Phantomosaurus is an extinct genus of ichthyosaur that lived during the late Anisian stage of the Middle Triassic. Fossils have been found in southern Germany. It was discovered in 1965 and named in 1997 as a species of Shastasaurus by Sander in the rocks of the Upper Muschelkalk. More recently, in 2005, the braincase was studied by Maisch and Matzke. They found it to be unique among all known ichthyosaurs in terms of braincase morphology. Despite its close relation to many other ichthyosaurs, in particular Cymbospondylus, Phantomosaurus appears to have a very primitive braincase which resembles other diapsids more than other ichthyosaurs.
Morphology Phantomosaurus is known from a partial skull, the lower jaw, some of the vertebrae and ribs (pictured) and the hind fin.
Hind fin The hind fin is very long and slender, with elongated femur, tibia and fibula. These features suggest a long but not particularly powerful limb, which would have generated a reasonable quantity of lift. Presumably the fore fins also generated lift, or the ichthyosaur would have found itself permanently heading downwards.
Vertebrae and ribs The anterior centrum of each vertebra has two symmetrical hollows in, possibly to aid buoyancy. The vertebrae also have ventro-lateral keels, for reasons unknown. Each zygapophysis is very long and extends a long way posteriorly - their articulatory facets are almost totally horizontal.
Lower jaw The lower jaw is around 40 cm long and has many conical teeth for spearing fish. On the lateral surface, the articular is anterior to the suprangular.
Skull and braincase The skull is mostly disarticulated. Bones we have found are: left quadratojugal, postfrontal, supratemporal, postorbital, basioccipital and both tubera basioccipitalia, the parabasisphenoid, the otic capsule containing both opisthotic and prootic bones, exoccipital, supraoccipital and pterygoid.
Quadratojugal This is very similar in shape to the quadratojugals of other large cymbospondylid ichthyosaurs, hence its classification as related to them. It was originally mistaken for the quadrate bone.
Postfrontal, supratemporal and postorbital The postfrontal and supratemporal together form the border of the upper temporal fenestra, but unusually, the postorbital has no contact with this fenestra. Again, it is similar to Cymbospondylus and not to Shastasaurus or Mikadocephalus in this respect.
Basioccipital As is common in basal ichthyosaurs, the basioccipital can be divided into two sections, the condylus occipitalis and the area extracondylaris. The condylus occipitalis is flattened dorsoventrally, and was probably concave, with a rather saddle-like shape. In this respect it closely resembles that of Cymbospondylus, and probably had an articulation with the atlas vertebra which was more flexible than some ichthyosaurs. Anterior to this part of the bone, the surface is very flat with a slight concavity. The area extracondylaris has high lateral margins which become well-formed tubera basioccipitalia. These are laterally sutured to the opisthotics and extend as far as these bones posteriorly, and about 8 mm in front of the condylus anteriorly. Each tuber basioccipitale is 13 mm in length, 10 mm in width, and rises at least 8 mm above the ventral surface of the basioccipital at the highest point. In life they would have been partially covered by the pterygoid, and also by a layer of cartilage. They would not have been attached to much of the otic capsule.
Parabasisphenoid This is only partly preserved, with the cultriform process almost entirely missing. Part of its base is still attached, indicating that it would have been around 15 mm wide at the most posterior point, and not strongly set off from the basal plate of the parasphenoid. This is considered primitive in ichthyosaurs. The basal plate of the parabasisphenoid is roughly rectangular and has the processus basipterygoidei projecting from it. Only the left one of these can be observed due to the left pterygoid disarticulation - in life they would have been attached. The pterygoid has strong facets on the upper surface where this attachment would have taken place. The basal plate was wider than its length of 27 mm, but the actual width cannot be measured as the right pterygoid is still articulated in the natural position. The suture between parabasisphenoid and basioccipital was straight. It is probable that the parabasisphenoid was in contact with the prootic bone, but it was definitely not attached to the opisthotic. Two small foramina, resembling slits, are immediately anterior to the parabasisphenoid-basioccipital suture. They extend anteriorly as canals into the bone and are probably the entrances of the cerebral carotid arteries, although positioned unusually far posteriorly.
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