Protoichthyosaurus

Protoichthyosaurus; Temporal range: Early Jurassic, Hettangian PreꞒ Ꞓ O S D C P T J K Pg N ↓
	Reconstructed skull
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	†Ichthyosauria
Family:	†Ichthyosauridae
Genus:	†Protoichthyosaurus; Appleby, 1979
Type species
	Protoichthyosaurus prostaxalis; Appleby, 1979
Other species
	Protoichthyosaurus applebyi Lomax et al., 2017;

Protoichthyosaurus is a genus of ichthyosaur from the early Jurassic of southern England. Two species are known, P. prostaxalis—the type species, named by Appleby in 1979^[1]—and P. applebyi.^[2] A third species, P. prosostealis, was named by Appleby,^[1] but it was removed from the genus in 2017 due to its similarity to Ichthyosaurus.^[2] The genus Protoichthyosaurus was synonymized with Ichthyosaurus by Maisch and Hungerbuhler in 1997, and again by Maisch and Matzke in 2000.^[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. The species most likely lived during the Hettangian stage, but may have lived as early as the Rhaetian and as late as the Sinemurian.^[2]

Species belonging to the genus were medium-sized, with P. prostaxalis measuring no more than 2.5 metres (8 ft 2 in) in length and P. applebyi reaching 2 metres (6 ft 7 in) at most. P. prostaxalis can be distinguished from P. applebyi and from other ichthyosaurs by the large, tall, and triangular maxilla that extends beyond the nasal bones at its front end; a vertically short but thick postorbital bone; and the lacrimal bone having an upward projection longer than its forward projection. Meanwhile, P. applebyi can be distinguished by the narrow, crescent-shaped postorbital; the low maxilla; the nasal reaching to the front of the maxilla; the lacrimal having a forward projection the same length as or longer than the upward projection; and the presence of a plate-like upward projection on the humerus.^[2]

The nominal species Ichthyosaurus fortimanus Owen, 1849-1884 based on the holotype forefin NHMUK R.1063 and synonymized with Ichthyosaurus communis by McGowan (1974), was referred to Protoichthyosaurus, as P. fortimanus, based on comparisons with known Ichthyosaurus and Protoichthyosaurus species.^[5]

Description[]

Protoichthyosaurus prostaxilis is a medium-sized ichthyosaur, with the total length of the largest specimen likely somewhere between 3.2–4 metres (10–13 ft) in total length.^[6] P. applebyi is smaller, probably not exceeding 2 meters in total length.^[2]

Skull[]

The external nares (nostril openings) of Protoichthyosaurus are large and roughly triangular in shape. In some specimens,^[7] none of their borders are formed by the maxillae (back upper tooth-bearing bones) as they are cut off by the premaxillae (front upper tooth-bearing bones) and lacrimals (paired bones in front of the eye sockets).^[2] The maxillae bear elongate, slim front portions are not symmetrical.^[5] The maxillae of P. prostaxilis are unique in shape; they are large, tall, and triangular with at least half the length of their forwards-projecting processes in front of the external nares; these bones extend further forwards than the nasals (a pair of skull roof bones located towards the front of the skull). While the maxillae also extend in front of the external nares in P. applebyi, in this species, the maxillae are low and the nasals extend further forwards than they do. The teeth of Protoichthyosaurus bear large grooves reaching the crowns' bases^[5] and are firmly rooted.^[2]

The nasals have wide rear ends. There is an opening between the nasals towards their rear ends that, in some specimens,^[5] can be quite long and pronounced. The lacrimals of Protoichthyosaurus are three-pronged. The upper prong is much longer than the front prong in P. prostaxilis, a morphology unique to this species. In P. applebyi, the forwards-projected prong is at least as long, if not longer, than the upper one.^[2] The forwards projections of a pair of skull roof bones known as the prefrontals (a pair of skull roof bones) isolates the upper prongs of the lacrimals from the orbital rim, similar to some species of Ichthyosaurus.^[5] 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. Like Ichthyosaurus communis, they form less than half of the rear edges of the orbits in this species. In P. applebyi, the postorbitals are instead thin and crescent-shaped, tall but narrow from front to back.^[2]^[5] Additionally, like many species of Ichthyosaurus, the postorbitals of P. applebyi form significantly more than half of the rear rims of the orbits.^[5]

Measured from their tops to their bottoms, the prefronals have wide front ends but narrow rear ends in P. prostaxalis, as in some Ichthyosaurus species. The pineal foramen (a small opening on the top of the skull) is positioned between two pairs of skull roof bones, the frontals and parietals, as seen in many other Early Jurassic ichthyosaurs,^[2] with the rear edge of the foramen formed by the parietals.^[5] The squamosals (paired bones near the rear of the skull) of P. prostaxalis are shaped like rectangles and each bear a downwards-pointing, triangular projection on their rear lower corners, a morphology also seen in Ichthyosaurus somersetensis.^[2]

Postcranial skeleton[]

The scapulae (shoulder blades) of Protoichthyosaurus are elongate. While the front edges of each scapula are slightly expanded, they lack prominent eminences known as acromion processes. The coracoids (paired shoulder bones positioned below the scapulae) bear broad notches on both their front and back edges. Both ends of each humerus (upper arm bone) are about the same width, although the middle of the bone is slightly constricted, as also seen in Ichthyosaurus. The humeri of P. prostaxalis are robust. 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. larkini.^[2]

Uniquely in Protoichthyosaurus, distal carpal 3 articulates with the ulnare (the carpal below the ulna). The intermedium (middle upper wrist bone) does not touch distal carpal 4, another characteristic unique to Protoichthyosaurus. In fact, distal carpal 3 is the only one of the distal carpals to have extensive contact with the intermedium. There are three distal (lower) carpals in Protoichthyosaurus, but due to digit II bifurcating, there are four metacarpals, a configuration known only in Protoichthyosaurus. This bifurcation nearly entirely intrudes between distal carpals 2 and 3 and separates them, a trait unique to Protoichthyosaurus. There are a total of three primary digits (digits that originate from the wrist) in Protoichthyosaurus,^[5] although digit V is absent. Digit II forks again further down the flipper, similar to Ichthyosaurus.^[2] Thus, the two bifurcations provide a total of five digits per foreflipper.^[5] 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. The phalanges are tightly packed together. The upper end of each femur (thigh bone) is robust in P. prostaxalis, while the middle part is thinner and the lower end is wider than the upper. There are three bones in the distal tarsal row, the third row of bones below the femur, in each hindflipper of P. prostaxalis.^[2]