Ichthyosauria de Blainville, 1835
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publication ID |
https://doi.org/ 10.4202/app.00376.2017 |
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persistent identifier |
https://treatment.plazi.org/id/D95187B8-DB0A-5578-3F72-F9BDE642FB42 |
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treatment provided by |
Felipe |
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scientific name |
Ichthyosauria de Blainville, 1835 |
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Order Ichthyosauria de Blainville, 1835 Family Ichthyosauridae Bonaparte, 1841 Genus Ichthyosaurus De la Beche and Conybeare, 1821
Type species: Ichthyosaurus communis De la Beche and Conybeare, 1821 ; upper Hettangian– lower Sinemurian , Lower Jurassic of England, UK .
Ichthyosaurus somersetensis Lomax and Massare, 2017
Referred material.— NLMH 106234, a virtually complete, articulated skeleton lying on its left side, and missing almost the entire tail (see Material; Fig. 2). From Doniford Bay, Watchet, Somerset, UK. It was recovered from the Lower Jurassic (lower Hettangian) Blue Lias Formation, specifically the Caloceras johnstoni Ammonite Subzone of the Psiloceras planorbis Ammonite Zone (Bed 36).
Emended diagnosis.—As in Lomax and Massare (2017) with the following changes: total length> 300 cm but <350 cm; hindfin with three or four elements in third row, with one element (tarsal 3) in broad contact with astragalus; notching usually present in some elements of the leading edge of the hindfin, but absent in the tibia.
Description.—The specimen can be assigned to I. somersetensis , because it possesses the following autapomorphies: a broad, asymmetric maxilla with a fairly long anterior process, and delicate, very long, and slender posterior process in lateral view; a triradiate lacrimal with a posterior shelf at the base of the dorsal process; a jugal dorsal ramus that is only slightly curved, lacking a right angle dorsal bend; and an ilium that is wide relative to its length and more oblong than rib-like (Lomax and Massare 2017). The specimen also possesses the unique combination of characters found in I. somersetensis , including: a prefrontal that excludes the dorsal process of the lacrimal from the orbit margin; the anterior process of the jugal extends slightly beyond the anterior inner edge of the orbit; the premaxilla supranarial and subnarial processes are about equal in length, extending about half way across the dorsal and ventral margins of the external naris, with the nasal making up about half of the dorsal margin; and a humerus that is long relative to its width, with a small dorsal process that does not extend far down the shaft (Lomax and Massare 2017).
Skull, mandible, and dentition: The skull is 57.5 cm long, and is preserved in dorsolateral view ( Fig. 3). It is complete, but damaged posteriorly and dorsoventrally crushed, with some bones missing and others difficult to identify. The skull has also been restored in the posterior region, where a crack runs across the skull and mandible ( Figs. 2B, 3). The orbit is crushed and the shape is not original, although a complete sclerotic ring is preserved and does not fill the orbital margin. Unless otherwise stated, the skull morphology concurs with other specimens of the species.
The skull roof is damaged and crushed, but some information can be gleaned. A low medial ridge is formed by the parietals and the lateral border of the temporal opening is formed largely by the supratemporal. The contact of the latter with the parietal and postfrontal is indiscernible.
An element that is probably the squamosal is displaced posterior to the postorbital and separated by a large crack. This element is roughly rectangular and positioned ventral to the supratemporal. Irrespective of crushing, the prefrontal forms a small portion of the dorsal and at least half of the anterior margin of the orbit. The prefrontal extends ventrally to the level of the external naris, excluding the dorsal process of the lacrimal from the anterior margin of the orbit.
The lacrimal is triradiate, similar to the condition in the holotype of the species ( ANSP 15766 About ANSP ), but the dorsal process is larger and more robust in NLMH 106234 . This difference may be due to the crushing or represent individual variation. There appears to be a shelf at the base of the lacrimal, which is found in all examples of the species (Lomax and Massare 2017), although this is somewhat obscured due to the crushing. The dorsal process of the lacrimal makes up the posterior margin of the external naris and the anterior process of the lacrimal, although broken, forms about half of the ventral border of the external naris. The posteroventral process of the lacrimal makes up less than half of the anterior margin of the orbit. A thickened anterior process of the jugal extends slightly beyond the anterior margin of the orbit (although some of it may be buried by the maxilla) and alongside the posteroventral process of the lacrimal. The dorsal ramus of the jugal is damaged, but has a robust shape and clearly lacks a dorsal bend, as is characteristic for I. somersetensis (Lomax and Massare 2017) .
In dorsal view, the nasal is wide posteriorly, with the anterior-most portion extending as far forward as the anterior process of the maxilla. In lateral view, the nasal forms about half of the dorsal border of the external naris. The supranarial and subnarial processes of the premaxilla make up at least half of the external naris borders. The maxilla is very large, as in all specimens of the species, and its maximum (dorsoventral) height is about even with the mid-posterior edge of the external naris. The maxilla is asymmetric in lateral view, with a fairly long and abruptly narrowing anterior process that extends beyond the external naris. The posterior process of the maxilla is delicate, very long, slender, and extends well under the orbit.
In lateral view, the angular forms a small portion of the mandible and the anterior end extends forward to about even with the posterior edge of the orbit. The surangular, however, extends farther forward than the angular, as far anteriorly as the highest point of the maxilla. The posterior end of the dentary ends abruptly, about half-way across the orbit, where it overlaps the surangular.
The teeth have largely slender crowns with longitudinal striations. Most of the roots are continuous with the crown, although some of the roots are wider than the crown. The roots have coarse, longitudinal grooves. The posterior maxillary tooth crowns are much smaller than the premaxillary teeth.
Axial skeleton: There are at least 40 precaudal centra, including the atlas-axis, although six are probably missing (see Material and methods). At least one of the posterior dorsals or anterior caudals, probably the latter based on shape, is disarticulated and positioned beneath some of the caudal ribs. There are eight caudal vertebrae preserved in articulation, and an isolated caudal centrum in the matrix. The isolated caudal centrum may not belong with the specimen as it appears smaller than the preserved caudal vertebrae, although it could conceivably be from a more posterior section. In addition, the last two caudal vertebrae in articulation are partially reconstructed and all of the vertebrae beyond that point have been added to the specimen and are not discussed further (see Material and methods; Fig. 2).
The neural spine apices of most of the cervical through mid-dorsal neural spines appear to have been “carved” during preparation, as the tips are squared and differ in colour from the rest of the neural spine. However, it is difficult to confirm whether this is an artefact of preparation or genuine. Regardless, the portions of the neural spines that are definitely genuine are at least twice as high as the centrum height. The posterior dorsal and anterior caudal neural spines are unusual in having a V-shape notch at the tip ( Fig. 5 View Fig ), which is uncommon in Ichthyosaurus . This morphology is genuine, although some of the neural spines have been reconstructed to mirror the morphology ( Fig. 5 View Fig ). The neural spines of the posterior dorsals and anterior caudals are also wider and about the same height as the centra.
The longest rib measures 49 cm along its curvature, but the distal end is buried.
Pectoral girdle and forefin: Most of the pectoral girdle is obscured by matrix. However, the complete right scapula is preserved and visible in lateral view. It is a long, slender element, measuring 15.5 cm along the long-axis. The anterior end is wide and the shaft is narrow, whereas the posterior end is slightly flared, as is typical of all species of the genus ( Massare and Lomax 2017a).
The right forefin is preserved in dorsal view, but has been partially reconstructed (see Material and methods; Fig. 4 View Fig ). The humerus is crushed, in that a portion of the proximal region has been displaced posteriorly and some of the shaft surface is missing. The proximodistal length of the humerus measures 10.4 cm, its proximal and distal widths are about equal and the dorsal process is small. There is an irregular depression on the head, ventral and slightly anterior to the dorsal process. The dorsal process does not form a prominent ridge. The depression appears deeper than is typical of Ichthyosaurus somersetensis (Lomax and Massare 2017: fig. 7B), but this could be due to crushing, which gives the appearance of a tall proximal region ( Fig. 4 View Fig ). Alternatively it might be a pathological deformation ( Rothschild and Storrs 2003). About midway down the shaft is a slight expansion of the anterior side. This may be an artefact of crushing. The anterodistal end of the humerus is expanded slightly, but there is no anterior facet.
There are at least five primary digits, identified by the presence of an anterior digital bifurcation ( Fig. 4 View Fig ). Irrespective of the reconstruction, the bifurcation is present in digit II, at the third phalangeal row. The elements of the bifurcation are much smaller, more circular than the other digits, as is often seen in some specimens of Ichthyosaurus , including in the holotype of I. somersetensis ( DRL personal observation of ANSP 15766). A posterior accessory digit contacts the ulna and extends to almost the distal end of the fin. A second posterior accessory digit is present at the fifth phalangeal row. Digit V is prominent, as in all specimens of the genus. Although a large portion of the forefin is reconstructed, it is clear that the intermedium contacts both distal carpals 3 and 4. This contact rules out any suggestion of the specimen being an example of Protoichthyosaurus ( Appleby 1979; Lomax and Massare in press).
Pelvic girdle and hindfin: The pelvis is tripartite, as in all species of Ichthyosaurus (Lomax and Massare 2017; Fig. 6). Both ilia are present and probably exposed in lateral view. The right ilium, identified as that closest to the skull, is damaged proximally, thus the description is based upon the left ilium (Fig. 6). The ilium is more oblong, than rib-like, as in other species of the genus (Lomax and Massare 2017). It is slightly curved, with the concave side presumably facing posteriorly, although the orientation of the element makes this difficult to confirm. It is marginally shorter than the ischium, but about the same length as the pubis. The ilium has a fork-like proximal end, which is an unusual trait that, to our knowledge, has not been reported in any other Lower Jurassic ichthyosaur. A lateral ridge runs from the middle of the ilium to the proximal end, where it is flared. This ridge is also present on the damaged right ilium. The central portion of the fork-like proximal end is separated from both lateral ridges and extends further proximally. This morphology has not previously been reported in any species of Ichthyosaurus , although the lateral ridge is reminiscent of the ilium in BRSMG Ce 16611, another Ichthyosaurus specimen with an embryo. It is possible that this morphology may be due to pathology. The ischium is the longest of the pelvic elements. It is elongate and robust, compared to either the ilium or pubis, and is only slightly flared proximally. The pubis has a very narrow shaft with a widely expanded, “fan” shaped, distal end. This pubis morphology is present in several examples of Ichthyosaurus spp. , including BRSMG Ce16611 and NHMUK R 3372, which both contain embryos.
Both hindfins are preserved. The more complete, preserved closest to the skull and identified as the right, is pu is il il pu is fi ti fi ti c a 2 4 3 ii bi c a 2 3 ii bi 4 Fig. 6. Hindfins and pelvis of Ichthyosaurus A somersetensis Lomax and Massare, 2017 ( NLMH 106234) from the Lower Jurassic (lower Hettangian) of Doniford Bay, Watchet, Somerset, UK. The right hindfin (B) is the more complete of the two, exposed in dorsal view. The left hindfin (A) is in ventral view. Abbreviations: 2, tarsal two; 3, tarsal three; 4, tarsal four; a, astrag- 50 mm alus; bi, bifurcation; c, calcaneum; fi, fib- B ula; ii, metatarsal 2; il, ilium; is, ischium; pu, pubis; ti, tibia.
exposed in dorsal view, whereas the left hindfin is exposed in ventral view. The femur is long relative to its distal width. Its anterodistal end has an anterior facet and the posterodistal end is expanded posteriorly. The dorsal process is offset anteriorly. The ventral process is more centrally located, but is slightly offset anteriorly. The fibula is anteroposteriorly wider than the tibia but is proximodistally about the same length. Tarsal 2, the first element of the bifurcation, and the first two phalanges are notched, although the shape of the notch differs, as has been reported in some specimens of Ichthyosaurus ( Massare and Lomax 2016a: fig 10). There is one element (tarsal 3) in broad contact with the astragalus. However, a bifurcation of tarsal 2 is present, which results in four elements in the third row. We identify the anterior branch of digit II as the bifurcation. A small portion of metatarsal 2 contacts the astragalus. This differs from the condition described by Lomax and Massare (2017), who found that I. somersetensis had only one element (tarsal 3) directly in contact with the astragalus, with three elements in that row, and a bifurcation in a more distal row. In NLMH 106234, a distal bifurcation is present at the fourth phalangeal row, similar to that seen in NHMUK OR2013*. A posterior accessory digit is also present in the right hindfin, but is incomplete.
Embryo: The embryo is positioned between the ribs, near the mid-posterior dorsal vertebrae, on the block posterior to the crack ( Figs. 2B, 7 View Fig ). There are 23 centra preserved in the embryo, although only 16 are articulated or associated. Of the articulated section, the 11th vertebra, counted from the left, is poorly preserved and/or partly missing; it may have been restored ( Fig 7A View Fig ). The articulated vertebral column is 6.37 cm long. It is not possible to identify the exact portion within the column. However, some loose and isolated centra are very round and could belong to dorsal vertebrae. This may suggest that the articulated section also comprises dorsal centra, which would be consistent with the interpretation of the fin as a forefin. Numerous delicate ribs are preserved; some fragments are in articulation with the vertebral column whereas others are lying in the matrix. A very small partial fin is preserved, which is probably the forefin. The total length is 1.84 cm, but the humerus is missing or buried. Four primary digits are evident, although a fifth primary digit may be present, indicated by a possible distal bifurcation ( Fig. 7B View Fig ). It appears that the?radiale,?distal carpal, and?metacarpal are notched. This may, however, be an artefact of preservation ( Fig. 7 View Fig ). All of the forefin elements are highly cancellous, displaying a spongious texture. The rim of many of the phalanges possesses a “bottle-cap” like morphology, suggestive of poorly ossified bone, or perhaps calcified cartilage. Similar preservation is also present in very small examples of Ichthyosaurus (e.g., BU 5289). A large element, proximal to the fin, is too long to be the humerus and is probably the scapula, which is expanded proximally.
Stratigraphic and geographic range.— Hettangian (lowermost Jurassic), Blue Lias Formation of Somerset and possibly Dorset, UK (see Lomax and Massare 2017, for more details) .
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