Macroprosopon hiltoni Capobianco et al., 2025

† Macroprosopon hiltoni Capobianco et al. , sp. nov.

ZooBank LSID: urn:lsid:zoobank.org:act:7F3B3C86-99CC-4D11-9997-D2408220646D .

Holotype: FSAC CP 330 , an almost complete and three-dimensionally preserved skull articulated with part of pectoral girdle and axial skeleton. This specimen was previously catalogued as UMMP 118216, and has been mentioned under that number by Capobianco et al. 2021. Reference casts are kept at the UMMP .

Etymology: Specific name in honour of Eric J. Hilton (Virginia Institute of Marine Science and College of William and Mary), in recognition of his fundamental contributions on bonytongue comparative anatomy and systematics, and on ichthyology in general.

Type locality/horizon: Due to non-specialist private collection of the specimen, information on the locality for FSAC CP 330 is limited to the early Eocene (Ypresian) phosphates of the Ouled Abdoun Basin , Morocco ( Fig. 1 View Figure 1 ). The surrounding matrix provides two lines of corroborative evidence. First, a slightly deformed and damaged shark tooth embedded in the matrix was tentatively identified as a posterior tooth of † Brachycarcharias atlasi Arambourg 1952 ( Fig. 1C View Figure 1 ), which occurs in the Thanetian–Ypresian strata of the Ouled Abdoun phosphates (C. Underwood, Birkbeck College, pers. comm. 2020). Second, the matrix includes poorly sorted peloids, and is thus lithologically consistent with Ypresian phosphates in the basin (Beds I and 0; Yans et al. 2014, Zouhri 2017).

Diagnosis: Osteoglossiform with roughly triangular skull profile, relatively long jaws and terminal mouth; bulbous antorbital with strong ornamentation; two semicircular scleral rings; approximately 23 maxillary teeth;>26 dentary teeth; bony collars at tooth base less than half the tooth height; lower jaw more than three times longer than deep; very long posterior process of the hyomandibula; opercle with dorsally-oriented concavity above the articular facet; 18 branchiostegals. † Macroprosopon hiltoni differs from † Furichthys in having the retroarticular included in (instead of excluded from) the articulation between lower jaw and quadrate; and the posterior process of the hyomandibula longer (rather than shorter) than the dorsal articulating surface of the hyomandibula. † Macroprosopon hiltoni differs from both † Phareodus and † Brychaetus in having proportionally much longer lower jaws; supraorbital shelf of the frontal not extending to the anterior margin of the frontal; posterior toothless portion of maxilla not substantially deeper than toothed portion; cleithrum extending anteriorly to the level of the angular (rather than extending to just below the preopercle).

Description

The holotype is broken transversely into two blocks that meet at the level of the opercle ( Figs 2–9 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 View Figure 9 ). It is strongly medio-laterally compressed; this compression caused the collapse of the skull roof on the left side of the specimen, which in turn resulted in several bones on the left side being crushed or completely missing. The anteriormost part of the skull is completely missing. Bones in the specimen have a widely varying state of preservation: some are heavily damaged and/or delaminated such that their surface is often missing (e.g. infraorbitals, opercular series, and left angular); others are almost pristine (e.g. parts of the skull roof and branchiostegal rays). The posterior block of the holotype is covered in layered, broken scales.

Neurocranium ( Figs 3 View Figure 3 , 7 View Figure 7 ): The anteriormost portion of the neurocranium (including nasals, vomer, and part of the ethmoid region) is missing from the specimen. A thin, rounded bone antero-medial to the antorbital on the left side of the specimen is tentatively interpreted as the lateral ethmoid. It is slightly concave medio-laterally. Several fragments of bone located medially to the lateral ethmoid and antero-ventrally to the frontals might represent the sole exposed portion of the parasphenoid. The frontal is very long, accounting for approximately two-thirds of the skull roof length when excluding the nasals. It possesses a broad supraorbital shelf overlying the orbit and articulating antero-ventrally with the antorbital. The shelf bears a radial pattern of ornamentation on its dorsal surface, consisting of furrows and shallow pits. The anterior margin of the frontal appears to be only slightly broader than its posterior margin, as the supraorbital shelf does not seem to extend anteriorly to the articulation with the nasal. At the level of the orbit, the frontal is at least 1.5 times broader than its posterior margin. The supraorbital canal does not run throughout the whole length of the frontal. The suture between the two frontals is not visible, because the right frontal partially overlaps the left one due to taphonomic distortion of the specimen. The suture between frontal and parietal is at least partially interdigitated. The parietal is short and bears a transverse crest dividing it in two portions: the anterior one is ornamented, while the posterior one is depressed with respect to the rest of the skull roof and forms part of a dorso-occipital fossa [‘dépression dorso-occipitale’ or ‘fosse dorso-occipitale’ of Taverne (1978)]. This fossa is bounded antero-ventrally by the parietal, medially by the supraoccipital, and postero-laterally by the epioccipital. The central portion of the dorso-occipital fossa is occupied by an open fenestra. The external surface of the left parietal is partially broken, revealing a transverse canal-like structure that is probably the supratemporal commissure extending through the parietal. Although the temporal fossa is not exposed in the specimen and it is not possible to determine the bones that border it, the parietal clearly does not contribute to its margins. The epioccipital is a large bone forming the postero-lateral corner of the skull roof; it bears a strong ridge in continuity with the dorsal ridge of the pterotic that terminates posteriorly with a marked thickening. The antero-lateral margin of the epioccipital sutures with the pterotic. The supraoccipital bears a crest that is partially broken in the specimen, such that its full extent cannot be determined. A broken and flattened piece of tubular, canal-bearing bone overlying the medial part of epioccipital and dorso-occipital fossa is interpreted as the extrascapular. The sphenotic is relatively short and has a marked lateral projection (partially broken in the specimen) perpendicular to the antero-posterior axis of the skull. The pterotic is very long, overlies the sphenotic anteriorly and sutures with frontal, parietal and epioccipital medially. It bears a strong dorsal ridge on its posterior half. The lateral surface of the pterotic is smooth and lacks large pits or foramina. Ventromedial to the sphenotic and pterotic, the prootic forms at least part of the articular surface for the anterior head of the hyomandibula. Posterior to the pterotic and ventral to the epioccipital, the intercalar bears a triangle-shaped posterior projection.

Orbital region ( Figs 3 View Figure 3 , 5 View Figure 5 , 7 View Figure 7 ): There is no identifiable supraorbital. The antorbital is bulbous and presents a heavily ornamented surface, with two different ornamentation fields: a postero-dorsal one with chevron-like patterns, and an antero-lateral one with radial furrows and shallow pits. The first infraorbital is slender and tapers posteriorly. It defines most of the ventral margin of the orbit and contributes partially to its anterior margin. The anterior portion of the first infraorbital is ventrolateral to the antorbital. Posterior to the first infraorbital and lining the remaining portion of the ventral margin of the orbit there is a short and thin second infraorbital. The third and fourth infraorbitals are very large, covering most of the lateral postorbital area (‘cheek’) of the skull. The third and fourth infraorbitals are at least twice as long as they are deep. Their surface is ornamented with thin radial ridges. The fourth infraorbital is deeper than the third one and partially overlaps it. The infraorbital sensory canal is completely enclosed in a bony canal that extends through all the infraorbitals. Bone fragments in an elongated and flattened area postero-dorsal to the orbit on the right side of the specimen are tentatively interpreted as fragments of the dermosphenotic. Two semicircular ossified scleral rings (anterior and posterior) surround the eye.

Jaws ( Figs 3 View Figure 3 , 7 View Figure 7 ): The premaxillae are not preserved in the specimen, except for a broken splinter of bone antero-lateral to the right maxilla. Two broken teeth appear to be associated with this premaxillary fragment. The maxilla is long and slightly curved with ventral concavity. It tapers anteriorly into an elongated, narrow, and arched anteromedial process. This process, which articulates with the premaxilla, is missing its anterior tip. The length of the process—coupled with the length and proportions of the lower jaw—suggests that the premaxilla was a relatively long bone, especially when compared with other osteoglossids. There is no distinct dorsal swelling in the maxilla behind the anteromedial process. There are 19 maxillary teeth arranged in a single row that are visible on the right maxilla, with a complete maxillary set consisting of approximately 23 teeth when accounting for empty spaces left by tooth replacement. The teeth decrease in size from the anterior to the posterior portion of the maxilla. They are hollow, sub-conical in shape, with a short (less than a third of the tooth height) bony collar at the base and a small conical acrodin cap at the tip. The posterior, toothless portion of the maxilla, which overlies the angular, is rounded and not substantially deeper than the rest of the bone. Based on tomograms, broken pieces of bone dorsal to the posterior portion of the maxilla probably belong to the third infraorbital and potentially the maxilla itself, rather than to a supramaxilla. The lower jaws are incomplete, missing their anteriormost portions. They are straight and elongated, with a low coronoid process and a relatively long post-coronoid region. The dentary is lightly ornamented with parallel lines running along its length. Few large pores of the mandibular canal are visible on the external surface of the dentary. A complete dentary would include more than 23 teeth arranged in a single row. Dentary teeth are larger on average than the maxillary ones, and they are markedly compressed antero-posteriorly. The relative size of the bony collar at the tooth base varies from a third to half the tooth height. Dentary tooth collars are longer than maxillary tooth collars for teeth of the same size. Several replacement tooth crowns are visible on the right lower jaw. The angular is very long and extends anteriorly at the level of the anterior orbital margin. It presents a large postero-dorsal flange that laterally covers the quadrate articular condyle and the articular surface of the lower jaw. The articular and the retroarticular are not fused with the angular. They both contribute to the surface of the jaw joint ( Figs 7 View Figure 7 , 10C View Figure 10 ).

Palate and suspensorium ( Figs 3 View Figure 3 , 5 View Figure 5 , 7 View Figure 7 , 9 View Figure 9 ): Only a small part of the palate can be seen in the specimen. Anteriorly, an exposed platelike bone with a multitude of small teeth is interpreted as either the endopterygoid or the palatine–ectopterygoid. The quadrate is approximately triangular in lateral view and likely longer than deep. The ‘peg-like’ head of the quadrate articulates with the articular and retroarticular of the lower jaw ( Figs 7 View Figure 7 , 10C View Figure 10 ). Posterodorsal to its head, a strong ridge marks a portion of the posterior edge of the quadrate. The symplectic can be tentatively identified as a wedge-shaped bone overlying the posterodorsal surface of the quadrate. The hyomandibula is mostly covered by the third and fourth infraorbital and by the preopercle on both sides of the specimen. The anterior hyomandibular head is clearly distinct from the posterior one on the partially exposed left hyomandibula. The articular surface with the opercle can also be seen on the left side of the specimen. Its posterior position, distant from the heads articulating with the braincase, suggest a very long posterior (=opercular) process. This is confirmed by examination of the tomograms, which show the posterior process being slightly longer than the dorsal articulating surface of the hyomandibula.

Opercular series ( Figs 5 View Figure 5 , 7 View Figure 7 , 9 View Figure 9 ): The ventral half of the right preopercle is well preserved on the right side of the specimen. It presents a curved anterior margin, with an angle larger than 90° between its vertical arm and a very short horizontal arm that does not anteriorly reach the level of the orbit. The preopercular sensory canal opens in the horizontal arm through six large, antero-ventrally directed pores, arranged in a straight horizontal line. It is unclear whether these were originally covered by a thin lamina of bone that might have broken off post-mortem. Smaller preopercular pores extend posteriorly back to the posterior broken margin of the bone. Two large openings for the preopercular sensory canal are visible on the vertical arm of the right preopercle. Small portions of the interopercle are visible underneath the preopercle ventrally. It contacts the subopercle posteriorly. The subopercle is partially exposed antero-ventrally to the opercle on the left side of the specimen, but its size and shape are difficult to determine. The opercle is incompletely preserved and fractured in several pieces on both sides. Its anterior margin has a distinct dorsally-oriented concavity just above the articular facet for the hyomandibular process. In correspondence to the articular facet, the opercle bears a thickened opercular ridge on its medial side, with several small foramina on the surface. The dorsal margin of the opercle is almost flattened, with only a moderate amount of curvature. Based on its imprint on the right side of the specimen, the ventral margin of the opercle was likely straight or very slightly curved.

Branchial skeleton ( Figs 3 View Figure 3 , 5 View Figure 5 , 7 View Figure 7 , 9 View Figure 9 ): The ceratohyals and the urohyal are the only exposed bones of the ventral hyoid arch. The anterior ceratohyal has a broad anterior head. The urohyal has a distinct head and a narrow ventral margin. An isolated tooth crown embedded in the sediment anterior to the ceratohyals might be part of the basibranchial toothplate dentition. There are 18 branchiostegals, with the posterior ones notably deeper than the anterior ones.

Pectoral girdle and fin ( Figs 5 View Figure 5 , 7 View Figure 7 , 9 View Figure 9 ): The posttemporal can be seen on the left side of the specimen.It bears a broad and flattened dorsal arm that articulates with the back of the neurocranium. The ventral arm of the posttemporal can be identified in the tomograms; it is laterally compressed and relatively short, reaching around half the length of the dorsal arm. The supracleithrum is a slightly curved, laterally flattened bone with a thickened head antero-dorsally. Its medial surface bears a few distinct foramina anteriorly. The cleithrum and scapula are partially exposed on both sides of the specimen. However, anatomical details of these bones cannot be discerned. The coracoid extends anteriorly beyond the lower jaw joint, as inferred by its exposed ventral margin. Six pectoral rays are partially preserved. The first one is greatly enlarged and thickened. Two radials are visible ventral to the first two pectoral rays.

Postcranial axial skeleton ( Figs 5 View Figure 5 , 7 View Figure 7 , 9 View Figure 9 ): The first 14 vertebrae (or at least their centra) are preserved in the specimen, as evaluated from the tomograms. All the vertebrae are amphicoelous and much deeper than long. Examination of the tomograms reveals the presence of a paired autogenous structure ventral to the first vertebra that extends anteriorly a few centimetres below the occipital region of the neurocranium. We interpret this structure as a greatly expanded first parapophysis that is wedge-shaped in lateral view (see ‘Modified coding and scoring of characters’ on character 88 for further discussion of this feature). The second and third vertebrae are partially exposed on the left side of the specimen, and clearly illustrate the autogenous nature of the neural arches ( Fig. 10B View Figure 10 ). Nine abdominal ribs are partially exposed on the right side of the specimen ( Fig. 5 View Figure 5 ).

Scales: Scales are poorly preserved in the specimen, with small scale fragments, including their surface texture, found in the anterior block, and whole scales (often fractured and delaminated) in the right side of the posterior block, posterior to the pectoral girdle. Scales appear to be subcycloid, a few centimetres in diameter, and partially overlapping. They seem to lack reticulate furrows; instead, small tubercles ornament their surface.