Grendel strikes again!

Remember this little critter?

Brachypterygius sp., a largely complete specimen (Ce 16696) in the Bristol Museum [composite image taken through glass].
This is the ichthyosaur formerly known as Grendelius mordax McGowan, 1976.  This name comes from the monster Grendel from the Old English epic Beowulf; roughly, it means ‘Grendel-like bite’.  Before 1997, all the material known of Grendelius mordax was a crushed, but pretty nicely complete skull, and some bits of vertebrae and shoulder girdle; almost all of which is in the Sedgwick Museum, Cambridge (see below).

Brachypterygius mordax, a large, complete but crushed skull in the Sedgwick Museum, Cambridge (SMC J68516) (from McGowan and Motani 2003).

Way back in 1904 (a,b), Boulenger gave the name Ichthyosaurus extremus to a paddle from the Kimmeridge Clay (Upper Jurassic) of Weymouth, Dorset, UK.  The shape of the bones, made it obvious that this was a type of ichthyosaur.  However, its humerus was unusual in joining to three paddle bones (three distal facets, similar to Ophthalmosaurus): the ulna, radius and intermedium.

Brachypterygius extremus forepaddle, specimen in the Natural History Museum (specimen R3177).

What does Ichthyosaurus extremus have to do with Grendelius mordax, I hear you cry?

These ichthyosaur names show a good example of how scientific names for animals change, based on new discoveries.  This is managed by the International Commission on Zoological Nomenclature (ICZN; which can be translated as ‘the Lords of Animal Names’, although I’d imagine this is the first time it’s happened).  This body has published guidelines on how to name, write and change names of animals.  It is the ICZN, following the rules first contrived by Linnaeus, who requires animal names to contain two parts (binomial nomenclature): a generic name, e.g. Ichthyosaurus, and a specific name, e.g. Ichthyosaurus extremus.  (The genus is the first part, but the species always contains both.)  Both the species and genus has a unique defining set of characters, but a genus can have many species, each subtly different — e.g. Ichthyosaurus has Ichthyosaurus communis, Ichthyosaurus breviceps, Ichthyosaurus conybeari and more.

Back to Ichthyosaurus extremus.  In 1922, Friedrich von Huene wrote a book about ichthyosaurs.  Here he noted that many ichthyosaurs were different enough from each other to split them into separate genera (sing. genus).  Before, most ichthyosaurs had been put into the genus Ichthyosaurus.  Huene (1922) changed the names of Ichthyosaurus enthekiodon to Nannopterygius enthekiodon (Huene 1922, p. 98), based upon it’s tiny paddles, and Ichthyosaurus extremus became Brachypterygius extremus (Huene 1922, p. 97), for the short and wide paddle.

When the fossil of Grendelius mordax above was described and named, only isolated paddles had been found that were certainly Brachypterygius.  Only a head was known for Grendelius mordax, and so the two were thought to be different, hence the different names.

Then, in 1997, a small note came out in Journal of Vertebrate Paleontology (McGowan 1997).  This showed a new specimen in the Bristol Museum (Ce 16696); the one in the picture at the top.  As an ichthyosaur specimen, this is a good example: preserved on its side, substantially complete with little disruption of the bones.  The skull was prepared, and upon studying this, Christopher McGowan could confirm it as Grendelius mordax — but now, the skull came complete with a body, and most importantly: a paddle! (I’m sure you can guess where I’m going with this.)

Of course, this paddle was characteristic of Brachypterygius extremus.  So this ichthyosaur, with the head of Grendelius and the paddle of Brachypterygius, showed that what was thought to be two different animals were the same.  The problem then was: what do you call it?

Brachypterygius sp., skull piece and interpretation, from the Bristol Museum (undescribed & unfigured specimen Ce 16696). This is the treat I spoke of in a previous post. [Thanks to Paul for commenting on the labelling — what I had labelled as the angulars are more likely splenials.]
Fortunately, the ICZN have guidelines for this too!  Unfortunately (in this case), the rule is that the earliest valid name has priority — there can be exceptions, as is likely in the case of Ophthalmosaurus icenicusBrachypterygius is the older name — both originally and its changing — so Grendelius became Brachypterygius, depriving us of such a good name (to save confusion, disused names cannot be applied to other animals).

What about the species?  Does it become Brachypterygius extremus, or Brachypterygius mordax.  McGowan (1997, p. 430) left this unsettled pending further preparation, study and comparison to the type specimens of Brachypterygius extremus and Brachypterygius “Grendelius” mordax.  In this post, I’ve left it as Brachypterygius sp., i.e. a species of Brachypterygius.  One of my tasks, as part of my PhD is to take this on: watch this space…

Bibliography

BOULENGER, G. A. 1904a. On a new species of ichthyosaur from Bath. Proceedings of the Zoological Society of London, 1904, 424–426.

—— 1904b. Abstracts to the Proceedings of the Zoological Society of London, March 15th, 1904. Proceedings of the Zoological Society of London, 1904, 18.

HUENE, F. R. V. 1922. Die Ichthyosaurier des Lias und ihre Zusammenhänge.  Verlag von Gebrüder Borntraeger, Berlin, 114 + 122 pl pp.

MCGOWAN, C. 1976. The description and phenetic relationships of a new ichthyosaur genus from the Upper Jurassic of England. Canadian Journal of Earth Sciences, 13, 668–683.

—— 1997. The taxonomic status of Grendelius mordax: a preliminary report. Journal of Vertebrate Paleontology, 17, 428–430.

—— and MOTANI, R. 2003. Ichthyopterygia. In Handbook of Paleoherpetology. Verlag Dr. Friedrich Pfeil, Munich, 175 pp.

[Edit log:
20120720—line 2 changed 1997 to 1976; general formatting improvements.]

Who’s useless at writing a blog: I am!

As I’m sure some of you may have noticed, I haven’t posted any new things recently — this month in fact.  It has been quite a busy time: I’ve taken visits to both London and Dorchester, acquired an ichthyosaur that I am planning to prepare, and spent an inordinate amount of time thinking about cristi supraorbitali and other obscure but important ichthyosaur features.

I also think that I have discovered my most favourite ichthyosaur yet: Besanosaurus leptorhynchus from the Middle Triassic (Upper Anisian–Lower Ladinian) of Besano, northern Italy.  I thank Christiano Dal Sasso for kindly sending me a copy of his and Giovanni Pinna’s paper (Dal Sasso and Pinna 1996).  It features the most wonderful map of the complete specimen (below; from Dal Sasso and Pinna 1996 fig. 8).

Besanosaurus leptorhynchus from the Besano Formation (Grenzbitumenzone) near Besano, northern Italy. From Dal Sasso and Pinna (1996).

In London, I went to the Natural History Museum (NHM), which is a treasure trove of fossils — a palaeontologist’s paradise.  There I looked through some of the Kimmeridgian ichthyosaur material (there’s too much to do in a day!); particularly looking for bits of Nannopterygius.  Aside from the main skeleton on the wall of the Marine Reptile Gallery, there is very little material for this genus.  The isolated hindpaddle was my main concern (below; layout suggested by SV-POW), but there’s also several vertebrae.

Isolated hindpaddle of Nannopterygius enthekiodon, from the Kimmeridgian, in most views as suggested on SV-POW. There has been some slight dorsoventral crushing, hence the significant difference in size of the proximal femoral processes. Abbreviations: car: carpal, dp: dorsal process, fem: femur, fib: fibula, int: intermedium, tbl tibiale, tib: tibia, vp: ventral process.
Unidentified ‘ichthyosaur’ vertebrae in the collections of the Natural History Museum, London. Still in their plaster wrapping, centra with articulated neural spines.

Whilst in the NHM, I also took a look through the drawers and found a nice set of vertebrae, which were unmarked and only partially removed from their cast, and a very crushed skull of AegirosaurusIchthyosaurus’ leptospondylus from the Solnhofen Limestone of Germany (Bardet and Fernández 2000): the same place the famous ‘missing link’ Archaeopteryx comes from.

Ichthyosaurs leptopterygius from the Solnhofen Limestone of southern Germany. Significantly crushed, in dorsal view.

References

BARDET, N. and FERNÁNDEZ, M. S. 2000. A new ichthyosaur from the Upper Jurassic lithographic limestones of Bavaria. Journal of Paleontology, 74, 503–511.

DAL SASSO, C. and PINNA, G. 1996. Besanosaurus leptorhynchus n. gen. n. sp., a new shastasaurid ichthyosaur from the Middle Triassic of Besano (Lombardy, N. Italy). Paleontologia Lombarda, New Series, 4, 1–22.

MCGOWAN, C. and MOTANI, R. 2003. Ichthyopterygia. In SUES, H.-D. (ed.) Handbook of Paleoherpetology. Vol. 8. Verlag Dr. Friedrich Pfeil, Munich, 175 pp.

Ichthyosaurs in the news!

Hello there!

I hope that you have enjoyed/considered/comprehended the last few posts on my PhD plans.  I’ve tried to explain simply, but some of the concepts are unusual or obscure.  From next week I plan to ‘start again’ from the basics of palaeontology and then move on to what vertebrates and evolution etc actually are!

Some of you may have noticed this piece on the BBC news site yesterday.  A new paper, published in online journal PLoS ONE, by Fisher et al. has described a new species of ichthyosaur from the Lower Cretaceous (~130 Ma): Acamptonectes densus.  Darren Naish, one of the coauthors, has written about this on his wonderful Tetrapod Zoology blog.

The paper also shows that ichthyosaurs were not as strongly affected by the Jurassic–Cretaceous extinction (JCE) as previously thought.  Ophthalmosaurine thunnisaurs (e.g. Ophthalmosaurus) are present either side of the boundary, whereas they were considered to only be present in the Jurassic.

My PhD: Part 3: the return of the ichthyosaurs

In this, the final of my original trilogy describing my planned studies, I will talk about the ‘spin-offs’ from looking at the British genera and phylogeny.  The other posts are:

This blog will look at two subsequent projects in my thesis.  The first, a disparity analysis, follows from the descriptive and phylogenetic work.  The second, biomechanics, follows mainly from the descriptions.

Summary

  • Disparity is the study of variance
    • Ichthyosaurs showed varying levels of disparity through their existence
    • High disparity did not always mean high diversity
  • Biomechanics is the study of animal movement
    • Finite Element Analysis (FEA) is a powerful tool to study this
    • Studies on skulls can tell a lot about animals’ feeding strategies
    • Ichthyosaurs have yet to be studied using FEA

Disparity analysis

Disparity is the study of variation within a group of organisms.  This is different from diversity, which looks at classification and systematics.  Disparity uses variation to estimate differences in the ecology of an organism: how it lives in, and interacts with its environment.  The two can be interlinked — greater disparity may indicate greater diversity, and vice versa.  This does not always hold true, and ichthyosaurs, being as odd as they are, show a good example of this.

Thorne, Ruta and Benton (2011) looked at ichthyosaurs over the Triassic–Jurassic boundary (TJB; ~200 Ma).  Using morphospace plots — charts that show morphological variation — they showed that post-TJB ichthyosaurs had a lower disparity than pre-TJB ichthyosaurs (fig. 1).  The TJB marks the spread of the thunnisaur (tuna-shaped) ichthyosaurs.

Fig. 1 Ichthyosaur disparity decreased over the TJB more than diversity. From Thorne et al. 2011.

The method used in this type of study is fairly simple: measure various dimensions of the organisms.  These can include length, width, height, radius, curvature etc. of individual bones, or body parts.  The data are then compared using computer algorithms that assess the influence of each aspect of variation.  This is usually spat out as a matrix of seemingly meaningless numbers.  These numbers can then be plotted onto morphospace plots.

Usually, these morphospace plots are two-dimensional.  The algorithms produce variables that show different forms of variation: e.g. longer or shorter, wider or narrower.  These variables can then be plotted as principle coordinates.  The two coordinates with the greatest effect or variation are most frequently chosen.  The data can be further analysed to take all coordinates into account.  A ‘mean sum of ranges’, for instance, is a simple way to show how great is the spread (variable).

Biomechanical studies

As I’m sure many of you will have noticed, animals have a tendency to move about.  This can be by walking, running, jumping, swimming and flying.  To do all of these, the animal must use its muscles and skeleton to apply forces through the feet, tail and arms.  When the animal works harder more force is applied: e.g. doing a full press-up is more difficult than bending at the knees.

Using muscles and bones to apply forces, doesn’t just make you move.  The bones themselves bend slightly too.  The greater the forces, the more the bone is deformed.  If the force is too great, the bone breaks.  Studies into bone deformation are one aspect of biomechanics (Rayfield 2007).

In vertebrates, the most interesting applications of biomechanics centre on the skull.  It is here where eating is done.  Also, the large number of bones in the skull makes for complicated, and sometimes unexpected, interactions.  As of writing, there have been no skull-biomechanics studies on ichthyosaurs (that I’ve found).  My research will hopefully change that.

‘Why should you study skull biomechanics?’ I hear you cry.  The skulls tell us a lot about how an animal lived.  In particular, what and how it ate and how it could see.  Sight in ichthyosaurs has been covered by Motani (1999).  Feeding is usually compared to modern dolphins, including the schooling behaviour.  Some ichthyosaur fossils have remains of belemnites — relatives of squid with a hard ‘bullet’ shell — in the stomach region.

Using biomechanics, there have been many changes in perceptions of vertebrate feeding strategies.  Humphries et al. (2007) showed that pterosaurs — a group of flying reptiles, extant during the Mesozoic — could not skim feed.  Skim feeding requires dipping the lower jaw into water then closing the jaws when food is caught.  A pterosaur that tried to skim feed would probably break its bill!

Humphries et al. (2007) used a sequence of calculations to predict the outcome.  A more complex technique used is finite element analysis (FEA).  This has been used in engineering for many years.  Its use in biology and palaeontology is more recent.  One of the major proponents is Dr Emily Rayfield here at the University of Bristol.

FEA uses computer tomography (CT) scans of skulls as a basis model then overlays a mesh composed of an arbitrary number of fixed points.  This model can then be put through computer software that applies forces to it.  It location and direction of the forces is specified, as are any joins, sutures and pivots.  The stress and strain experienced by the model are then calculated (fig. 2).

Fig. 2 Finite Element analysis of three theropod dinosaurs, Coelophysis, Allosaurus, and Tyrannosaurus. Top diagrams show the fossils. Middle diagrams are the surface mesh, arrows show fulcra. Lower diagrams show stresses under FEA analysis, warmer colours indicate higher forces, arrows show force transferral. Modified from Rayfield (2005).

An advantage of FEA is that it is a more sophisticated technique than simpler rigid bar calculations.  It takes more account of the object’s shape and the variations in strength that this causes.  More accurate responses to the effects of sutures are also possible.  The model and results produced are generally a more accurate representation of real life.

FEA studies have been used to great effect in studying dinosaurs (Rayfield 2004, 2005; Rayfield et al. 2007).  These have been used to show that some that some ‘top predators’ (e.g. Allosaurus and Tyrannosaurus) could not use the same strategy in killing prey.

References

HUMPHRIES, S., BONSER, R. H. C., WITTON, M. P. and MARTILL, D. M. 2007. Did pterosaurs feed by skimming? Physical modelling and anatomical evaluation of an unusual feeding method. PLoS Biology, 5, 1647–1655.

MOTANI, R. and (null). 1999. Large eyeballs in diving ichthyosaurs. Nature, 402, 747.

RAYFIELD, E. J. 2005. Aspects of comparative cranial mechanics in the theropod dinosaurs Coelophysis, Allosaurus and Tyrannosaurus. Zoological Journal of the Linnean Society, 144, 309–316.

—— 2004. Cranial Mechanics and Feeding in Tyrannosaurus rex. Proceedings of the Royal Society of London Series B-Biological Sciences, 271, 1451–1459.

—— 2007. Finite Element Analysis and understanding the biomechanics and evolution of living and fossil organisms. Annual Review of Earth and Planetary Sciences, 35, 541–576.

——, MILNER, A. C., XUAN, V. B. and YOUNG, P. G. 2007. Functional Morphology of Spinosaur “Crocodile-Mimic” Dinosaurs. Journal of Vertebrate Paleontology, 27, 892–901.

THORNE, P. M., RUTA, M. and BENTON, M. J. 2011. Resetting the evolution of marine reptiles at the Triassic-Jurassic boundary. Proceedings of the National Academy of Sciences, 108, 8339–8344.

My PhD plans: Part 1: a new ichthyosaur

Well, having given a presentation on the plans for my PhD, I feel as though I can now share it with the world.  That and I’ve had agreement on what I plan to do and how I plan to do.  Thus, here is an edited pseudo-transcript (mostly made up) on my PhD plans for the next year or so.  This will be in the form of three posts:

My thesis is based upon an unpublished thesis of Angela Kirton (1983).  As it stands, it is currently divided into two main projects.  These will certainly lead on to several further projects.  The initial part is to redescribe the three genera (group of species) of Late Jurassic ichthyosaurs from Great Britain.  Concurrent with this, I will be comparing the three main phylogenetic analyses from 1999a (Motani) and 2000 (Maisch and Matzke; Sander).

Summary

As these posts will be quite long, I will put a small summary at the beginning of each.  This way you can get the gist and come back to the details later.

The first part of my PhD will be to describe the three genera of Late Jurassic ichthyosaurs from Britain:

    • Ophthalmosaurus
    • Brachypterygius
    • Nannopterygius
  • These can be identified based upon features of the skull and forelimb:
    • Ophthalmosaurus has a very large eye
    • Brachypterygius has three equal-sized facets on the humerus
    • Nannopterygius has very tiny paddles

Ichthyosaur basics

[Specimen abbreviations: BMNH—Natural History Museum, London (originally British Museum (Natural History))]

There are a few small things that I must clear up and add from my ‘Introduction to ichthyosaurs’ blog.  Firstly, how we describe different parts of vertebrates, and where the features of interest are (fig. 1).

The front of the animal (where the head is) is called anterior.  The rear, or tail end, is posterior: your hands are anterior to your legs.  The belly-side is ventral whereas the back is dorsal: your spine is dorsal to your belly button.  Each side is called the right or left lateral side: your right hand is on the right lateral side.  Closer to the body is proximal whereas distal is farther: your hand is distal to your elbow.

Fig. 1 Reconstruction of Ophthalmosaurus icenicus (length about 3.3 m; modified from Kirton 1983).

The ichthyosaurs that I am looking at show different features of the skull and paddle.  In the skull, the size of the eye is the most obvious difference.  The forepaddle (arm) varies in relative size, as do the number and size of the distal facets (concave faces) on the humerus (fig. 2).

Fig. 2 Right forepaddle of Ophthalmosaurus icenicus from the Oxford Clay of Peterborough (BMNH R3702; length about 650 mm) in dorsal view. Note the three facets on the distal end of the humerus and the large number of digits (arab numerals) and phalanges (roman numerals). The anterior- and posteriormost digits are accessory digits (modified from Kirton 1983).

Ichthyosaurs can have many digits (fingers) and phalanges (finger bones).  Humans have five digits on each hand, each with three phalanges (the thumb has two).  Some ichthyosaurs could have eight digits, each with over 20 phalanges.  Oddly enough, these are not all the same as our five fingers.  Ophthalmosaurus (fig. 2) has six digits and up to nine phalanges on each.  The middle four digits (fig. 2, numbered) are analogous to ours.  However, the other two are ‘accessory digits’ from different origins.  These definitions will be important in describing the ichthyosaurs below.

British Upper Jurassic ichthyosaurs

The three ichthyosaur genera from the British Late Jurassic are: Ophthalmosaurus, Brachypterygius and Nannopterygius.  I will give the most visible differences of these.

Ophthalmosaurus (oph-THAL-mo-SORE-us) is found in the Oxford Clay of the Callovian and Oxfordian stages (~162–~155 Ma).  Some possible remains are from the Kimmeridgian Stage (~155.6–150.8 Ma).  As the name suggests, this ichthyosaur is famous for having a large eye (‘ophthalmo-’ is from the Latin for eye; fig. 3).  It is the largest eye compared to body size known (Motani 1999b)!  The humerus has three distal facets.  These articulate with the pre-axial accessory digit, the radius and the ulna (Seeley 1874; Kirton 1983; McGowan and Motani 2003).  The anterior facet is the smallest.  The posteriormost is oblique to the other two.


Fig. 3 Skull of Ophthalmosaurus icenicus showing the very large orbit, where the eye sits (based upon BMNH R3893 & R 4753; length about 800 mm). The ring of bones (sclerotic ring) would have kept the shape of the eye and assisted in focussing underwater. Modified from McGowan and Motani (2003).

Ophthalmosaurus icenicus, the single British species, is the most common Late Jurassic ichthyosaur.  Many specimens have been found near Peterborough, collected by the Leeds brothers in the last half of the 19th Century.  The Leeds’ Collection was largely donated to the Natural History Museum in London, but parts went to museums around the UK.

Brachyterygius (BRACK-ip-ter-I-gi-us) is known from the Kimmeridgian (~155.6–150.8 Ma) of Dorset but there are also some possible finds from the Purbeck Beds (Lower Cretaceous, ~143 Ma) (Ensom et al. 2009) and even later (Lydekker 1888; McGowan and Motani 2003).  This genus was originally described from a forepaddle (fig. 4).  Brachypterygius’s paddle also has three distal facets on the humerus, each is subequally sized and oblique to each other (McGowan 1997).  These facets articulate (anterior to posterior) with the radius, (unusually) intermedium and ulna.  As with Ophthalmosaurus there are four digits with one pre- and one post-axial accessory digit.

Fig. 4 Brachypterygius extremes forepaddle showing the three equally sized distal humeral facets articulating with the radius, intermedium and ulna. Specimen BMNH R3177 (length about 400 mm; modified from Kirton 1983).

A skull in the Sedgwick Museum, Cambridge, described as Grendelius, was suggested to be Brachypterygius.  This was confirmed by a specimen in Bristol Museum.  This whole body specimen posses the skull of Grendelius with the forepaddle of Brachypterygius (McGowan 1997).  Because of this, Grendelius was renamed Brachypterygius (the earlier name gets precedence).  There are two species of Brachypterygius from the British Late Jurassic: B. extremus (the original) and B. mordax (originally Grendelius; McGowan 1997).

Nannopterygius (NAN-op-ter-I-gi-us) is the most uncommon, and unusual, of British Late Jurassic ichthyosaurs.  Only one whole body specimen, and one forepaddle, is known.  As the ‘nanno-’ in the name suggests, there is something small on this ichthyosaur: the paddles (see fig. 5; Hulke 1871).  Nannopterygius has significantly reduced fore- and hindpaddles (McGowan and Motani 2003).  These contain less than half the number of bones of other ichthyosaurs.  The humerus of Nannopterygius is also different from Ophthalmosaurus and Brachypterygius: it has only two distal facets.  These articulate with the radius and ulna (McGowan and Motani 2003).  The single British species, Nannopterygius enthekiodon, is from the Kimmeridgian of Kimmeridge Bay, Dorset (Hulke 1871).


Fig. 5 Nannopterygius enthekiodon from the Kimmeridge Clay of Dorset (specimen BMNH 46497; length about 3.5 m). Note the very small paddles compared to the rest of the body. Modified from McGowan and Motani (2003).

Rationale

Whereas Ophthalmosaurus has a lot of material available, Brachypterygius and Nannopterygius are known from only a few good specimens each.  These were described when first found, in the late 19th and early 20th centuries, but briefly, and have escaped extended descriptions since.  The largest recent work was done by Angela Kirton (1983) in her PhD thesis but is unpublished.  The recent surge in ichthyosaur interest, particularly in this millennium, has meant that the need for definite and accurate descriptions has increased.  This is especially important for conducting phylogenetic analyses, which is what I will talk about in my next post.

References

ENSOM, P. C., CLEMENTS, R. G., FEIST-BURKHARDT, S., MILNER, A. R., CHITOLIE, J., JEFFERY, P. A. and JONES, C. 2009. The age and identity of an ichthyosaur reputedly from the Purbeck Limestone Group, Lower Cretaceous, Dorset, southern England. Cretaceous Research, 30, 699–709.

HULKE, J. W. 1871. Note on an Ichthyosaurus (I. enthekiodon) from Kimmeridge Bay, Dorset. Quarterly Journal of the Geological Society, 27, 440–441.

KIRTON, A. M. 1983. A review of British Upper Jurassic ichthyosaurs. Unpublished PhD Thesis, University of Newcastle-upon-Tyne, 239 pp.

LYDEKKER, R. 1888. Note on the classification of the Ichthyopterygia (with a notice of two new species). Geological Magazine, 5, 309–313.

MAISCH, M. W. and MATZKE, A. T. 2000. The Ichthyosauria. Stuttgarter Beiträge zur Naturkunde, Serie B (Geologie und Paläontologie), 298, 1–160.

MCGOWAN, C. 1997. The taxonomic status of Grendelius mordax: a preliminary report. Journal of Vertebrate Paleontology, 17, 428–430.

MCGOWAN, C. and MOTANI, R. 2003. Ichthyopterygia. In SUES, H.-D. (ed.) Handbook of Paleoherpetology, Vol. 8. Verlag Dr. Friedrich Pfeil, Munich, 175 pp.

MOTANI, R. 1999a. Phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology, 19, 473–496.

MOTANI, R. 1999b. Large eyeballs in diving ichthyosaurs. Nature, 402, 747.

SANDER, P. M. 2000. Ichthyosauria: their diversity, distribution, and phylogeny. Paläontologische Zeitschrift, 74, 1–35.

SEELEY, H. G. 1874. On the pectoral arch and fore limb of Ophthalmosaurus, a new ichthyosaurian genus from the Oxford Clay. Quarterly Journal of the Geological Society, 30, 696–707