Phalarodon fraasi Merriam, 1910

Roberts, Aubrey Jane, Engelschiøn, Victoria Sjøholt & Hurum, Jørn Harald, 2022, First three-dimensional skull of the Middle Triassic mixosaurid ichthyosaur Phalarodon fraasi from Svalbard, Norway, Acta Palaeontologica Polonica 67 (1), pp. 51-62 : 54-58

Phalarodon fraasi Merriam, 1910

Figs. 1–3 .

Diagnosis.—Modified from Merriam (1910) and Ji et al. (2015). Marginal teeth ankylosed; significant heterodonty between anterior and posterior teeth in the maxilla; dental groove absent anteriorly and posteriorly (not clear in premaxilla of PMO 235.393); number of primary digits in forelimb increased to six; six distal carpals; metatarsal III gives rise to two digits.

Material.— PMO 235.393, skull, mandible and some axial and appendicular elements, the Isfjorden area (Spitsbergen), Middle Triassic.

Description.—The skull, mandible and some axial and appendicular elements are present. Using CT, it is possible to see that the skull is slightly distorted, with the right side positioned lower than the left side ( Fig. 2 ). Ontogeny is not possible to determine as no bone surface is preserved. However, based on the size of the specimen and the relative size of the orbit, it is thought to be osteologically mature ( Fernández et al. 2005). No braincase or palatal elements are visible for description on the specimen or by using CT. PMO 235.393 is referred to Mixosauridae based on the long sagittal crest which reaches the nasal; the large anterior terrace of the upper temporal fenestra; and posterior teeth more robust than anterior teeth. The specimen is referred to Phalarodon based on the presence of significant heterodonty a labial mandibular shelf and the lack of a dental groove in the maxillary region ( Ji et al. 2015).

Skull: Premaxilla: Part of the premaxilla is damaged and missing on the left side. Using the CT scan, the right can be described in more detail. No over- or underbite is visible; however, the rostral tip is enclosed in matrix on slab D ( SOM: fig. S4). Numerous teeth are set in the element (see dentition). The dentition cannot be clearly defined due to poor contrast, but there is some indication of ankylosed thecodonty ( Fig. 3 ). This is likely a resolution and/or preservation issue, as the three-dimensional Phalarodon fraasi premaxilla ( PMO 210.122) described by Økland et al. (2018) shows sockets. In PMO 210.122, the sockets are separated by alveolar walls and the length of the tooth crowns nearly equals the length of the roots. The slight compression and distortion of the specimen described herein might be enough to destroy the partitions as these are very thin. Økland et al. (2018) also observed that the alveolar walls became more robust and better preserved posteriorly. Small sockets are situated adjacent to larger sockets in PMO 210.122, which may have contained smaller teeth or demonstrate an uneven pattern of tooth replacement. These are not visible in PMO 235.393.

The presence of supra- or subnarial processes of the premaxilla of PMO 235.391, are not visible on the left side due to the preservation of this region. Using the CT scan data of the covered right side ( Fig. 2 ), these processes appear absent as in all Phalarodon species ( Schmitz et al. 2004; Jiang et al. 2007; Liu et al. 2013). Here, the posterior margin of the premaxilla only forms the anterior margin of the external naris ( Jiang et al. 2007), a feature which is also present in PMO 235.391. This feature is also present in Mixosaurus panxianensis and may be a feature of all mixosaurids ( Jiang et al. 2006).

Similar to the Phalarodon cf. fraasi specimen ( GMPKU-P- 1032) the premaxilla of PMO 235.391 borders the maxilla along a diagonal suture. The suture is visible within a small area, although distorted by teeth and not clear. Using CT data for the right side, the suture is shorter than maxilla-nasal suture, a feature differing from Mixosaurus panxianensis and Phalarodon avatus , where the sutures to the nasal and maxilla are near equal in length ( Motani 1999, Ji et al. 2015). In the P. fraasi specimen PMO 210.122, Økland et al. (2018) described a three-dimensional premaxilla with anteroposteriorly-oriented coarse surface striations, which might also encompass foramina. This is not possible to see on PMO 235.393 due to preservation.

Maxilla: In lateral view, the maxilla is triangular with a long posterior process. It borders the premaxilla anteromedially, the nasal dorsally, and the lacrimal posteriorly. The anterior process of the maxilla appears short on the specimen, although CT analysis revealed this to be broken. The right side shows that maxilla terminates several centimetres anterior to the external naris and forms the entire ventral border of the external naris as in other specimens of Phalarodon fraasi and also Mixosaurus panxianensis ( Jiang et al. 2007; Ji et al. 2015).

The maxilla articulates to the prefrontal posterodorsally, the lacrimal posteriorly, and the jugal posteroventrally. The posteriorly-pointed maxilla (common in mixosaurids, see Motani 1999 and Jiang et al. 2007) is often overlapped by the jugal just anteriorly to the orbital margin, as in other specimens of Phalarodon fraasi and also Phalarodon atavus . However, due to distortion in PMO 235.393, the jugal appears to have been pushed anteromedially and is therefore partially disarticulated. Although slightly displaced, the position of this suture differs from that seen in Mixosaurus panxianensis , where it is located just posterior to the anterior orbital margin ( Ji et al. 2015).

From the CT scan, there are clear alveoli present in the maxilla. At least seven teeth are present in a single row on the better-preserved right maxilla. Replacement teeth are visible lingually. This confirms the observation in the Phalarodon fraasi specimen from Svalbard ( PMO 210.122, Økland et al. 2018) of a single row of teeth in the maxilla as in the type specimen UCMP 9853 ( Motani 2005a). Resultingly, we removed this character from the species diagnosis for P. fraasi ( Schmitz et al. 2004) . The posterior part of the maxilla bears significantly more robust and bulbous teeth compared to those in the premaxilla and the anterior part of the maxilla (see dentition). This is a dividing feature between Mixosaurus and Phalarodon , as Mixosaurus lacks true heterodont teeth which are entirely set in a dental groove ( Ji et al. 2015; see also discussion in Økland et al. 2018). As such, PMO 235.393 shares these diagnostic features with Phalarodon species. The CT images also revealed the lingual surface of the maxilla ( Fig. 3A 5), which has a clear lingual shelf for articulation with the palate, likely the pterygoid. The maxilla is placed labially in respect to the dentary, so occlusion surfaces between the posterior maxillary teeth and posterior dentary teeth appear to be present ( Fig. 3 ). This maxillary position also seems to be present in the type specimen of Phalarodon fraasi ( Merriam 1910: fig. 2).

Nasal: The nasal of PMO 235.393 appears reduced in mediolateral extent on slab A. This is likely an artefact of the slightly skewed view and preservation, as the right nasal has a larger mediolateral extent seen in CT ( Fig. 2 ). The nasal has a long anterior process which surpasses the anterior margin of the external naris and maxilla. The nasal forms the dorsal border of the external naris as in Phalarodon atavus ( Liu et al. 2013) . There is an indication of an anteroposteriorly orientated ridge on the CT scans, directly above the external naris. There is no descending process of the nasal. The posterior extent of the nasal is difficult to discern on the specimen or the CT scans, due to calcite crystal growth inside the concretion in that region. However, a contact between the prefrontal and nasal appears present, although its extent is not visible ( Fig. 1 ). The nasal appears to terminate posterolaterally with a contact to the frontal where it forms the anteriormost part of the sagittal crest.

Lacrimal: The lacrimal forms the anteroventral part of the orbit. Schmitz et al. (2004) described an antorbital ridge on the lacrimal of Phalarodon callawayi . In Phalarodon fraasi , this has been described as “prominent, triangular knob” in the type specimen ( Merriam 1910: 387). This is more-clearly figured in Motani (2005a: fig. 2). No clear antorbital ridge is visible on PMO 235.393 due to matrix covering the surface in slab D (see SOM: fig. S4). However, in lateral view, a slight eminence is raised where the prefrontal overlaps with the lacrimal, indicating an antorbital ridge may be present. Similar to Phalarodon atavus ( CCCGS LPV 30872, Liu et al. 2013), a small and narrow posteroventral process is overlapped by the jugal in lateral view. The anterior part of the orbit is not possible to describe in detail and the border between the prefrontal and the lacrimal is indiscernible. Based on observations from PMO 235.393 and descriptions of the Phalarodon fraasi type ( Merriam 1910; Motani 2005a), we consider it unlikely that the lacrimal is as small as shown in Jiang et al. (2007) for Phalarodon fraasi GMPKU-P-1032. The element presented there as the prefrontal is more likely to be largely formed by the lacrimal.

Jugal: Visible on the CT segmentation ( Fig. 2 ), the left jugal is slightly displaced and moved further anteriorly than in life. This would have likely fanned out and covered part of the maxilla and lacrimal as is visible on the well-segmented right side ( Fig. 2 ). The left jugal is partly exposed on the left side in lateral view on slab A, with part of the posterior ramus preserved on counter slab C ( SOM: fig. S3). It forms at least one third of the posterior margin of the orbit.

Prefrontal: The prefrontal forms the anterodorsal part of the orbit. A supraorbital crest is present on the prefrontal, although it is uncertain how far posteriorly this crest stretches due to preservation. This is feature is present in many mixosaurids and is clear on Mixosaurus kuhnschnyderi ( Maisch and Matzke 2000) , Mixosaurus cornalianus ( Maisch and Matzke 1997, 1998) Phalarodon atavus and Phalarodon fraasi ( Ji et al. 2015) .

Orbit and sclerotic ring: The orbital margin is circular anteriorly and more irregular posteriorly. This differs from the more irregular orbits seen in some other Triassic ichthyosaurs ( Fernández et al. 2005). The sclerotic ring is partially preserved on the left side, there are at least five elements forming the anterodorsal part of the ring. In cross-section, these curve inwards towards the socket and would have followed the shape of the eye itself, as described in Fernández et al. (2005).

Frontal: Most of the frontal is covered and not visible on the specimen or in the CT images. Only part of the left frontal is preserved on slab B ( SOM: fig. S1) and as an impression on slab A in dorsal view. This preserved portion of the frontal is slightly concave and meets the prefrontal laterally. No part of the frontal-forming sagittal crest is visible.

Postfrontal: The postfrontal is only preserved as an impression on slab A, but is well preserved in dorsal view on slab C ( SOM: fig. S2). In dorsal view, the postfrontal has an interdigitating suture with the prefrontal anteriorly (anterior third of orbit). Posteriorly, the postfrontal meets the anterodorsal process of the postorbital.

Postorbital: Most of the postorbital is visible in medial view on slab C ( SOM: fig. S3). An impression and dorsal section of the element is present on slab A. Dorsally, the postorbital is overlapped by the supratemporal. Due to significant breakage in this area, the nature of this articulation and the extent of the border with the postfrontal is not entirely clear. Using slab C ( SOM: fig. S2), there appears to be small overlap of the postfrontal at the anterodorsal part of the postorbital. Ventrally on slab C, a separate bone fragment which is likely a part of the jugal is present, although these do not clearly articulate ( SOM: fig. S3).

Parietal and temporal fenestrae: The parietal is partly preserved as an impression on slab A ( Fig. 1 ) and as an element on slab B ( SOM: fig. S1). The temporal fenestra is visible on slab B, with parts of the parietal preserved along the margin. It is likely part of the sagittal crest, but the height and crest-forming elements are equivocal due to matrix cover and preservation.

Quadrate: The quadrate is preserved on part C and exposed in medial view, but partly encased in sediment ( SOM: fig. S2). The outline of the element as shown in Fig. 1 , is an impression from slab C preserved on slab A. The element is rather large, stretching to most of the dorsoventral height of the postorbital region. As in Phalarodon fraasi (GMPKU-P-1032), the quadrate condyle is massive ( Jiang et al. 2007). In lateral view, most of this element would be covered by the postorbital, quadratojugal, squamosal and/or supratemporal.

Other cranial elements: No parts of the braincase, palate or quadratojugal are visible on the specimen or on the CT scan images due to the poor contrast in the posterior region of the skull. A possible part of the squamosal is marked in Fig. 1 .

Mandible: Both lower jaws are preserved, although damaged, as a portion is preserved on counter slab D and E. There is no clear transition between the dentary and angular/surangular. In PMO 210.122, a small horizontal shelf of the dentary (labial mandibular shelf, Jiang et al. 2007) on the labial side offsets the teeth slightly towards the lingual margin ( Økland et al. 2018). This is also preserved in PMO 235.393, but is more posteriorly pronounced. This is clearly visible in the segmented image of the maxilla and dentary Fig. 3 ). This feature is also observed in other Phalarodon fraasi and Phalarodon callawayi specimens ( Motani 2005a; Jiang et al. 2007).

Phalarodon fraasi has been described as having a dorsally inflected and posteriorly extending angular which is larger than the surangular ( Jiang et al. 2007). The variation within Phalarodon is not known. The suture between the angular and surangular has not been described for Phalarodon callawayi ( Schmitz et al. 2004) . It is not possible in PMO 235.393 to determine the exact suture between the angular and surangular. However, the angular seems to have contributed to at least half the posterior portion of the mandible laterally. Part of the posterior section of the mandible including the articular and quadrate joint, is visible on slabs A, C ( SOM: fig. S3). The element is skewed, making it appear more extensive than the actual extent. Part of the outline is exposed and shows the curved contact to the quadrate.

The dentition of the mandible is heterodont ( Figs. 2 , 3 ). Anteriorly, conical teeth match those in the premaxilla. Posteriorly, the teeth increase in size and become rounded crushing teeth. These are significantly larger than their counterparts in the maxilla. All teeth are preserved in separate alveoli as in PMO 210.122 ( AJR personal observation).

Hyoids: Two hyoids are preserved in place, between the two lower jaws. These are only an impression on slab A, but are preserved on counter slab E. They are slightly medially convex with a larger anterior than posterior head.

Dentition: Although obscured on the specimen itself, teeth from PMO 235.393 are clearly visible in the CT scan Figs. 2 , 3 ). It is unclear how many teeth would have been present in the premaxilla and mandible.

The teeth on PMO 235.393 vary from conical to slightly anteroposteriorly elongated in the anterior region of the jaw. In the posterior region, they are strongly anteroposteriorly elongated with oval cross-sections. This differs from Phalarodon callawayi , which has circular teeth in cross-section in the anterior part of the jaw, and from Phalarodon atavus which has conical tooth crowns with a figure-of-eight cross-section in posterior maxillary teeth ( Schmitz et al. 2004; Ji et al. 2015).

The dentition in the anterior part of the premaxilla and dentary consists of elongated piercing teeth which end in a sharp point ( SOM: fig. S5). This differs significantly from the flattened and anteroposteriorly elongated crushing teeth in the posterior parts of the maxilla and dentary. The teeth in the posterior end of the dentary are twice the size of the teeth in the maxilla, both in height and anteroposterior length. An anteriorly positioned tooth in the maxilla is 3 mm anteroposteriorly long, which indicates a size around 5 mm in anteroposterior length for the more posterior teeth in the maxilla. Posterior teeth in the maxilla and in the dentary can be studied in CT scans. The posterior teeth in the dentary are approximately twice the size of the posterior teeth in the maxilla, approximately 10 mm. There are at least seven teeth preserved in the right maxilla.

There is a distinct difference between the tooth attachment in the anterior parts of the premaxilla and the dentary. From CT scans, the teeth are clearly visible in sockets in the dentary ( Fig. 3 ). In the premaxilla, these are diffuse and hard to identify. This fits the description for Phalarodon fraasi by Motani (1997) as having subthecodont dentition anteriorly only in the upper jaw. In the maxilla, alveoli are clearly visible, and the dentition appears to be anchylosed thecodont as described by Motani (1997). This is not as clear posteriorly in the dentary.

Postcranial material: At least six vertebrae are seen on the different slabs, together with disarticulated neural arches. Many of these are better preserved on slab C. Several ribs are partially preserved. In addition, there are some broken appendicular elements, either the scapula or the coracoid. Due to the fragmented nature this is not possible to determine. For a better-preserved pectoral girdle of Phalarodon fraasi from Svalbard see Økland et al. (2018).

Stratigraphic and geographic range.—Middle Triassic, China, Central Europe, North-America, Svalbard.

Figures

Fig. 1. Mixosaurid ichthyosaur Phalarodon fraasi (Merriam, 1910) PMO 235.393, from the Botneheia Formation, Middle Triassic of the Isfjorden area in Spitsbergen, Svalbard. A. Slab A in concretion, photograph (A1), line drawing (A2) showing the outline and identification of the visible bone elements. B. Hypothetical reconstruction of PMO 235.393.

Fig. 2. The segmented model of right lateral side of the skull of mixosaurid ichthyosaur Phalarodon fraasi (Merriam, 1910) PMO 235.393, from the Botneheia Formation, Middle Triassic of the Isfjorden area in Spitsbergen, Svalbard.

Fig. 3. The computed tomography rendered segmentation of rostral elements of mixosaurid ichthyosaur Phalarodon fraasi (Merriam, 1910) PMO 235.393, from the Botneheia Formation, Middle Triassic of the Isfjorden area in Spitsbergen, Svalbard. Right dentary and maxilla in lingual (A1) and labial (A2) views, right dentary (A3) in dorsal view, right maxilla in dorsal (A4) and lateral (A5) views. Note that the dorsal extent of the maxilla is not complete. Scale bars 10 mm.