[3][4] However, it was found to be distinct in 2017 by Dean Lomax and colleagues, who separated it from Ichthyosaurus on account of differences in the arrangement and shape of the carpal ossifications, as well as the absence of the fifth digit.
[2] In 1979, Robert M. Appleby noted that some ichthyosaur foreflippers showed unique configurations of bones, and named a new genus, Protoichthyosaurus, for these specimens.
[2] In 2000, Maisch and Andreas Matzke elaborated on this synonymy, noting that the foreflippers of Ichthyosaurus are highly variable and that while those of Protoichthyosaurus were unusual, they did not seem sufficiently different to support the latter's status as a separate genus.
They also noted that the traits Appleby used to distinguish Protoichthyosaurus only pertained to foreflipper anatomy and represented the ancestral condition for Ichthyosaurus rather than being novel evolved features.
[4] In 2017, Dean Lomax, Judy Massare, and Rashmiben Mistry instead found Protoichthyosaurus to be distinct from Ichthyosaurus, based on features of the skull and shoulder girdle, in addition to those of the foreflipper.
Specimens from Dorset, Somerset, Nottinghamshire, Leicestershire, and Warwickshire in England as well as Glamorgan in Wales were considered attributatable to Protoichthyosaurus, making it one of the most widespread Early Jurassic ichthyosaurs of Britain.
Save for an additional flipper bone, this specimen is very similar to Ichthyosaurus and therefore probably an aberrant individual of that genus, rather than a distinct species.
[2] Additionally influencing the apparent variability of Ichthyosaurus, many skeletons from historical collections actually represent composites of multiple individuals, and sometimes contain reconstructed parts or were dissassembled then reassembled incorrectly.
However, they noted that there were differences between its skull and humerus (upper arm bone) and those of P. prostaxalis, so they placed it in a new species, P. applebyi, named in honor of Appleby and his work on ichthyosaurs.
[8] In 2019, Lomax, Laura Porro, and Nigel Larkin found that BMT 1955.G35.1, a partial skeleton initially identified as belonging to Ichthyosaurus communis, instead pertained to Protoichthyosaurus prostaxalis.
[8] The postorbitals (paired bones behind the orbits) of P. prostaxalis are unique in shape, being short vertically but wide from front to back and roughly rectangular.
The coracoids (paired shoulder bones positioned below the scapulae) bear broad notches on both their front and back edges.
In this species, a small projection known as the dorsal process on each humerus is located along the bone's midline and does not reach very far down its length, similar to the condition in Ichthyosaurus somersetensis.
The dorsal processes of P. applebyi are also located along the midlines of the humeri, though in this species they are vaguely plate-like and are present as thin ridges, similar to the condition in I.
The intermedium (middle upper wrist bone) does not touch distal carpal 4, another characteristic unique to Protoichthyosaurus.
[8] The individual bones composing these digits, the phalanges, are roughly rectangular in shape in the upper part of the flipper, though those further towards it tip are rounded.
[12] However, in 1979, Appleby noted that Protoichthyosaurus, as well as Leptonectes,[13] both had foreflipper morphologies intermediate between the latipinnate and longipinnate conditions, despite living in the Early Jurassic.
[1] The latipinnate-longipinnate distinction in ichthyosaurs was abandoned as a method of classification since studies such as Appleby's revealed that forefin morphology alone did not always follow trends shown elsewhere in the skeleton.
[4] In their 2017 revision of Protoichthyosaurus, Lomax and colleagues placed it within the family Ichthyosauridae due to the similarities between it and Ichthyosaurus and the results of their phylogenetic analysis, which found the two genera to be sister taxa.
[2] Mikadocephalus gracilirostris Macgowania janiceps Leptonectes tenuirostris Hudsonelpidia brevirostris Temnodontosaurus Eurhinosaurus longirostris Excalibosaurus costini Suevoleviathan disinteger Hauffiopteryx typicus Malawania anachronus Ichthyosaurus communis Protoichthyosaurus applebyi Protoichthyosaurus prostaxalis Stenopterygius quadriscissus Acamptonectes densus Ophthalmosaurus icenicus Sveltonectes insolitus Aegirosaurus leptospondylus Brachypterygius extremus Platypterygius australis Caypullisaurus bonapartei