Thunnosauria

Reconstruction of Platypterygius bannovkensis, by Olorotitan.

Belongs within: Merriamosauria.

The clade Thunnosauria, defined by Maisch & Matzke (2000) as the most exclusive clade including Ichthyosaurus communis and Stenopterygius quadriscissus, includes the latest-surviving ichthyosaurs. Thunnosaurs had a compact body form relative to other ichthyosaurs with a short, powerful tail that would have provided most of the propulsive force when swimming.

The tuna-lizards
Published 28 November 2011
The classic ichthyosaur Ichthyosaurus communis, from here.

Ichthyosaurs have long been one of the most famous examples of convergent evolution. These Mesozoic marine reptiles, as any textbook will tell you, evolved a body form similar to that of modern dolphins and sharks, and presumably held a similar niche as fast-swimming apex predators. But interesting as that might be, it’s certainly not all there is to be said about ichthyosaurs.

The classic ichthyosaurs that said textbooks will usually depict are members of the clade Thunnosauria that first appeared in the upper Triassic (Thorne et al. 2011). Thunnosaurs differ from other ichthyosaurs in having a relatively short tail, shorter than the trunk, and hindfins that are much shorter than (usually less than half as long as) the forefins (Maisch & Matzke 2000). The name ‘Thunnosauria’ appropriately means ‘tuna-lizards’: as with modern tunas, the compact body of the thunnosaurs indicates greater specialisation for more powerful, tail-driven swimming.

Cast of the short-beaked Ichthyosaurus breviceps, from Charmouth Heritage Coast Centre.

In the Lower Jurassic, thunnosaurs are represented by the genera Ichthyosaurus and Stenopterygius, though the known fossil record for the former is earlier than that of the latter. Both genera are represented by hundreds (if not thousands in the case of Stenopterygius) of known specimens from Europe (Motani 2005): primarily England for Ichthyosaurus, Germany for Stenopterygius. Stenopterygius grew up to 4 m in length; Ichthyosaurus would have been somewhat smaller (Maisch & Matzke 2000). One species of Ichthyosaurus, I. breviceps, stands out for its particularly short and robust rostrum in comparison to other species. Another potential Lower Jurassic thunnosaur is Hauffiopteryx typicus, which also has a distinctively small rostrum, but in this case a particularly fine and slender one (Maisch 2008).

Mounted skeleton of Ophthalmosaurus icenicus, from the British Natural History Museum.

During the Lower Jurassic, the thunnosaurs were among a number of ichthyosaur lineages present. By the time of the Upper Jurassic, all surviving ichthyosaurs (with one possible exception*) belonged to a single thunnosaur lineage, the Ophthalmosauridae. Unfortunately, for most of the Middle Jurassic the ichthyosaur fossil record is missing, and a gap of more than ten million years separates Stenopterygius from Ophthalmosaurus. The only break in this gap is the Argentinan Chacaicosaurus cayi, which sits a few million years later than Stenopterygius. Intriguingly, Chacaicosaurus is not only intermediate in age, it is intermediate in morphology: while its skull is similar to that of Ophthalmosaurus, its forefin is more similar to that of Stenopterygius. As noted by Maisch & Matzke (2000), “It appears as if Chacaicosaurus cayi is one of the rare forms that are true structural intermediates”.

*The possible exception is the Upper Jurassic Nannopterygius enthekiodon, some features of which suggest that it occupies a more basal Stenopterygius-grade position (Maisch & Matzke 2000). Unfortunately, it has not yet been adequately described and included in a formal phylogenetic analysis. This is rather frustrating: Nannopterygius promises to be a quite distinctive animal, with greatly reduced fins and long spinal processes on the anterior tail vertebrate.

The ophthalmosaurids survived from the late Middle Jurassic to the early Upper Cretaceous. Ophthalmosaurus had a slender rostrum with reduced dentition, while other genera such as Brachypterygius and Platypterygius had higher, more robust rostra with their full complement of teeth. Some ophthalmosaurids grew very large: Platypterygius reached up to 9 m. The name Ophthalmosaurus means ‘eye lizard’, and reference to the large eyes of this ichthyosaur seems to be de rigeur for any popular book in which it features, together with some speculation that it may have been a nocturnal hunter. However, a quick scan through the various ichthyosaur skulls illustrated by Maisch and Matzke (2000) indicates that ichthyosaur eyes were generally large. Those of Ophthalmosaurus were not the largest; the eyes of Eurhinosaurus longirostris are particularly ridiculous, with orbits filling almost the entire side of the cranium! So perhaps the question should not be why Ophthalmosaurus had large eyes, but why those ichthyosaurs without large eyes had reduced them.

Systematics of Thunnosauria

Synapomorphies (from Maisch & Matzke 2000): Quadratojugal small; squamosal small, delicate plate or absent; rib articulation in thorax bicipital; tail short; hindlimbs much shorter than forelimbs.

Thunnosauria [Ichthyosauridae]
|--StenopterygiidaeMM00
| |--Chacaicosaurus Fernández 1994MM00
| | `--*C. cayi Fernández 1994MM00
| `--Stenopterygius Jaekel 1904FF13, MM000
| |--*S. quadricissus (Quenstedt 1856)MM00
| |--S. acutirostris (Owen 1840)B93
| |--S. hauffianus von Huene 1922MM00
| |--S. longifrons (Owen 1881)MM00
| `--S. megalorhinus von Huene 1922MM00
`--+--Ichthyosaurus De La Beche & Conybeare 1821FF13, MM00 (see below for synonymy)
| |--*I. communis Conybeare 1822MM00
| |--I. acutirostrisN85
| |--I. australisF71
| |--I. breviceps Owen 1881MM00
| |--I. conybeari Lydekker 1888MM00
| |--I. intermedius Conybeare 1822MM00
| |--I. marathonensisF71
| |--‘Protoichthyosaurus’ porostealis Appleby 1979MM00
| `--*Protoichthyosaurus’ postaxalis Appleby 1979MM00
`--+--Aegirosaurus Bardet & Fernández 2000MM03, MM00
| |--*A. leptospondylus (Wagner 1873) [=Ichthyosaurus leptospondylus]MM00
| `--‘Ophthalmosaurus’ monocharactus Appleby 1956MM00
`--Ophthalmosauridae [Ophthalmosauria]MM00
|--Brachypterygius von Huene 1922FF13, MM00 [incl. Grendelius McGowan 1976MM00, Otschevia Efimov 1998MM00]
| |--*B. extremus (Boulenger 1904)MM00
| |--‘Otschevia’ alekseevi Arkhangelsky 2001H02
| |--B. mordax (McGowan 1976) [=Grendelius mordax]MM00
| `--B. pseudoscythius (Efimov 1998)MM00 (see below for synonymy)
`--+--Ophthalmosaurus Seeley 1874FF13, MM00 (see below for synonymy)
| |--*O. icenicus Seeley 1874MM00
| |--*Khudiakovia’ calloviensis Arkhangelsky 1999 (n. d.)MM00
| |--O. discusD07
| |--O. gorodischensis (Efimov 1999) (see below for synonymy)MM00
| |--O. natans (Marsh 1879)MM00
| |--O. periallus (Fernández 1999) [=Mollesaurus periallus]MM00
| |--‘Paraophthalmosaurus’ saratoviensisMM00
| |--O. saveljeviensis (Arkhangelsky 1997) [=*Paraophthalmosaurus saveljeviensis]MM00
| `--O. yasykovi (Efimov 1999) [=*Yasykovia yasykovi; incl. Y. kabanovi, Y. mittai, Y. sumini]MM00
`--+--Caypullisaurus Fernández 1997FF13, MM00
| `--*C. bonapartei Fernández 1997MM00
`--Platypterygius von Huene 1922FF13, MM00 (see below for synonymy)
|--*P. platydactylus (Broili 1907)MM00
|--P. americanus (Nace 1939)MM00
|--P. bannovkensis Arkhangelsky 1998 [=P. (Pervushovisaurus) bannovkensis]MM00
|--P. bedengensis (Efimov 1998) [=*Plutoniosaurus bedengensis]MM00
|--P. birjukovi (Otschev & Efimov 1985) [=*Simbirskiasaurus birjukovi]MM00
|--P. campylodon (Carter 1846)B93
|--P. hauthali (von Huene 1926)MM00
|--P. hercynicus Kuhn 1946MM00
|--P. kiprijanoffi Romer 1968MM00
`--P. longmani Wade 1990MM00

Brachypterygius pseudoscythius (Efimov 1998)MM00 [=*Otschevia pseudoscythiaMM00; incl. B. zhuravlevi Arkhangelsky 1998MM00, O. zhuraleviH02]

Ichthyosaurus De La Beche & Conybeare 1821FF13, MM00 [incl. Eurypterygius Jaekel 1904MM00, Protoichthyosaurus Appleby 1979MM00]

Ophthalmosaurus Seeley 1874FF13, MM00 [incl. Ancanamunia Rusconi 1942MM00, Apatodontosaurus Mehl 1927MM00, Baptanodon Marsh 1880MM00, Khudiakovia Arkhangelsky 1999MM00, Mollesaurus Fernández 1999MM00, Paraophthalmosaurus Arkhangelsky 1997MM00, Sauranodon Marsh 1879 (preoc.)MM00, Undorosaurus Efimov 1999MM00, Yasykovia Efimov 1999MM00]

Ophthalmosaurus gorodischensis (Efimov 1999) [=*Undorosaurus gorodischensis; incl. U. khorlovensis, U. nessovi]MM00

Platypterygius von Huene 1922FF13, MM00 [incl. Longirostria Arkhangelsky 1998MM00, Myobradypterygius von Huene 1926MM00, Myopterygius von Huene 1922MM00, Pervushovisaurus Arkhangelsky 1998MM00, Plutoniosaurus Efimov 1997MM00, Simbirskiasaurus Ochev & Efimov 1985MM00, Tenuirostria Arkhangelsky 1998MM00]

*Type species of generic name indicated

References

[B93] Benton, M. J. 1993. Reptilia. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 681–715. Chapman & Hall: London.

[D07] Dixon, D. 2007. The Complete Illustrated Encyclopedia of Dinosaurs & Prehistoric Creatures. Hermes House: London.

[F71] Fletcher, H. O. 1971. Catalogue of type specimens of fossils in the Australian Museum, Sydney. Australian Museum Memoir 13: 1–167.

[FF13] Fröbisch, N. B., J. Fröbisch, P. M. Sander, L. Schmitz & O. Rieppel. 2013. Macropredatory ichthyosaur from the Middle Triassic and the origin of modern trophic networks. Proceedings of the National Academy of Sciences of the USA 110 (4): 1393–1397.

[H02] Haubold, H. 2002. Literaturbericht: Ichthyosaurier und Sauropterygier. Zbl. Geol. Paläont. Teil II 5-6: 367–403.

Maisch, M. W. 2008. Revision der Gattung Stenopterygius Jaekel, 1904 emend. von Huene, 1922 (Reptilia: Ichthyosauria) aus dem unteren Jura Westeuropas. Palaeodiversity 1: 227–271.

[MM00] Maisch, M. W., & A. T. Matzke. 2000. The Ichthyosauria. Stuttgarter Beiträge zur Naturkunde Serie B (Geologie und Paläontologie) 298: 1–159.

[MM03] Maisch, M. W., & A. T. Matzke. 2003. Observations on Triassic ichthyosaurs. Part XII. A new Early Triassic ichthyosaur genus from Spitzbergen. Neues Jahrbuch für Geologie und Paläontologie—Abhandlungen 229 (3): 317–338.

Motani, R. 2005. True skull roof configuration of Ichthyosaurus and Stenopterygius and its implications. Journal of Vertebrate Paleontology 25 (2): 338–342.

[N85] Norman, D. 1985 (reprinted 2000). The Illustrated Encyclopedia of Dinosaurs. Salamander Books: London.

Thorne, P. M., M. Ruta & M. J. Benton. 2011. Resetting the evolution of marine reptiles at the Triassic-Jurassic boundary. Proceedings of the National Academy of Sciences of the USA 108 (20): 8339–8344.

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