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[[File:Ichthyovenator vertebrae.jpg|thumb|alt=White casts of spinal column with wave-like sail laid out on a dark background in a museum exhibit|Casts of the vertebrae seen from behind]]
[[File:Ichthyovenator vertebrae.jpg|thumb|alt=White casts of spinal column with wave-like sail laid out on a dark background in a museum exhibit|Casts of the vertebrae seen from behind]]


Though no skull remains have been found for ''Ichthyovenator'', all known spinosaurids had elongated, low and narrow snouts which allowed them to reach far for food and close their jaws quickly, akin to modern [[Crocodilia|crocodilians]]. The tips of their upper and lower jaws fanned out into a [[Rosette (design)|rosette]]-like shape that bore long teeth, behind which there was a notch in the upper jaw; this formed a natural trap for prey.<ref name="hone2017" /> ''Ichthyovenator''{{'}}s straight and unserrated teeth,<ref name=":0" /> like those of other spinosaurids, would have been suitable for impaling and capturing small animals and aquatic prey. This type of jaw and tooth morphology, also observed in today's [[gharials]] and other fish-eating predators, has led many palaeontologists to believe that spinosaurids were largely piscivorous (as implied in ''Ichthyovenator''{{'}}s name).<ref name="hone2017" /><ref>{{Cite journal|last=Cuff|first=Andrew R.|last2=Rayfield|first2=Emily J.|date=2013|title=Feeding Mechanics in Spinosaurid Theropods and Extant Crocodilians|journal=PLOS One|language=en|volume=8|issue=5|pages=e65295|doi=10.1371/journal.pone.0065295|issn=1932-6203|pmc=3665537|pmid=23724135|bibcode=2013PLoSO...865295C}}</ref> This is also evidenced by ''[[Scheenstia]]'' fish scales discovered in the stomach cavity of one ''Baryonyx'' skeleton,<ref name="hone2017" /><ref>{{cite journal|last1=López-Arbarello|first1=A.|year=2012|title=Phylogenetic Interrelationships of Ginglymodian Fishes (Actinopterygii: Neopterygii)|journal=PLOS ONE|volume=7|issue=7|pages=e39370|bibcode=2012PLoSO...739370L|doi=10.1371/journal.pone.0039370|pmc=3394768|pmid=22808031}}</ref> and a ''Spinosaurus'' snout found with a vertebra from the [[sawfish]] ''[[Onchopristis]]'' embedded in it.<ref name="dalsassoetal05">{{cite journal|last=dal Sasso|first=C.|author2=Maganuco, S.|author3=Buffetaut, E.|author4=Mendez, M.A.|year=2005|title=New information on the skull of the enigmatic theropod ''Spinosaurus'', with remarks on its sizes and affinities|journal=Journal of Vertebrate Paleontology|volume=25|issue=4|pages=888–896|doi=10.1671/0272-4634(2005)025[0888:NIOTSO]2.0.CO;2|issn=0272-4634}}</ref> A more [[generalist diet]] for spinosaurs has also been proposed, based on fossils such as juvenile ''[[Iguanodon]]'' bones also found in the aforementioned ''Baryonyx'' specimen, and an ''Irritator'' tooth embedded in the vertebrae of a [[pterosaur]]. Thus, it is likely that spinosaurids were also scavengers and hunters of larger prey than fish.<ref name="hone2017" /><ref name="milner2007">{{Cite journal|last=Milner|first=Andrew|last2=Kirkland|first2=James|date=September 2007|title=The case for fishing dinosaurs at the St. George Dinosaur Discovery Site at Johnson Farm|url=https://www.researchgate.net/publication/285906225|journal=Utah Geological Survey Notes|volume=39|pages=1–3|via=}}</ref><ref>{{cite journal|last1=Sales|first1=M. A. F.|last2=Lacerda|first2=M. B.|last3=Horn|first3=B. L. D.|last4=de Oliveira|first4=I. A. P.|last5=Schultz|first5=C. L.|last6=Bibi|first6=F.|date=2016|title=The "χ" of the Matter: Testing the Relationship between Paleoenvironments and Three Theropod Clades|journal=PLOS One|volume=11|issue=2|pages=e0147031|doi=10.1371/journal.pone.0147031|pmc=4734717|pmid=26829315|bibcode=2016PLoSO..1147031S}}</ref> Though no limb bones are known from ''Ichthyovenator'', all known spinosaurids had well-built arms with enlarged thumb claws, which they likely used to hunt and process prey.<ref name="hone2017">{{Cite journal|last=Hone|first=David William Elliott|last2=Holtz|first2=Thomas Richard|date=June 2017|title=A century of spinosaurs – a review and revision of the Spinosauridae with comments on their ecology|journal=Acta Geologica Sinica – English Edition|language=en|volume=91|issue=3|pages=1120–1132|doi=10.1111/1755-6724.13328|issn=1000-9515}}</ref>
Though no skull remains have been found for ''Ichthyovenator'', all known spinosaurids had elongated, low and narrow snouts which allowed them to reach far for food and close their jaws quickly, akin to modern [[Crocodilia|crocodilians]]. The tips of their upper and lower jaws fanned out into a [[Rosette (design)|rosette]]-like shape that bore long teeth, behind which there was a notch in the upper jaw; this formed a natural trap for prey.<ref name="hone2017" /> ''Ichthyovenator''{{'}}s straight and unserrated teeth,<ref name=":0" /> like those of other spinosaurids, would have been suitable for impaling and capturing small animals and aquatic prey. This type of jaw and tooth morphology, also observed in today's [[gharials]] and other fish-eating predators, has led many palaeontologists to believe that spinosaurids were largely piscivorous (as implied in ''Ichthyovenator''{{'}}s name).<ref name="hone2017" /><ref>{{Cite journal|last=Cuff|first=Andrew R.|last2=Rayfield|first2=Emily J.|date=2013|title=Feeding Mechanics in Spinosaurid Theropods and Extant Crocodilians|journal=PLOS One|language=en|volume=8|issue=5|pages=e65295|doi=10.1371/journal.pone.0065295|issn=1932-6203|pmc=3665537|pmid=23724135|bibcode=2013PLoSO...865295C}}</ref> This is also evidenced by ''[[Scheenstia]]'' fish scales discovered in the stomach cavity of one ''Baryonyx'' skeleton,<ref name="hone2017" /><ref>{{cite journal|last1=López-Arbarello|first1=A.|year=2012|title=Phylogenetic Interrelationships of Ginglymodian Fishes (Actinopterygii: Neopterygii)|journal=PLOS ONE|volume=7|issue=7|pages=e39370|bibcode=2012PLoSO...739370L|doi=10.1371/journal.pone.0039370|pmc=3394768|pmid=22808031}}</ref> and a ''Spinosaurus'' snout found with a vertebra from the [[sawfish]] ''[[Onchopristis]]'' embedded in it.<ref name="dalsassoetal05">{{cite journal|last=dal Sasso|first=C.|author2=Maganuco, S.|author3=Buffetaut, E.|author4=Mendez, M.A.|year=2005|title=New information on the skull of the enigmatic theropod ''Spinosaurus'', with remarks on its sizes and affinities|journal=Journal of Vertebrate Paleontology|volume=25|issue=4|pages=888–896|doi=10.1671/0272-4634(2005)025[0888:NIOTSO]2.0.CO;2|issn=0272-4634}}</ref> A more [[generalist diet]] for spinosaurs has also been proposed, based on fossils such as the bones of a juvenile [[Iguanodontidae|iguanodontid]] also found in the aforementioned ''Baryonyx'' specimen, an ''Irritator'' tooth embedded in the vertebrae of a [[pterosaur]], and tooth crowns from ''Siamosaurus'' found in association with [[Sauropoda|sauropod]] dinosaur bones. Thus, it is likely that spinosaurids were also scavengers or hunters of larger prey.<ref name="hone2017" /><ref name="milner2007">{{Cite journal|last=Milner|first=Andrew|last2=Kirkland|first2=James|date=September 2007|title=The case for fishing dinosaurs at the St. George Dinosaur Discovery Site at Johnson Farm|url=https://www.researchgate.net/publication/285906225|journal=Utah Geological Survey Notes|volume=39|pages=1–3|via=}}</ref><ref>{{cite journal|last1=Sales|first1=M. A. F.|last2=Lacerda|first2=M. B.|last3=Horn|first3=B. L. D.|last4=de Oliveira|first4=I. A. P.|last5=Schultz|first5=C. L.|last6=Bibi|first6=F.|date=2016|title=The "χ" of the Matter: Testing the Relationship between Paleoenvironments and Three Theropod Clades|journal=PLOS One|volume=11|issue=2|pages=e0147031|doi=10.1371/journal.pone.0147031|pmc=4734717|pmid=26829315|bibcode=2016PLoSO..1147031S}}</ref><ref name=":12">{{Cite journal|last1=Buffetaut|first1=Eric|last2=Suteethorn|first2=Varavudh|date=1999|title=The dinosaur fauna of the Sao Khua Formation of Thailand and the beginning of the Cretaceous radiation of dinosaurs in Asia|url=https://www.sciencedirect.com/science/article/pii/S0031018299000048|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|language=en|volume=150|issue=1–2|pages=13–23|bibcode=1999PPP...150...13B|doi=10.1016/S0031-0182(99)00004-8|issn=0031-0182|via=}}</ref> Though no limb bones are known from ''Ichthyovenator'', all known spinosaurids had well-built arms with enlarged thumb claws, which they likely used to hunt and process prey.<ref name="hone2017">{{Cite journal|last=Hone|first=David William Elliott|last2=Holtz|first2=Thomas Richard|date=June 2017|title=A century of spinosaurs – a review and revision of the Spinosauridae with comments on their ecology|journal=Acta Geologica Sinica – English Edition|language=en|volume=91|issue=3|pages=1120–1132|doi=10.1111/1755-6724.13328|issn=1000-9515}}</ref>


[[File:CrocoLoco-swimming-01.jpg|thumb|left|Three images of a crocodile in the water at different stages of swimming sequence as it propels itself with its tail|''Ichthyovenator'' might have used its tail, which had elongated neural spines, for propulsion through water, similar to [[crocodilians]] such as this [[Nile crocodile]]<ref name="arden2018" />]]
[[File:CrocoLoco-swimming-01.jpg|thumb|left|Three images of a crocodile in the water at different stages of swimming sequence as it propels itself with its tail|''Ichthyovenator'' might have used its tail, which had elongated neural spines, for propulsion through water, similar to [[crocodilians]] such as this [[Nile crocodile]]<ref name="arden2018" />]]
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[[File:Caudal vertebrae Tangvayosaurus hoffeti.jpg|thumb|Fossils of the [[sauropod]] ''[[Tangvayosaurus]]'', another dinosaur from the [[Gres superieurs Formation]], [[Savannakhet]] Dinosaur Museum]]
[[File:Caudal vertebrae Tangvayosaurus hoffeti.jpg|thumb|Fossils of the [[sauropod]] ''[[Tangvayosaurus]]'', another dinosaur from the [[Gres superieurs Formation]], [[Savannakhet]] Dinosaur Museum]]


''Ichthyovenator'' is known from the Barremian to [[Cenomanian]] Grès supérieurs Formation, and was found in a layer probably dating to the Aptian [[Stage (stratigraphy)|stage]] of the [[Early Cretaceous]] [[Geological period|Period]], between 125 to 113 million years ago.<ref name="AXRK12" /> It coexisted with other dinosaurs such as the [[Sauropoda|sauropod]] ''[[Tangvayosaurus]]'', and an indeterminate sauropod, [[Iguanodontia|iguanodontian]], and [[neoceratopsian]].<ref name=":2">{{Cite journal|last=Cavin|first=Lionel|last2=Deesri|first2=Uthumporn|last3=Veran|first3=Monette|last4=Khentavong|first4=Bounsou|last5=Jintasakul|first5=Pratueng|last6=Chanthasit|first6=Phornphen|last7=Allain|first7=Ronan|date=2019-03-04|title=A new Lepisosteiformes (Actinopterygii: Ginglymodi) from the Early Cretaceous of Laos and Thailand, SE Asia|journal=Journal of Systematic Palaeontology|volume=17|issue=5|pages=393–407|doi=10.1080/14772019.2018.1426060|issn=1477-2019}}</ref><ref name="cretaceousdistribution" /> [[Fossil trackway|Trackways]] of sauropod and [[Ornithopoda|ornithopod]] dinosaurs, as well as plant remains, are also known from the formation.<ref name="AXRK12" /><ref name="cretaceousdistribution">Weishampel, David B; et al. (2004). "Dinosaur distribution (Early Cretaceous, Asia)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 563-570. {{ISBN|0-520-24209-2}}.</ref> Fossils of non-dinosaurian fauna are represented by [[Ray finned fish|ray-finned fish]] like ''[[Lanxangichthys]]''<ref name=":2" /> and ''[[Lepidotes]]'', as well as turtles, including ''[[Shachemys]]'', ''[[Xinjiangchelys]]'', and an indeterminate [[carettochelyid]] and [[Trionychidae|trionychid]].<ref>{{Cite journal|last=de Lapparent de Broin|first=France|date=2004-09-01|title=A new Shachemydinae (Chelonii, Cryptodira) from the Lower Cretaceous of Laos: preliminary data|url=http://www.sciencedirect.com/science/article/pii/S1631068304000934|journal=Comptes Rendus Palevol|language=en|volume=3|issue=5|pages=387–396|doi=10.1016/j.crpv.2004.05.004|issn=1631-0683}}</ref> The [[Trigoniida|trigoniid]] [[Bivalvia|bivalves]] ''[[Trigonioides]]'' and ''[[Plicatounio]]'' have also been recovered from the formation.<ref name=":2" /> The Grès supérieurs Formation is the lateral equivalent of the neighboring Khok Kruat and [[Phu Phan Formation]]s in Thailand; from the former of which fossils of theropods (including spinosaurids), sauropods, iguanodontians, and freshwater fish have also been recovered.<ref name="AXRK12" /><ref name=":7">{{Cite journal|last=Buffetaut|first=Eric|last2=Suteethorn|first2=Varavudh|last3=Le Loeuff|first3=Jean|last4=Khansubha|first4=Sasa-On|last5=Tong|first5=Haiyan|last6=Wongko|first6=K|date=2005-01-01|title=The dinosaur fauna from the Khok Kruat Formation (Early Cretaceous) of Thailand|url=https://www.researchgate.net/publication/312976683|journal=Proceedings of the International Conference on Geology, Geotechnology and Mineral Resources of Indochina|pages=575–581}}</ref>
''Ichthyovenator'' is known from the Barremian to [[Cenomanian]] Grès supérieurs Formation, and was found in a layer probably dating to the Aptian [[Stage (stratigraphy)|stage]] of the [[Early Cretaceous]] [[Geological period|Period]], between 125 to 113 million years ago.<ref name="AXRK12" /> It coexisted with other dinosaurs such as the sauropod ''[[Tangvayosaurus]]'', and an indeterminate sauropod, [[Iguanodontia|iguanodontian]], and [[neoceratopsian]].<ref name=":2">{{Cite journal|last=Cavin|first=Lionel|last2=Deesri|first2=Uthumporn|last3=Veran|first3=Monette|last4=Khentavong|first4=Bounsou|last5=Jintasakul|first5=Pratueng|last6=Chanthasit|first6=Phornphen|last7=Allain|first7=Ronan|date=2019-03-04|title=A new Lepisosteiformes (Actinopterygii: Ginglymodi) from the Early Cretaceous of Laos and Thailand, SE Asia|journal=Journal of Systematic Palaeontology|volume=17|issue=5|pages=393–407|doi=10.1080/14772019.2018.1426060|issn=1477-2019}}</ref><ref name="cretaceousdistribution" /> [[Fossil tracks|Tracks]] of theropod, sauropod and [[Ornithopoda|ornithopod]] dinosaurs, as well as plant remains, are also known from the formation.<ref name="AXRK12" /><ref name="cretaceousdistribution">Weishampel, David B; et al. (2004). "Dinosaur distribution (Early Cretaceous, Asia)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 563-570. {{ISBN|0-520-24209-2}}.</ref> Fossils of non-dinosaurian fauna are represented by [[Ray finned fish|ray-finned fish]] like ''[[Lanxangichthys]]''<ref name=":2" /> and ''[[Lepidotes]]'', as well as turtles, including ''[[Shachemys]]'', ''[[Xinjiangchelys]]'', and an indeterminate [[carettochelyid]] and [[Trionychidae|trionychid]].<ref>{{Cite journal|last=de Lapparent de Broin|first=France|date=2004-09-01|title=A new Shachemydinae (Chelonii, Cryptodira) from the Lower Cretaceous of Laos: preliminary data|url=http://www.sciencedirect.com/science/article/pii/S1631068304000934|journal=Comptes Rendus Palevol|language=en|volume=3|issue=5|pages=387–396|doi=10.1016/j.crpv.2004.05.004|issn=1631-0683}}</ref> The [[Trigoniida|trigoniid]] [[Bivalvia|bivalves]] ''[[Trigonioides]]'' and ''[[Plicatounio]]'' have also been recovered from the formation.<ref name=":2" /> The Grès supérieurs Formation is the lateral equivalent of the neighboring Khok Kruat and [[Phu Phan Formation]]s in Thailand; from the former of which fossils of theropods (including spinosaurids), sauropods, iguanodontians, and freshwater fish have also been recovered.<ref name="AXRK12" /><ref name=":7">{{Cite journal|last=Buffetaut|first=Eric|last2=Suteethorn|first2=Varavudh|last3=Le Loeuff|first3=Jean|last4=Khansubha|first4=Sasa-On|last5=Tong|first5=Haiyan|last6=Wongko|first6=K|date=2005-01-01|title=The dinosaur fauna from the Khok Kruat Formation (Early Cretaceous) of Thailand|url=https://www.researchgate.net/publication/312976683|journal=Proceedings of the International Conference on Geology, Geotechnology and Mineral Resources of Indochina|pages=575–581}}</ref>


In 2010, American palaeontologist [[Stephen L. Brusatte|Stephen Brusatte]] and colleagues noted that the discovery of spinosaurids in Asia, a family previously known only from Europe, Africa, and South America, suggests faunal interchange between the [[Supercontinent|supercontinents]] [[Laurasia]] (in the north) and [[Gondwana]] (in the south) during the early Late Cretaceous. Though it may also be possible that spinosaurids already had a [[cosmopolitan distribution]] before the Middle Cretaceous, preceding the breakup of Laurasia from Gondwana. However, the authors noted that more evidence is needed to test this hypothesis.<ref name=":18">{{Cite journal|last=Brusatte|first=Stephen|last2=B. J. Benson|first2=R|last3=Xu|first3=Xing|date=2010-12-10|title=The evolution of large-bodied therood dinosaurs during the Mesozoic in Asia|url=https://www.researchgate.net/publication/260714806|journal=Journal of Iberian Geology|volume=36|issue=2|pages=275–296|doi=10.5209/rev_JIGE.2010.v36.n2.12}}</ref> In 2012, Allain and colleagues suggested such a global distribution may have occurred earlier across [[Pangaea]], before its breakup starting in the [[Late Jurassic]], even if Asia became separated first.<ref name="AXRK12" /> In 2019, Spanish palaeontologist Elisabete Malafaia and colleagues also indicated a complex [[biogeographical]] pattern for spinosaurs during the Early Cretaceous, based on anatomical similarities between ''Ichthyovenator'' and ''Vallibonavenatrix''.<ref name=":3" />
In 2010, American palaeontologist [[Stephen L. Brusatte|Stephen Brusatte]] and colleagues noted that the discovery of spinosaurids in Asia, a family previously known only from Europe, Africa, and South America, suggests faunal interchange between the [[Supercontinent|supercontinents]] [[Laurasia]] (in the north) and [[Gondwana]] (in the south) during the early Late Cretaceous. It may also be possible that spinosaurids already had a [[cosmopolitan distribution]] before the Middle Cretaceous, preceding the breakup of Laurasia from Gondwana, however, the authors noted that more evidence is needed to test this hypothesis.<ref name=":18">{{Cite journal|last=Brusatte|first=Stephen|last2=B. J. Benson|first2=R|last3=Xu|first3=Xing|date=2010-12-10|title=The evolution of large-bodied therood dinosaurs during the Mesozoic in Asia|url=https://www.researchgate.net/publication/260714806|journal=Journal of Iberian Geology|volume=36|issue=2|pages=275–296|doi=10.5209/rev_JIGE.2010.v36.n2.12}}</ref> In 2012, Allain and colleagues suggested such a global distribution may have occurred earlier across [[Pangaea]] before the [[Late Jurassic]], even if Asia became separated first during the breakup of the supercontinent.<ref name="AXRK12" /> In 2019, Spanish palaeontologist Elisabete Malafaia and colleagues also indicated a complex [[biogeographical]] pattern for spinosaurs during the Early Cretaceous, based on anatomical similarities between ''Ichthyovenator'' and the European genus ''Vallibonavenatrix''.<ref name=":3" />


==References==
==References==

Revision as of 20:34, 22 April 2020

Ichthyovenator
Temporal range: Early Cretaceous, (Aptian) 125–113 Ma
White casts of fossil spinal column with a wave-like sail on display at a museum
Casts of the known spinal column at the National Museum of Nature and Science, Tokyo. The thirteenth dorsal spine and the lower part of the sacrum have been partially reconstructed.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: †Spinosauridae
Subfamily: †Spinosaurinae
Genus: †Ichthyovenator
Allain et al., 2012
Type species
†Ichthyovenator laosensis
Allain et al., 2012

Ichthyovenator is a genus of spinosaurid theropod dinosaur that lived in what is now Laos during the Aptian stage of the Early Cretaceous Period, sometime between 125 to 113 million years ago. It is known from fossils collected in the Grès supérieurs Formation of the Savannakhet Basin, the first of which were found in 2010, and comprised 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 further remains from the dig site had been recovered; these fossils included teeth as well as more vertebrae (backbones) and a pubic bone from the same individual.

The holotype specimen is estimated to have been between 8.5 to 10.5 metres (28 to 34 ft) long and weighed 2 to 2.4 tonnes (2.2 to 2.6 short tons). Ichthyovenator's teeth were straight and conical, and its neck resembled that of the closely related Sigilmassasaurus. Like others in its family, Ichthyovenator had tall neural spines forming a sail on its back. Unlike other known spinosaurids, Ichthyovenator's sail had a sinusoidal (wave-like) shape, curving downwards over the hips into two separate sails. The pelvic girdle was reduced, with the illium, the uppermost body of the pelvis, being proportionately longer than both the pubis and ischium than in other known theropod dinosaurs. The animal was initially thought to belong to the Baryonychinae subfamily, but more recent analyses place it as a primitive member in the Spinosaurinae.

As a spinosaur, Ichthyovenator would have had a long, shallow snout and robust forelimbs. Its diet likely consisted mainly 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 that spinosaurids, especially spinosaurines, were adapted for semiaquatic lifestyles. The vertebral spines of Ichthyovenator's tail were unusually tall, suggesting that—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

Drawing of fossil rib, backbones and pelvis superimposed on silhouette of a dinosaur
Skeletal diagram with holotype elements in place, excluding the additional fossil material announced in 2014

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 bones were recovered from a red sandstone layer, within a surface area of less than 2 metres (6 ft 7 in) squared. Designated under the specimen numbers MDS BK10-01 to 15, it consists 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 bone), 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, and the centra (bodies of the vertebrae) of the sacrals are largely incomplete due to erosion, but preserved all their accompanying spines with their upper edges intact. At the time of Ichthyovenator's description, excavations at the site were still ongoing.[1]

After undergoing preparation in 2011, the skeleton was used as the basis, or holotype, for the type species Ichthyovenator laosensis; named and described in 2012 by palaeontologists Ronan Allain, Tiengkham Xeisanavong, Philippe Richir, and Bounsou Khentavong. The generic name is derived from Greek ἰχθύς (ichthys), "fish", and Latin venator, "hunter", in reference to its likely piscivorous (fish-eating) lifestyle. The specific name refers to its provenance from Laos.[1] Ichthyovenator is the third named spinosaurid from Asia, after the Thai genus Siamosaurus in 1986, and the Chinese species "Sinopliosaurus" fusuiensis in 2009.[1][2][3] The latter may represent the same animal as Siamosaurus.[4][5] In 2014, a conference paper on the genus was published by Allain. This abstract indicated that additional remains from the original individual had been found after continued excavations 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.[6] 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 multiple similarities with the vertebrae of the African spinosaurid Sigilmassasaurus.[7]

Description

Size of various spinosaurids (Ichthyovenator in yellow, second from right) compared with a human

In 2016, American palaeontologist Gregory S. Paul estimated Ichthyovenator to have been approximately 8.5 metres (28 ft) long and to have weighed 2 tonnes (2.2 short tons).[8] The same year, Spanish palaeontologists Rubén Molina-Pérez and Asier Larramendi gave a higher 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) in weight.[9]

Ichthyovenator's teeth were conical, straight, and bore no serrations.[6] The front carinae (edges) of the maxillary and dentary teeth were evident on the base of the tooth crown.[7] 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, which were filled by air sacs in life. The parapophysis (a process that articulated with the capitulum of the ribs) increased in height from the rear cervicals to the first dorsal, with its underside remaining in contact with the front lower edge of the centrum. This is unlike the condition in most theropods, where 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 all 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 all shared with the spinosaurids Baryonyx, Suchomimus, Sigilmassasaurus,[7] and Vallibonavenatrix.[10] 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.[7] The single known dorsal rib, which was found near the twelfth dorsal vertebra, had a head typical of the ribs of other moderately to large-sized theropods. The rib shaft was strongly curved, forming a half circle. The lower end of the rib 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 known theropods, suggests that the rearmost dorsal ribs articulated with the complex of the sternum (breast bone).[1]

Casts of the vertebrae at the Muséum National d'Histoire Naturelle, Paris

Like many other spinosaurids, Ichthyovenator had a sail on its back and hips, formed by the elongated neural spines of its vertebrae. Uniquely among known members of the family however, Ichthyovenator's sail was divided in two over the hip, 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, 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, with its highest point 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 a trapezoidal shape, whereas the spines of other spinosaurids were roughly rectangular. It also lacked the forward or backward inclination of Spinosaurus's neural spines. Its front corner formed a 3 cm (1.2 in) narrow process, pointing upwards. The spine of the thirteenth dorsal vertebra has only been partly preserved, its upper and lower ends having broken off. From its general shape the describers inferred however, that it was about as long as the preceding spine. This would imply that the back edge of the front sail would form a rectangular corner, as the spine of the much lower first sacral vertebra is about 21 cm (8.3 in), creating a sudden hiatus in the profile of the sail. 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 most other theropods, including the sail of Suchomimus, the sacral spines were not fused, nor did they have any extensive contacts. The second and third sacral centra were fused, with the suture connecting them still visible. Since only the rearmost two dorsal vertebrae are preserved, it is unknown how far the dorsal sail continued towards the front of the animal's back.[1]

Colour life reconstruction of a left-facing spinosaur
Speculative life restoration, with unknown regions such as the head and limbs based on relatives

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 a 30 degree angle, 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, sturdy, and 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 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 not observed in other theropods.[1] Some of the caudal vertebrae also had unusually tall, forwardly-extended prezygapophyses.[7]

The 92 cm (36 in) ilium of the pelvis was blade-like, and longer in proportion to the pubic bone, which was 65 cm (26 in) long, than in any other known large theropod. The postacetabular ala (rear expansion) was much longer than the preacetabular ala (front expansion), the latter had a shelf on its central surface, which formed the middle edge of the preacetabular fossa. Viewed distally (from away from the centre of attachment), the lower end of the pubis was L-shaped, resembling that of Baryonyx. The pubic apron (the expanded lower end of the pubis) features 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 49.6 cm (19.5 in) long, making it shorter in relation to the pubis than in all other known tetanuran theropods. Its 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).[1] The ischium attached to the illium via a peg-and-socket-like articulation, unlike the flat and concave condition seen in Baryonyx. This feature was also present in Vallibonavenatrix.[10]

Classification

Three pairs of illustrated fossil pelvic bones and spinal columns compared to the silhouette of a human to their left, Ichthyovenator's pelvis and vertebrae first from the bottom
Comparison of the pelvic region and neural spine sails of Suchomimus, Spinosaurus, and Ichthyovenator

In 2012, the describers established the following autapomorphies (unique derived traits) in Ichthyovenator: its dorsal and sacral sinusoidal sail; the thirteenth dorsal neural spine being 410% the length of the centrum, and having a 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 centrodiapophyseal fossae and S-shaped transverse processes in top view; and the lengthy illium, with its accompanying pubis being shorter in comparison to it than in any other known theropod. Allain and colleagues also identified some anatomical features unique among 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, as well as a shaft that was flatenned sideways. The shrinking 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.[1]

Allain and his team considered Ichthyovenator as representing the first unequivocal spinosaurid from Asia.[1] Though prior spinosaurids had been named from the continent; including Siamosaurus from the Barremian Sao Khua Formation of Thailand, and "Sinopliosaurus" fusuiensis from the Aptian Xinlong Formation of China, the authors noted that the validity of these taxa has been debated by palaeontologists, due to them only being confidently known from isolated teeth.[1][11] Brazilian palaeontologists Marcos Sales and Caesar Schultz have suggested that these teeth may eventually be attributed to spinosaurids similar to Ichthyovenator.[12] In addition to tooth fossils, a spinosaurid skeleton, possibly belonging to Siamosaurus, was excavated from the Thai Khok Kruat Formation in 2004,[13][14] and identified as a definite spinosaur in a 2008 conference abstract by British palaeontologist Angela Milner and colleagues, eight years prior to Ichthyovenator's description.[15]

Reconstructed skeleton of a spinosaurid holding a pterosaur skeleton in its jaws against a painted backdrop
Reconstructed skeleton of the spinosaurine Irritator, mounted at the National Museum of Nature and Science, Tokyo

Ichthyovenator was assigned by Allain and colleagues in 2012 to the Spinosauridae, more precisely to the subfamily Baryonychinae in a basal position as the sister taxon of a clade formed by Baryonyx and Suchomimus.[1] Allain's 2014 abstract instead found that Ichthyovenator was a member of the Spinosaurinae, due to the lack of serrations on its teeth and the possession of vertebrae with some similarities to those of Sigilmassasaurus.[6] In 2015, a phylogenetic analysis by Evers and colleagues suggested that given the apparent presence of both baryonychine and spinosaurine characteristics in Ichthyovenator, the distinction between the two subfamilies may not be as clear as previously thought.[7] In 2017, American palaeontologist Mickey Mortimer informally hypothesized that Ichthyovenator may not have been a spinosaurid at all, instead possibly being a sail-backed carcharodontosaurid dinosaur closely related to Concavenator. She considered Ichthyovenator as incertae sedis (of uncertain taxonomic affinity) within the clade Orionides, pending description of the new material, which she states will likely confirm Ichthyovenator's spinosaurid identity.[16] A 2017 analysis by Sales and Schultz questioned Baryonychinae's validity, citing the morphology of Brazilian spinosaurids Irritator and Angaturama as suggesting they were possible transitionary forms between the earlier baryonychines and the later spinosaurines. The authors thus indicated that with further research, Baryonychinae may turn out to be a paraphyletic (unnatural) grouping.[12] Ichthyovenator's spinosaurine classification was supported by British palaeontologist Thomas Arden and colleagues in 2018, who resolved it as a basal member of the group due to its tall dorsal sail. Their cladogram can be seen below:[17]

Spinosauridae

Palaeobiology

White casts of spinal column with wave-like sail laid out on a dark background in a museum exhibit
Casts of the vertebrae seen from behind

Though no skull remains have been found for Ichthyovenator, all known spinosaurids had elongated, low and narrow snouts which allowed them to reach far for food and close their jaws quickly, akin to modern crocodilians. The tips of their upper and lower jaws fanned out into a rosette-like shape that bore long teeth, behind which there was a notch in the upper jaw; this formed a natural trap for prey.[11] Ichthyovenator's straight and unserrated teeth,[6] like those of other spinosaurids, would have been suitable for impaling and capturing small animals and aquatic prey. This type of jaw and tooth morphology, also observed in today's gharials and other fish-eating predators, has led many palaeontologists to believe that spinosaurids were largely piscivorous (as implied in Ichthyovenator's name).[11][18] This is also evidenced by Scheenstia fish scales discovered in the stomach cavity of one Baryonyx skeleton,[11][19] and a Spinosaurus snout found with a vertebra from the sawfish Onchopristis embedded in it.[20] A more generalist diet for spinosaurs has also been proposed, based on fossils such as the bones of a juvenile iguanodontid also found in the aforementioned Baryonyx specimen, an Irritator tooth embedded in the vertebrae of a pterosaur, and tooth crowns from Siamosaurus found in association with sauropod dinosaur bones. Thus, it is likely that spinosaurids were also scavengers or hunters of larger prey.[11][21][22][23] Though no limb bones are known from Ichthyovenator, all known spinosaurids had well-built arms with enlarged thumb claws, which they likely used to hunt and process prey.[11]

Ichthyovenator might have used its tail, which had elongated neural spines, for propulsion through water, similar to crocodilians such as this Nile crocodile[17]

Many possible functions, including thermoregulation and energy storage, have been proposed for spinosaurid sails.[11][24] In 2012, Allain and colleagues posited that given the high diversity in neural spine elongation observed in different theropod dinosaurs, as well as histological research done on the sails of synapsids (stem mammals), Ichthyovenator's sinusoidal sail may have instead been used for courtship display or for recognising members of its own species.[1] In a 2013 blog post, British palaeontologist Darren Naish considered the latter function unlikely, favouring the hypothesis of sexual selection for Ichthyovenator's sail, since it appears to have evolved on its own, without very close relatives. Naish also notes that it is possible that similar relatives have not been discovered yet.[25]

Spinosaurids appear to have had semiaquatic lifestyles, spending much of their time near or in water, as has been inferred by the high density of their limb bones (which would have made them less buoyant), and the isotope ratios of their teeth being closer to those of aquatic animals like turtles, crocodilians, and hippopotamuses than those of other, more terrestrial theropods.[11] Semiaquatic adaptations seem to have been more developed in spinosaurines than baryonychines.[17][26][27] Ichthyovenator's short pubis and ischium relative to its illium, coupled with the elongated neural spines in the tails of early spinosaurs, were suggested by Arden and colleagues in 2018 as indications that spinosaurids may have progressively employed further use of their tails to propel themselves underwater (similarly to crocodilians) as they grew more adapted towards an aquatic lifestyle.[17] A similar, though more extreme shrinkage of the pelvis and hind limbs was observed in Spinosaurus, which appears to have been more semiaquatic than any other known spinosaur species.[28]

Palaeoecology and palaeobiogeography

Fossils of the sauropod Tangvayosaurus, another dinosaur from the Gres superieurs Formation, Savannakhet Dinosaur Museum

Ichthyovenator is known from the Barremian to Cenomanian Grès supérieurs Formation, and was found in a layer probably dating to the Aptian stage of the Early Cretaceous Period, between 125 to 113 million years ago.[1] It coexisted with other dinosaurs such as the sauropod Tangvayosaurus, and an indeterminate sauropod, iguanodontian, and neoceratopsian.[29][30] Tracks of theropod, sauropod and ornithopod dinosaurs, as well as plant remains, are also known from the formation.[1][30] Fossils of non-dinosaurian fauna are represented by ray-finned fish like Lanxangichthys[29] and Lepidotes, as well as turtles, including Shachemys, Xinjiangchelys, and an indeterminate carettochelyid and trionychid.[31] The trigoniid bivalves Trigonioides and Plicatounio have also been recovered from the formation.[29] The Grès supérieurs Formation is the lateral equivalent of the neighboring Khok Kruat and Phu Phan Formations in Thailand; from the former of which fossils of theropods (including spinosaurids), sauropods, iguanodontians, and freshwater fish have also been recovered.[1][32]

In 2010, American palaeontologist Stephen Brusatte and colleagues noted that the discovery of spinosaurids in Asia, a family previously known only from Europe, Africa, and South America, suggests faunal interchange between the supercontinents Laurasia (in the north) and Gondwana (in the south) during the early Late Cretaceous. It may also be possible that spinosaurids already had a cosmopolitan distribution before the Middle Cretaceous, preceding the breakup of Laurasia from Gondwana, however, the authors noted that more evidence is needed to test this hypothesis.[33] In 2012, Allain and colleagues suggested such a global distribution may have occurred earlier across Pangaea before the Late Jurassic, even if Asia became separated first during the breakup of the supercontinent.[1] In 2019, Spanish palaeontologist Elisabete Malafaia and colleagues also indicated a complex biogeographical pattern for spinosaurs during the Early Cretaceous, based on anatomical similarities between Ichthyovenator and the European genus Vallibonavenatrix.[10]

References

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