Etymology: From Wessex, an ancient British kingdom which incorporated the Isle of Wight; and the Greek erpeton, meaning creeping animal, also used in the construction of the name of the type genus Albanerpeton Fox and Naylor, 1982.

Type and only species: Wesserpeton evansae; see below.

Diagnosis.—As for species.

Stratigraphic and geographic range.—As for the type species; see below.

Etymology: For Professor Susan E. Evans (University College London, UK) in recognition of her outstanding contributions in the field of microvertebrate palaeontology and to our understanding of Albanerpetontidae.

Holotype: NHMUK PV R36521, substantially complete fused frontals (Fig. 3C, 4A, 12D).

Type locality: Yaverland, south-east coast of the Isle of Wight, southern England, NGR SZ 61693 85223 (Figs. 1, 2).

Type horizon: Bed 38 (Radley 1994) occurring about 4 m below the top of the Wessex Formation, Barremian, Lower Cretaceous.

Referred material.—NHMUK PV R36522-36568 and R36595-36611 (Figs. 4–11) complete the hypodigm.

Stratigraphic and geographic range.—Known only from the Barremian Wessex Formation of the Isle of Wight, southern Britain, where it has been recorded from seven horizons (Fig. 2). All figured specimens are from the type locality and horizon with the exception of the frontals NHMUK PV R36539 (Fig. 4F) and R36522 (Fig. 4H) and the dentaries R36553-36558 (Fig. 10M–R) which are all from bed L2 (Fig. 2) exposed high in the cliff at Sudmoor Point (Fig. 1) on the south-west coast, NGR SZ 39458 82709.

Differential diagnosis.—Monotypic albanerpetontid species without any recognized autapomorphies, but which differs from other albanerpetontid genera and species in a unique combination of primitive and derived characters relating to the frontals, premaxilla, maxilla and dentary and to body size.

Frontals: primitively resembles Anoualerpeton and Celtedens, but differs from Albanerpeton in having frontals that are elongate and with the anterior limit of the orbital margin in front of the anteroposterior midpoint of the bone; resembles Albanerpeton and differs from Anoualerpeton and Celtedens in having a more derived, triangular outline of the frontals; differs from Celtedens and resembles Anoualerpeton and Albanerpeton in having a pointed internasal process (primitive).

Premaxilla: differs from Anoualerpeton and resembles Celtedens and Albanerpeton in having suprapalatal pit in premaxilla facing lingually (derived); differs from Anoualerpeton, Celtedens, and some Albanerpeton species, but resembles A. nexuosum, A. inexpectatum, A. pannonicum, and the Paskapoo species in having dorsal edge of premaxilla sutured to nasal in larger individuals (derived); differs from Anoualerpeton, Celtedens, and some Albanerpeton species, but resembles A. inexpectatum, A. pannonicum, and the Paskapoo species in having suprapalatal pit in premaxilla subdivided in at least one third of individuals (derived).

Maxilla and dentary: differ from Anoualerpeton and one Albanerpeton species (A. nexuosum), but resembles other Albanerpeton species and Celtedens in having dentaries and maxillae with relatively straight occlusal margins and teeth weakly heterodont in size (both primitive).

Body size: differs from Anoualerpeton, Celtedens, and some Albanerpeton species, but resembles A. arthridion, A. pannonicum, and the Paskapoo species in small body size (derived).

Description

The only complete specimens are “axis” vertebrae among which are NHMUK PV R36534 (Fig. 11C) and R36609 (Fig. 11D), and a humerus NHMUK PV R36610 (Fig. 11F), none of which possesses characters that are diagnostic at the genus or species level. However, the available frontals and jaws document almost all of the structure of these elements and do exhibit diagnostically informative characters. Detailed descriptions are provided and a large number of specimens are figured in order to demonstrate the considerable variation in osteological characters of the new species. Characters described include those that may have no phylogenetic or functional significance but some of which might be thought to in the absence of multiple examples of individual elements. Such descriptions are rarely presented, yet these are informative for recording the range of variability that might be seen in other albanerpetontid taxa if similarly large samples of individual elements were available for those. For ease of reference variations in significant characters of the premaxilla are also set out in Table 1.

Frontals (Figs. 3C, 4, 12D).—The most nearly complete specimen is the holotype NHMUK PV R36521 (Figs. 3C, 4A, 12D). It lacks only the anterolateral processes, the extreme posterior part of the ventrolateral crest on the left side and a smaller fragment from the right side. The two halves are solidly fused medially but posteriorly the suture can be seen in ventral view where it is visible for approximately half the distance to the anterior end. The midline length is 3.4 mm (excluding the posterior protuberances described below) and the width across the posterior margin is estimated (due to breakage) to be 2.6 mm. This yields a ratio of midline length:posterior width of 1.3. It is also possible to estimate the midline length and posterior width in another specimen, R36536 (Fig. 4B). In this specimen the midline length is about 3.6 mm and the posterior width about 2.7 mm giving a midline length:posterior width ratio of 1.33. The internasal process is triangular in dorsal outline and somewhat rounded anteriorly. In those specimens in which the internasal process is preserved, R36521 (Fig. 4A), R36536 (Fig. 4B), R36537 (Fig. 4C) and R36522 (Fig. 4H), it is somewhat variable in terms of both the ratio of midline length:posterior width and in the degree of roundness at the anterior end. This reflects intraspecific variation and not abrasion postmortem. The internasal process bears deep lateral slots for reception of the nasals and in one specimen, R36522 (Fig. 4H), the ventral surfaces of these slots are expanded laterally so that they are visible in dorsal view. The anterolateral processes are distinct from the main body of the bone but are incomplete anteriorly in all specimens. Their morphology can best be seen in R36523 (Fig. 4D) and R36540 (Fig. 4G). Reconstruction of the anterolateral processes suggests they were not as prominent as in some other albanerpetontids (Fig. 3C versus D). The posterior lateral slot for receipt of the posterior end of the prefrontal is also deep. This slot occupies a position between the posterior margin of the anterolateral process and the anterior end of the orbital margin. The dorsal margin of the posterior slot is excavated posteromedially (Fig. 4A₁). The anterior end of the orbital margin, taken to be the posterior end of the slot for reception of the prefrontal, lies anterior to the midpoint of the bone. R36521 is triangular in outline, with the posterior margin approximately 2.5 times wider than the distance between the slots for reception of the prefrontals. Behind the base of the anterolateral process, the lateral edge of the bone extends posteriorly in a gentle medially concave curve. Anteriorly a tangent to this curve intersects the midline of the bone at about 18° whereas from a position approximately one third of the distance from the posterior margin this increases to about 33°. The posterior margin of the bone is transverse and shallowly anteriorly concave to either side of the midline but is expanded posteriorly either side of that to produce rounded protuberances separated by a U-shaped valley (Fig. 4A). Dorsally, the bone is ornamented with shallow, polygonal pits of variable shape enclosed by a network of narrow ridges. In ventral view, the ventrolateral crests are broadest anteriorly but they do not meet across the midline in any of the available specimens. However, in some, e.g., R36523 and R36538 (Fig. 4D₂ and E respectively) they approach each other closely. They are strongly tapered posteriorly and bear a shallow groove of variable extent along their ventral surfaces. These grooves do not extend to the posterior end of the ventrolateral crests in any specimen. A minute foramen is present at the anterior extremity of both ventrolateral crests. Neither of these foramina open into the closely adjacent slots posterior to the anterolateral processes and they may be for emissary veins travelling between the intra- and extracranial surfaces via the diploe of the skull (Susan Evans, personal communication 2011). A larger foramen occupies the extreme posterior end of both ventrolateral crests and represents the posterior opening of canals underlying them. The canals are somewhat variable in diameter but can be clearly seen in all broken specimens (e.g., Fig. 4D₂ arrowed). Anteriorly the sub-ventrolateral crest canals open as slits at the medioposterior extremity of the slots posterior to the anterolateral processes. Posteriorly the crests are strongly convex ventrally and ridge-like in transverse profile. The more lateral part along the orbital margin is bevelled and faces ventrolaterally. Posteriorly the medial face of each ventrolateral crest bears a grooved facet for reception of a complementary process from the parietal. In R36536 (Fig. 4B₂) the complete left ventrolateral crest bears a sculpted facet at its posterior extremity for contact with the parietal. Gardner (1999d) estimated that the snout-pelvic length in albanerpetontids is about ten times the midline length of the frontals. Therefore, those available for Wesserpeton suggest a snoutpelvic length of about 35 mm.

Premaxilla (Figs. 5–7).—A substantial number of premaxillae have been recovered. Of these, 19 of the most complete and/or informative with regard to general morphology and intraspecific variation in characters are shown in Figs. 5–7. Two of the most nearly complete specimens NHMUK PV R36524 and R36526 (Fig. 5A and B respectively) document some of the morphological variation observed in this element and provide a framework from which to discuss variations observed in other specimens.

NHMUK PV R36524 (Fig. 5A) is from the right side. It is about 2.6 mm high. The bone is complete with the exception of parts of the left and right sides of the pars palatinum. The medial edge bears an elongate groove, which extends below the labial surface of the bone from a position just above the base of the dorsal boss to the level of the base of the dentition. The lateral extremity of the groove (its base) contains five evenly spaced and sized foramina and a minute foramen just above the level of the dentition. A pronounced flange separates the labial groove from a lingual groove extending from a position just below the level of the base of the boss to a position approximately one third of the distance from the base of the medial-most tooth to its apex. The ventral extremity of the groove is deepened to form a pit and on the lingual side at the dorsal extremity of the pit there is a small peg (Fig. 5A₂). Dorsal to this, the groove is continuous dorsally for the rest of its length and is expanded lingually over the medial surface of the pars palatinum. The presence or absence of foramina in this groove cannot be determined because of adhering matrix. In no specimen is there evidence of fusion between the right and left premaxillae but the deep grooves and the ventrolingual pit and prong seen in this specimen suggest a strong interlock between the bones. However, although all premaxillae in which this part of the bone is preserved display grooves of the type described above their morphology is very variable. A detailed description of some of these differences is provided below in connection with R36526. Another noteworthy example of differences in this aspect of the morphology of premaxillae occurs in R36596 (Fig. 6G), which is also from the right side. In this specimen there is a deep labial groove extending from almost the dorsal extremity of the bone to a level equivalent to the base of the dentition. The lingual groove is shallow and narrow, extending only from just above the dorsal margin of the pars palatinum to a similar position to that described above for R36524. It is separated from the labial groove by a considerably more medially expanded flange (Fig. 6G₃) than that of R36524. Furthermore, R36596 does not possess a pit and peg of the sort seen in R36524 and these are variably present in other specimens.

Labially (Fig. 5A₁), R36524 bears a pronounced dorsal boss ornamented with shallow polygonal pits similar to those seen on the frontals. The boss is variably developed in other specimens and is absent from some (see Table 1 and Figs. 5B₁, 6B₂, C₂, D₂, E₂, G₂, 7A₂, B₂, 1₂, J₂). All specimens show that the remainder of the labial face of the bone is relatively smooth with only minor differences in rugosity and that it is perforated by small, scattered nutritive foramina. The dorsolateral corner of the pars dentalis bears a broad, smooth facet for contact with the overlapping complementary premaxillary lateral process of the maxilla. A shallow laterodorsal notch occupies a little less than half of the dorsal margin of the pars dorsalis on its lateral side. A facet extending along the lateral margin of the notch is labiolingually expanded in the region of the ventral extremity of the boss, reflecting labiolingual thickening of the bone in this region.

Lingually, the apex of the pars dorsalis bears a sculpted facet for reception of the anterior margin of the nasal. This is also variably developed in other specimens, being particularly pronounced in R36597 (Fig. 7A₃). On the dorsolateral margin of the pars dorsalis on the lingual side there is a distinct but low, lingually convex ridge below the boss. A similar but narrower ridge also extends from the dorsal extremity of the mediolingual groove to the dorsal margin of the pars palatinum. Six minute foramina pierce the bone linguomedial to the narial margin and a more substantial foramen is present at the junction between the pars dorsalis and the lateral margin of the strut surrounding the lateral margin of the suprapalatal pit. In other specimens, e.g., R36526 described below, R36600 (Fig 7D₂) and R36604 (Fig. 7H₁) foramina in this area are larger and fewer in number (Table 1). The lingually opening suprapalatal pit is dorsoventrally elongate and oval in outline, and lies above the 4^th and 5^th tooth positions (as counted from the medial end following Gardner 1999c). Its medial and lateral margins comprise strongly raised struts rising from the dorsal surface of the pars palatinum. The orientation and height of these struts are very variable; so too is the shape of the suprapalatal pit, which is subdivided in at least one third of the specimens (Table 1) including R36526 described below (Fig. 5B) and others listed in Table 1, e.g., R36595 (Fig. 6F), R36600 (Fig7D₂) and R36604 (Fig. 7H₁). The dorsal opening of the palatal foramen is located at the labial margin of the pars palatinum immediately adjacent to the base of the medial face of the medial strut. In many others it is located between the struts, below the ventral margin of the suprapalatal pit but in one specimen, R36597 (Fig. 7A), there is some ambiguity. In this specimen the undivided suprapalatal pit is large, deep and extends to and below the dorsolabial margin of the pars palatinum. It is possible that in this specimen, as in some other albanerpetontids, e.g., Albanerpeton pannonicum Venczel and Gardner, 2005, the dorsal opening of the palatal foramen is located within the floor of the suprapalatal pit. However, this area is obscured by matrix and this condition is not observed in any of the other Wessex Formation premaxillae. A more likely interpretation is that the dorsal opening of the palatal foramen is a very small opening located medially at the base of the medial strut. This placement is seen both in R36524 described here and in a slightly more lateral position in R36600 (Fig. 7D₂). In close proximity to this but ventral and slightly lateral to it there is another opening suggesting that in this individual the dorsal opening of the palatal foramen may be divided. Another unusual feature of this specimen is the occurrence of an additional foramen close to the dorsal extremity of the medial strut where it is placed in the centre of the medial surface. In R36524 the dorsal opening of the palatal foramen is somewhat lateromedially elongate and oval in outline. No other foramina are present on the dorsal surface of the preserved part of the pars palatinum. A much smaller foramen, interpreted as the ventral opening of the palatal foramen, lies directly below the dorsal opening at the junction between the pars palatinum and pars dentalis. No other foramina can be seen on the ventral surface of the pars palatinum but the area above the 5^th tooth position is obscured by strongly cemented matrix. As in other albanerpetontids, a canal extends through the pars palatinum connecting the dorsal and ventral openings of the palatal foramen. Unlike the situation reported for other albanerpetontids (e.g., Gardner et al. 2003; Venczel and Gardner 2005), in Wessex Formation albanerpetontid premaxillae the orientation of this canal is variable (Table 1). In about half of the specimens, including R36524 (Fig. 5A₂) and R36566 (Fig. 6B₃), the canal extends vertically through the pars palatinum, as it does in all species of Albanerpeton (Gardner et al. 2003; Venczel and Gardner 2005). In the remaining specimens, e.g., R36601 (Fig. 7E₂), and as in Anoualerpeton (Gardner et al. 2003), the canal instead extends dorsolaterally—ventromedially through the pars palatinum as a result of a more medial placement of the ventral opening of the palatal foramen relative to the dorsal opening. Due to breakage, nothing further can be said about the morphology of the pars palatinum in R36524 other than that it appears to have comprised a lingually broad, horizontal shelf as in other albanerpetontids.

The pars dentalis of R36524 is deep and supports teeth for approximately three quarters of their length from the base to the apex. Nine tooth positions are present of which the 4^th, 6^th and 9^th loci are vacant, the last due to breakage of the tooth. At the 2^nd locus the basal part of the tooth on its lingual side is broken away. This reveals a hollow interior at the base of which there is what appears to be a replacement tooth at an early stage of development (Fig. 5A₃, A₄). The crowns of teeth are entirely discrete from but cemented to the overlying mediolaterally compressed and labiolingually expanded shafts. The shafts have flat medial and lateral surfaces and convex lingual surfaces. They are also labially convex above the ventral margin of the pars dentalis. The shafts thicken labiolingually from their bases towards the ventral margin of the pars dentalis and thereafter rapidly thin labiolingually, but not mediolaterally, as they approach the crowns. The latter are low relative to their mediolateral width and somewhat broader and wider at their bases than the shafts. They are chisel-like along their occlusal surfaces and faintly tricuspid with a central cusp that is slightly higher than the medial and lateral cusps. The crown of the tooth at the 3^rd locus appears to be worn as a result of dietary attrition. Other crowns appear unworn.

NHMUK PV R36526 (Fig. 5B) is from the left side. It is about 2.1 mm high, representing a somewhat smaller individual than R36524. This bone is also nearly complete, with the exception of the lateral part of the pars palatinum and a small part of the bone immediately dorsal to it. As in R36524, the medial edge bears an elongate groove that extends below the labial surface of the bone. However in this specimen it extends from the dorsal extremity, where it is shallow, to a position slightly dorsal to the level of the base of the dentition. The lateral extremity of the groove (its base) also contains evenly spaced and sized foramina of which four can be seen but the groove is partially filled with matrix possibly obscuring others. The minute foramen just above the level of the dentition in R36524 is not present in this specimen. The groove, which is shallow dorsally, becomes progressively deeper ventrally. At its ventral extremity it almost entirely covers the 1^st tooth position but here the labial roof is excavated laterally to produce a notch. At the ventrolingual extremity of the notch there is a small peg similar to that observed in R36524. As in R36524 a flange creates the lingual surface of the labial groove. However, this is of different morphology: it is entirely confluent with the medial edge of the labial surface of the groove whereas it extends medially beyond it in R36524; it is entirely discrete in R36524 whereas in R36526 it flows into and becomes the lingual surface of the pars dorsalis at about half its height above the pars palatinum (see also the description above of the grooves and flange present in R36596). Lingual to this flange there are what appear in medial view to be two similarly dorsoventrally orientated grooves. The dorsal extremity of the dorsal-most groove marks the point of confluence of the flange with the lingual surface of the pars palatinum. The dorsal groove is deep and very acutely triangular in medial view, the apex pointing dorsally and the base being expanded to excavate the bone between the dorsal and ventral surfaces of the pars palatinum. The ventral groove is in fact connected to the dorsal groove internally by a canal but is separated from it in medial view by a strut extending from the ventral surface of the pars palatinum to the flange separating the labial and lingual grooves. Immediately ventral to the strut the groove is deep but it rapidly becomes shallow ventrally where it comprises two shallow pits. Foramina are not present in the lingual groove(s).

Labially, R36526 bears a dorsal boss but this is dorsoventrally shorter and less prominent than that of R36524. One shallow polygonal pit is present on the lateral side but otherwise the surface of the boss is rugose and bears a number of minute nutritive foramina. As in other specimens, the remainder of the labial face of the bone is relatively smooth and perforated by small, scattered nutritive foramina. The dorsolateral corner of the pars dentalis bears a facet for contact with the overlapping complementary premaxillary lateral process of the maxilla. Unlike R36524, in which this facet is broad and smooth, that of R36526 is narrow and deeply excavated comprising a pit dorsally at the bottom of which there is a very small foramen. The laterodorsal notch is shallower than that of R36524 but similarly occupies a little less than half of the dorsal margin of the pars dorsalis on its lateral side. It also bears a facet.

Lingually, the apex of the pars dorsalis bears a sculpted facet for reception of the anterior margin of the nasal but this is less pronounced than that of R36524. Unlike R36524, on the dorsolateral margin of the pars dorsalis on the lingual side of R36526 there is only the faintest trace of a lingually convex ridge below the boss. Also in contrast to R36524, there is no ridge extending from the dorsal extremity of the mediolingual groove to the dorsal margin of the pars palatinum. In this specimen only two foramina pierce the bone linguomedial to the narial margin and these are both located at the junction between the pars dorsalis and the lateral margin of the strut surrounding the lateral side of the suprapalatal pit. The suprapalatal pit opens lingually but is subdivided in this specimen. The larger dorsal part is dorsoventrally elongate and oval in outline but is much smaller than that of R36524. The ventral opening is also oval in outline but is only about one third of the size of the dorsal opening. As in R36524 the suprapalatal pit lies above the 4^th and 5^th tooth positions. Its medial and lateral margins comprise raised struts rising from the dorsal surface of the pars palatinum but in this specimen the lateral strut is substantially more robust than the medial strut. Both lie diagonally ventrolaterally-dorsomedially across the face of the pars dorsalis. In this specimen the dorsal opening of the palatal foramen is located ventral to the suprapalatal pit at the junction with the pars palatinum and between the struts bordering the suprapalatal pit. It is somewhat smaller than the lower opening of the suprapalatal pit and is only about half the diameter of that of R36524. Also unlike R36524, two additional foramina are present at the junction of the pars dorsalis and pars palatinum. Both are small and of approximately equal diameter. One is located at the base of the lateral strut bordering the suprapalatal pit and dorsal opening of the palatal foramen and the other above the 7^th tooth position. Lateral to this the bone is broken but a small, circular area of calcite cement suggests that a third foramen may have been present above the lateral margin of the 8^th tooth. The ventral opening of the palatal foramen is slightly larger in diameter than the dorsal opening (the opposite condition occurs in R36524) and in this specimen it is located on the pars palatinum. It lies approximately one third of the distance from the junction between the pars palatinum and pars dentalis to the lingual extremity of the pars palatinum. It is located slightly medial to the dorsal opening suggesting that the canal connecting the openings is somewhat dorsolaterally-ventromedially inclined. Variation in the size and location of the ventral opening of the palatal foramen is seen in all specimens (Table 1), no two specimens being exactly the same in this respect. An extreme example is R36567 (Fig. 6C), in which the ventral opening is many times larger in area than the dorsal opening and several times larger than the suprapalatal pit. In R36526 the lateral part of the pars palatinum is broken away and it appears to be abraded medially as no lingually projecting vomerine process of the sort seen in R36603 (Fig. 7G but dorsal view not provided) is present. Between the 3^rd and 6^th tooth positions, the pars palatinum appears to be complete and is slightly labially excavated below the suprapalatal pit. Two foramina only slightly smaller in diameter than the ventral opening of the palatal foramen are present medially at the junction between the pars palatinum and the pars dentalis. One is placed above and between the 1^st and 2^nd tooth loci and the other above and between the 3^rd and 4^th. A third foramen of comparable diameter is similarly placed above the 8^th tooth locus. Four additional minute foramina also occur along the junction between the pars palatinum and the pars dentalis. As in R36524, the pars dentalis is deep and supports teeth for approximately three quarters of the distance from their bases to their apices. Also as in R36524, nine tooth positions are present. In this specimen the 1^st and 2^nd teeth are broken and polishing of the broken ends suggests that breakage may have occurred premortem. The 4^th and 9^th loci are vacant. The general morphology of the teeth is as described for R36524 but they are somewhat more gracile.

As outlined in the descriptions above, there is considerable intraspecific variation in the premaxilla. To further illustrate this, ten variable characters were chosen and recorded for each of the figured premaxillae (Table 1). Subjective terminology has been used to convey the observed range of variability for each character, but all specimens have been evaluated consistently. In cases where struts surrounding the suprapalatal pit are recorded as absent the pit is invariably located on a swelling on the pars dorsalis. In some cases this is somewhat raised medially and laterally but discrete struts as seen in the described specimens R36524 and R36526 are not present. No attempt has been made to numerically evaluate the areas occupied by the dorsal and ventral openings of the suprapalatal pit and only their relative sizes are recorded. In the table “additional foramina below pars palatinum” refers to those close to and immediately below the junction of the pars palatinum with the pars dentalis. The number recorded does not include minute foramina often present at or immediately below the bases of teeth. In the region close to and immediately below the junction of the pars palatinum with the pars dentalis, as in that medial to the orbital margin on the internal surface of the bone, the size of foramina referred to as “large” and “small” is variable between individual specimens. However, where differently sized foramina are present, they invariably fall into two discrete size categories rendering those recorded as “large” entirely distinct from those recorded as “small”. Counts recorded “as preserved” imply that part of the bone that in other specimens may bear foramina is either damaged, missing or partially obscured by matrix. With regard to tooth counts, two are recorded with question marks. In both specimens, the bones are damaged and their counts recorded as “?8”. This appears to be a reasonable estimate based on the preserved parts but no complete dentition has yet been recovered in which eight tooth positions are unambiguously preserved.

Maxilla (Fig. 8).—Maxillae are less commonly preserved than premaxillae for uncertain taphonomic reasons. Nevertheless two specimens, NHMUK PV R36528 and R36527, document all characters of this element. These and six further specimens also serve to demonstrate some of the ontogenetic and intraspecific variations that occur.

NHMUK PV R36528 (Fig. 8A) is from the right side and is complete save for a portion of the posterior end. As in other albanerpetontids, the maxilla is an elongate and dorsoventrally low bone. This specimen is 3.0 mm long, but based on the posterior morphology of R36527 (Fig. 8B), it was probably about 4 mm long when complete. The bone is a little under 1.1 mm high measured from the ventral margin of the pars dentalis to the dorsal margin of the nasal process. It includes loci for 21 teeth. Of these, counting from the anterior end (following Gardner 1999c), the 1^st, 6^th and 19^th are vacant. However, a replacement tooth at a very early stage of development is present at the 6^th locus and remnants of the base of a ?shed tooth are present at the 19^th. In terms of their general morphology, teeth are as described for the premaxilla. Teeth are somewhat variable in size with those occurring anteriorly being larger than those in the posterior half of the dentition. Nevertheless, heterodonty of the sort seen in Anoualerpeton (Gardner et al. 2003) and Albanerpeton nexuosum (Estes 1964; Gardner 2000a) does not occur. Tooth crowns show variable amounts of wear and some are unworn. The crown of the tooth at the 3^rd locus is missing and the apex is rounded. There is also a deep scratch extending anteroventrally-posterodorsally across the lingual side of the pedicel. These features suggest that the crown was lost some time pre-mortem, perhaps as a result of traumatic dietary attrition. The pars dorsalis comprises no more than a low ridge extending along the dorsolateral margin of the bone. Laterally it bears a broad facet for contact with the lacrimal. This commences at the base of the nasal process and extends to the equivalent of approximately half the distance to the posterior end, had the bone been complete. Posteriorly on the dorsal surface there is a facet for articulation with the jugal. This commences slightly posterior to the posterior end of the facet for articulation with the lacrimal and lies medial to it. The nasal process projects dorsally and in this specimen comprises a broad flange with an almost vertical anterior margin forming the posterior part of the external narial opening. The dorsal surface is rounded. In other specimens, e.g., R36559 (Fig. 8C), the anterior surface is posteriorly concave in lateral or medial profile. Furthermore, the lateral and medial outline of the nasal process is different in all specimens including those shown in Fig. 8 and in some, e.g., R36560 (Fig. 8D), R36562 (Fig. 8F), and R36564 (Fig. 8H), dorsally it more resembles a peg than a flange. Medially the nasal process bears a further facet for contact with the lacrimal. In the most robust specimen, R36561 (Fig. 8E), this facet extends onto the dorsal surface of the premaxillary dorsal process that is more horizontal in this specimen than that of R36528 which has an anteromedial margin that is more dorsally inclined. The anterior extremity of the nasal process is variably placed; it lies above the 3^rd tooth locus in R36528 (Fig. 8A) and R36527 (Fig. 8B); above the 2^nd tooth locus in R36560 (Fig. 8D); between the 1^st and 2^nd tooth loci in R36559 (Fig. 8C); and anterior to the dentition in R36564 (Fig. 8H). Difference in size between R36527 and R36528, in which placement of the anterior extremity of the nasal process is the same, indicates that this variability represents intraspecific variation rather than differences relating to ontogeny. The premaxillary dorsal process comprises a broad, lingually expanded and slightly ventrally concave shelf. As outlined above, it is of somewhat variable morphology, particularly with regard to inclination of the anteromedial extremity. The premaxillary lateral process is a prominent anteriorly projecting flange. It is also somewhat variable in shape and anterior extent but variations are less marked than those of the nasal process. The pars palatinum comprises a lingually expanded shelf, but anteriorly it is deeply indented by the internal narial margin. It also tapers posteriorly. In the central part between the 11^th and 17^th tooth loci it bears a facet on the medial edge for contact with a palatal bone. Dorsally four large foramina are present. The anteriormost lies dorsally at the base of the very low pars dorsalis. It is slit-like and is placed above the 7^th to 9^th tooth loci. Ventral to this and towards its posterior end an oval foramen lies above the 9^th tooth locus. Posterior and ventral to that location the third foramen lies above the 10^th tooth locus. The fourth foramen is placed dorsally above the 13^th tooth locus at a similar level to the first but is oval in outline. Ventrally, three large foramina lie at the junction between the pars palatinum and the pars dentalis. They are sub-circular in outline and placed above the 6^th tooth locus, between the 10^th and 11^th tooth loci and above the 17^th. A small, irregularly shaped foramen is also present at the base of the 7^th tooth. As with the premaxilla, the pars dentalis is dorsoventrally deep supporting teeth for approximately three quarters of the distance from their bases to their apices. It tapers posteriorly reflecting a progressive decrease in tooth height in that direction. The labial surface of the bone is smooth but it is pierced by a row of four very small nutritive foramina. These are located dorsally on the pars facialis above the 3^rd, 6^th, 9^th, and 14^th tooth loci.

NHMUK PV R36527 (Fig. 8B) represents an individual of intermediate size, being smaller than that represented by R36561 (Fig. 8E), the most robust specimen, and larger than the smallest represented by R36564 (Fig. 8H). It is substantially complete, lacking only the premaxillary dorsal and lateral processes. It is from the right side, is 3.1 mm long and, as measured for R36528 above, is 0.9 mm high. As in R36528 the labial surface of the bone is smooth but in this specimen it is pierced by a row of three very small nutritive foramina (Fig. 8B₃). These are similarly located dorsally on the pars facialis but above the 5^th, 7^th, and 9^th tooth loci. The dental row appears to be complete with loci for 26 teeth (Fig. 8B₁). Of these 16 are occupied and although the teeth have been somewhat abraded post-mortem it appears that loci without teeth were vacant pre-mortem. Replacement teeth at an advanced stage of development are present at the 13^th, 16^th, 18^th, and 21^st positions. They are identified as such because they almost, but do not completely, fill preexisting slots in the pars dentalis (Fig. 8B₂). Otherwise this bone is of similar morphology to that of R36528. Exceptions are the profile of the nasal process (described above) and the number and/or location of foramina perforating the pars palatinum dorsally, and at the junction between it and the pars dentalis ventrally. In this specimen the anteriormost dorsal foramen is anteroposteriorly elongate and located at the base of the pars dorsalis as it is in R36528. Here it extends from a position above the midpoint between the 6^th and 7^th tooth loci to that between the 8^th and 9^th. Posterior to this an oval foramen lies more ventrally above the 9^th tooth locus. The foramen posterior to this is located between the latter and the posteriormost foramen above the 10^th tooth locus. In this specimen it is very small. The posteriormost foramen is located dorsal to the preceding two, it is slit-like resembling the anteriormost foramen and extends from a position above the midpoint between the 10^th and 11^th tooth loci to that between the 12^th and 13^th. Despite differences of this sort, all specimens in which the dorsal foramina are preserved show the same general arrangement with the anteriormost and posteriormost foramina being placed dorsal to the two lying between them. The junction between the pars palatinum and the pars dentalis is perforated by at least 11 oval to round foramina. Most are very small but those above the 3^rd, 5^th, and 9^th tooth loci are more substantial. Similar morphological variations are observed in other specimens, including R36559 (Fig. 8C), which is from the left side. As in R36528, this lacks the posterior end and in this case also the premaxillary dorsal process. This specimen is intermediate in size between R36527 and R36228 described above. It is a little less than 2.5 mm long and 0.9 mm high and contains loci for 16 teeth of which 13 are occupied. Based on the posterior morphology of R36527, R36559 may have been about 3.7 mm long when complete. In this specimen all dorsal foramina are oval in outline and the middle two are large and of approximately equal area. Ventrally, only two foramina pierce the area between the pars palatinum and pars dentalis. One is located above the anterior margin of the 1^st tooth and the other above the 4^th tooth locus.

Dentary (Figs. 9, 10).—Dentary fragments are the most commonly encountered albanerpetontid remains in the Wessex Formation. No specimen is complete, but those available document all aspects of dentary morphology except for the region at the extreme posterior end. In only one specimen is a postdentary bone preserved in association with the dentary, a partial prearticular in R36549 (Fig. 10H), but a number of articulars have been recovered, one of which is almost complete and to which is fused an almost complete angular (see below, Fig. 11I). The most nearly complete dentaries are NHMUK PV R36541 and R36542 (Fig. 9). However, the latter (Fig. 9B) bears a pathology rendering the former the most useful for determining dentary morphology.

R36541 (Fig. 9A) is from the left side. At the anterior end part of the symphyseal region is missing and at the posterior end part of the area for attachment of the postdentary bones is missing. As preserved, the bone is approximately 4.2 mm long and at its posterior end the maximum height is a little less than 0.9 mm, measured from the dorsal margin of the dental parapet to the ventral margin of the bone. It tapers anteriorly and at the anterior end is a little less than 0.6 mm high. The complete dentition comprising 26 teeth is present and all loci are occupied. Counting from the anterior end, teeth at the 2^nd, 3^rd, and 8^th positions have been broken post-mortem, and the crowns of teeth at the 10^th and 15^th positions have also been damaged post-mortem. Elsewhere teeth show variable amounts of wear attributable to dietary attrition. Those at the posteriormost three positions lack crowns but have polished apices suggesting that the crowns were lost pre-mortem. Three of the teeth bear minor abnormalities: that at the 9^th locus has no mesial or distal cuspules and its main cusp is displaced mesially; that at the 12^th locus has a minute cuspule just below the crown on the labial surface; and that at the 18^th locus has a minute cuspule at the base of the crown on its mesial margin. The 26^th tooth is also abnormal in that its base is indented on the mesial margin and swollen distally. This is also seen in an anterior tooth of R36543 (Fig. 10A₂). Otherwise, teeth of R36541 are of the type seen in other albanerpetontids and are as described above for the premaxilla and maxilla; however, crown and shaft morphologies of dentary teeth appear to be more variable than those of the premaxilla and maxilla. For example, the crowns of R36557 (Fig. 10Q₃) are low and noticeably tricuspid whereas those of R36530 (Fig. 10C₂) are taller and less obviously tricuspid. Also, in the former specimen shafts are similar in width to, or mesiodistally narrower than, the crowns throughout their length whereas they are noticeably broader than the crowns in the latter specimen and R36546 (Fig. 10E₂), particularly in the region just below the dorsal margin of the dental parapet. In R36541, teeth increase in height from the 1^st to the 5^th locus and progressively decrease in height posteriorly from the 9^th locus. However, along most of its length the occlusal margin is essentially straight as is the dorsal margin of the dental parapet. This renders dentaries of Wesserpeton distinct from those of Anoualerpeton and Albanerpeton nexuosum (Estes 1964; Gardner 2000a; Gardner et al. 2003).

The labial surface is smooth and at the posterior end of the bone there is no trace of a scar for attachment of the external adductor musculature. It bears a single external nutritive foramen located below the 13^th tooth locus. However, the number and size of labial foramina are variable as R36549 and R36529 possess two small foramina and R36552 has three minute foramina in the posterior parts of these dentaries (not figured). In R36541 the external nutritive foramen occupies the anterior part of a slit-like anteroposteriorly orientated pit occurring a little more than half of the distance from the dorsal to the ventral margin. Below this foramen, and extending from a position below the 8^th tooth locus to the 17^th, there is a shallow scar that extends anteroposteriorly along the lateroventral and ventral surfaces of the bone. This marks the site of attachment of the intermandibularis musculature. The symphyseal region has been damaged post-mortem and the symphyseal prongs that interlock in a mortise and tenon fashion with complementary prongs on the opposite dentary as in other albanerpetontids are missing. They are, however, well preserved in other specimens, e.g., R36544, R36530, and R36555. These specimens display some variability in the structure of the symphyseal joint. In R36544 (Fig. 10B) and R36555 (Fig. 10O), both from the left side, two prongs of different size are present, the larger lying dorsally above the smaller. In contrast only one prong is present in R36530 (Fig. 10C), which is also from the left side. The symphyseal face in all specimens is vertical medially and posteriorly. On the posterior surface below the 1^st tooth locus in R36541 and similarly placed in other specimens, there is a foramen representing the anterior opening of the Meckelian canal (Fig. 9A₁). In dorsal view (Fig. 9A₃) the bone is broadly curved, the curvature being greatest along the anterior third of its length. In lingual aspect (Fig. 9 A₁) the dental parapet is shallow posteriorly and anteriorly but deepens in the region of the tallest teeth. The subdental shelf is labiolingually narrow posteriorly becoming broader anteriorly. The dorsolingual margin is somewhat raised along its length. Ventrally, the subdental shelf is dorsoventrally deep posteriorly but shallows progressively towards the symphysis. No large foramina are present at the junction between the dental parapet and the subdental shelf but numerous minute foramina are present at the bases of teeth. The posterior opening for the Meckelian canal extends anteriorly to underlie the 22^nd tooth locus. The lingual margin of the subdental shelf above this bears a facet marking the point of contact with the prearticular. This extends anteriorly terminating below the 20^th tooth locus. The remainder of the posterior part of the bone is missing.

R36542 (Fig. 9B) is of note because it bears a pathology resulting from severe trauma and possibly infection at the anterior end of the bone. This is one example of a number of dentaries from both North America and Europe displaying pathologies, and these are the subject of further study by the authors and co-workers. R36542 is from the right side. As preserved, it is approximately 4.8 mm long and at its posterior end the maximum height is a little more than 0.9 mm, as measured for R36541. This bone represents an individual of similar or slightly larger size than that represented by R36541. The greater length of the bone mostly reflects more complete preservation of the posterior end and the presence of abnormal bone growth at the anterior end. In its general morphology R36542 appears to have been similar to R36541 prior to trauma but may have had somewhat fewer teeth. Damage at the anterior end (Fig. 9B₁, B₂) renders a tooth count here problematic and the injured area posterior to about the 5^th tooth locus is obscured by siderite-cemented matrix to about the 10^th locus. A tooth count of about 23 seems likely. However, in this specimen, as in R36541, the anterior extremity of the posterior opening for the Meckelian canal extends anteriorly to underlie the 4^th tooth locus from the posterior end. This corresponds to the 22^nd tooth locus in R36541 for which the total count is 26. Damage to the dentary is primarily restricted to the dental parapet and the anterior end of the bone including the symphysis. Counting from the posterior end, the dental parapet has been displaced lingually and slightly ventrally in the region of the 3^rd to 5^th teeth. It is more substantially lingually displaced between the 11^th and 15^th teeth where a fracture between it and the subdental dental shelf has occurred (Fig. 9B₂). Anterior to this, part of the dental parapet has broken away and the bone appears to have been fractured dorsoventrally-mediolaterally immediately posterior to the symphyseal prong or prongs. However, abnormal bone growth, which may also reflect sites of infection, renders positive determination of a fracture here problematic. Despite these rather severe injuries, which must have caused feeding difficulties both at the time they were sustained and during the remainder of the animal's life, the individual clearly survived for some considerable time after being injured. The injuries have healed, leaving on the lingual side a flap of bone that projects ventrally from the site of fracture of the dental parapet but which does not adhere to the original bone medial to it (Fig. 9B₃). Anterior to this on the ventral surface, bone overgrowth also covers the site of injury adjacent to the symphyseal prong or prongs (Fig. 9B₃). Little of the original structure of this area remains and the prong or prongs have been replaced by an anteromedially projecting peg of deformed bone (Fig. 9B₂).

Articular and angular (Fig. 11I).—A number of articulars have now been recovered from the Wessex Formation, but most are fragmentary, comprising only the posterior end but in some part of the angular is also preserved. As in other albanerpetontids for which these elements have been described (Estes and Hoffstetter 1976; Estes 1981; McGowan and Ensom 1997; Gardner 1999c; Venczel and Gardner 2005), the angular is fused to the articular with no suture evident in available specimens. The most complete, from the right side, is NHMUK PV R36531 (Fig 11I), which lacks only part of the posterodorsal surface on the lateral side of the articular. This specimen exhibits a vertical surface on the articular for articulation with the quadrate and facet patterns typical of Albanerpetontidae (Evans et al. 2004). The surface for articulation with the quadrate faces posteriorly and, unlike the fragmentary specimen from the Wessex Formation described by Evans et al. (2004), it bears very distinct cotylar surfaces. The larger of these is on the lateral side and is dorsoventrally higher than it is mediolaterally wide. The surface on the medial side also comprises a rounded rectangle but the height to width ratio is lower than that of the lateral side, its dorsal margin being placed more ventrally. The dorsal and ventral margins are supported by lips of bone, but these are not present on the lateral and medial margins. The lateral surface (Fig. 1 11I₁) bears a deep facet for the posterior process of the dentary. A dorsal groove on this extends from the anterior end for a little over half the facet's length posteriorly. Slightly dorsal to this there is another groove extending anteriorly for about three quarters of the length of the facet and above this there is a similar groove which extends to the anterior limit of the facet dorsally. The posterior margin of the facet demonstrates that the articular was sheathed along most of its length by the dentary. Medially (Fig. 1 11I₂) there is a deep groove comprising the posterior part of the adductor fossa. Ventral to this the angular is fused to the articular with no suture being evident. On the posterodorsal surface of this, there is a rather indistinct facet for contact with the prearticular (Estes and Hoffstetter 1976; Estes 1981).

Prearticular (Fig. 10H).—In only one specimen (R36549) is part of a prearticular preserved. This is an anterior fragment that has become lodged in the posterior opening for the Meckelian canal so that its dorsal surface now lies laterally. Little can be said about the morphology of the prearticular because part of what remains is obscured and in addition to the posterior part, the anteroventral part is also broken away. However there is a swelling and thickening of the bone at the posterodorsal part of what remains.

Atlas and “axis“(Fig. 11C–E).—Unlike other lissamphibians in which the atlas is followed by a dorsal vertebra similar in morphology to others in the series, in albanerpetontids there are two joints in the neck. The atlanto-occipital joint permits movement of the head in a dorsoventral plane, as it does in other lissamphibians, whereas that between the atlas and the specialized first post atlantal vertebra (conventionally called the “axis”) permits lateral movement (McGowan 1998b). In this respect the first post-atlantal vertebra resembles the axis of amniotes although the two are not homologous.

Of the considerable number of atlantes available, NHMUK PV R36535 (Fig. 11E) is the most informative. As with all other Wessex Formation specimens, it lacks the neural arch and bears only the broken bases of the arch pedicels. Basally at the anterior margins of these and immediately posterior to the dorsal margin of the anterior cotyles there are deep but anteroposteriorly narrow grooves to accommodate the first spinal nerve. Open grooves rather than foramina for the first spinal nerve have also been recorded for Anoualerpeton priscus from the Middle Jurassic and Celtedens sp. from the Early Cretaceous of Britain (Gardner et al. 2003). As in other albanerpetontids (e.g., Estes and Hoffstetter 1976; Fox and Naylor 1982; McGowan 1996; Gardner 1999c, 2000a), the atlas is wider than it is anteroposteriorly long. The anterior cotyles are shallow and kidney-shaped, and the surfaces for articulation with the occipital bone extend across the ventral surface of the broad interglenoid tubercle. The dorsal and ventral surfaces of the latter are variably pierced by small foramina and some bear no foramina. In R36535 one is present on the left side and one on the right at the posterior margin of the ventral surface of the tubercle. The posterior cotyle bears no notochordal pit and has a dorsal concavity for articulation with a dorsal facet on the anterior cotyle of the “axis” (as further described below). Ventrolaterally the posterior cotyle bears two anterolaterally concave, semilunar facets for articulation with additional and corresponding facets on the anterior cotyle of the “axis”. These extend posteriorly, giving the impression in dorsal (Fig. 11E₂) and ventral view that the posterior cotyle is deeply anteriorly U-shaped.

Two “axes”, NHMUK PV R36534 (Fig. 11C) and R36609 (Fig. 11D), document characters of this element and some of the either intraspecific or ontogenetic variation in these characters. Both specimens are very similar in their general morphology to the “axes” of other albanerpetontids (e.g., Estes and Hoffstetter 1976; McGowan 1996, 1998b; McGowan and Ensom 1997). The bone is smaller than the atlas and comprises a centrum with no neural arch or other processes. Posteriorly there is a deep, anteriorly tapering, conical notochordal pit but the notochordal canal does not extend to the anterior end of the bone. The bone surrounding this pit is of even thickness and the cotyle slightly dorsoventrally compressed so that the posterior outline is broadly oval. Foramina are absent from the dorsal surfaces of both specimens, but ventrolaterally towards their dorsal margins both bear minute foramina which do not appear to open in the notochordal pit. Ventrally three small foramina are present in R36534. Two are located in a posterodorsally narrow slit occupying a position close to the midline of the bone at its extreme anterior end. The other minute foramen is located on the midline in the middle of the bone. In R36609 two foramina are present anteriorly as in R36534 but in this specimen they do not lie in a slit. Also in this specimen, there is no third foramen. The anterior surface of R36534, referred to elsewhere as a cotyle but which is functionally a condyle, bears three articulation surfaces. One is placed dorsally and the others ventrolaterally. These articulate, respectively, with the dorsal concavity and the semilunar paired facets of the posterior cotyle of the atlas. In R36534 the anterior cotyle is considerably dorsoventrally higher than that of R36609 although both are of similar width (Fig. 11C₁, D₁). Also, although both share the same pattern of articulatory surfaces, there are minor differences in their disposition (Fig. 11C, D). Whether or not these differences relate to ontogeny (apart from its anterior width R36609 is in other dimensions smaller than R36534) or to intraspecific variation awaits the recovery of a larger sample of “axes”.

Trunk vertebrae (Fig. 11A, B).—Most trunk vertebrae, e.g., NHMUK PV R36607 (Fig. 11A), have the neural arch broken away, but in one specimen, R36608 (Fig. 11B), part is preserved on the right side and the base remains on the left. From the preserved parts it is evident that the neural arch was at least as tall as or possibly slightly taller than the centrum. As in other albanerpetontids for which trunk vertebrae have been described (e.g., Estes and Hoffstetter 1976; McGowan 1996; McGowan and Ensom 1997), the centra are amphicoelous and hourglass-shaped, being very narrowly constricted in the centre of the bone, the anterior and posterior cotyles are circular in outline and have thickened rims, and the notochordal canal is anteroposteriorly continuous. In R36608, the transverse processes have been broken off close to their junction with the centrum but the bases can be seen to be anteroposteriorly narrow and dorsoventrally high. In R36607, the transverse processes are intact and, although similar in basal morphology to those of R36608, on R36607 the transverse processes additionally can be seen to be narrow, oval in cross section and unicipital at their lateral extremity. Ventrolaterally two minute foramina are present on the centrum on both the left and right sides of R36607. These are also present on the left side of R36608 but the right side is obscured by siderite overgrowth. In both specimens they are situated equidistant on either side of the anteroposterior mid point of the bone approximately one quarter of the distance from the centreline to the anterior and posterior ends. No other foramina or processes are present.

Humerus (Fig. 11F, G).—Numerous humeri have been recovered, but most lack the proximal end. However, one complete specimen NHMUK PV R36610 (Fig. 11F) and one almost complete specimen R36533 (Fig. 11G) are available. Both are from the left side and are of similar size and morphology. The shaft is slender and elongate and its axis passes through the proximodistal centreline of the humeral ball. The width of the proximal end in ventral view and that of the distal end in medial view are a little more than three times the diameter of the shaft in the middle part of the bone. As in other albanerpetontids (e.g., Estes and Hoffstetter 1976), the humeral ball is fully ossified and larger than the adjacent radial epicondyle and the less perfectly hemispherical condyle on the proximal end for articulation with the scapula. Above the humeral ball there is a deep triangular fossa (the fossa cubitus ventralis of some authors) at the proximal extremity of which there is a small foramen, as also reported for Albanerpeton inexpectatum (Estes and Hoffstetter 1976).

Ilium (Fig. 11H, J).—A small number of fragmentary ilia have been recovered, of which the most complete is NHMUK PV R36532 (Fig. 11J). This is from the left side and from which the distal part of the iliac shaft is missing. Unlike some anurans, there is neither a dorsal crest on the shaft nor a dorsal tubercle on the margin of the acetabular region. The shaft is straight and the preserved part shows no evidence of an ovoid or subcircular prominence on its lateral surface as seen in some urodeles (Gardner et al. 2010). The long axis of the shaft is slightly tilted posteriorly relative to the ventral surface (assuming a similar in-life orientation to urodeles), somewhat mediolaterally compressed and oval in cross section. The acetabulum occupies about one third of the acetabular region, which is triangular in lateral and medial outline (Fig. 11J₁, J₂). The acetabulum is elevated on a ramp-like pedestal that is highest adjacent to the base of the iliac shaft (i.e., dorsally). In lateral outline, the acetabulum is dorsoventrally elongated and its dorsal margin is convex. The acetabular surface is shallowly convex and its anterior, dorsal and posterior margins are in the form of a low rim. Relative to the long axis of the shaft, when viewed laterally, the somewhat laterally raised anterior edge of the acetabular region diverges at an angle of about 23°, whereas the more strongly laterally raised posterior edge diverges at about 47°. In another slightly larger specimen, R36611, from the right side and lacking most of the iliac shaft (Fig. 11H), the angles of divergence are about 30° and 55° respectively. The ventral surface, which in life presumably contacted the pubis and ischia as is typical for tetrapods in which the pubis is ossified, is somewhat sinuous, being dorsally concave anteriorly and ventrally convex posteriorly. The medial surface of the acetabular region is smooth, slightly laterally concave and bears a single foramen located somewhat anterior to the midline and about two thirds of the distance from the ventral margin to the base of the iliac shaft.