sauropods, iguanodontians, and freshwater fish have also been recovered.[1][30]
In 2010,Stephen L. Brusatte and colleagues noted the discovery of spinosaurids in Asia, a family previously known only from Europe, Africa, and South America, suggests there was faunal interchange between thesupercontinentsLaurasia in the north andGondwana in the south during the early Late Cretaceous. It may also be possible spinosaurids already had acosmopolitan distribution before the Middle Cretaceous preceding the breakup of Laurasia from Gondwana, but the authors noted more evidence is needed to test this hypothesis.[31] In 2012, Allain and colleagues suggested such a global distribution may have occurred earlier acrossPangaea before theLate Jurassic, even if Asia was the first landmass to be separated during the breakup of the supercontinent.[1] In 2020, Elisabete Malafaia and colleagues also indicated a complexbiogeographical pattern for spinosaurs during the Early Cretaceous based on anatomical similarities betweenIchthyovenator and the European genusVallibonavenatrix.[9]
Media related toIchthyovenator at Wikimedia Commons
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