Skull

The skull of Isthminia is relatively complete on its dorsal aspect, although it is missing the left side of the facial bones (Fig. 3). The skull is heavily eroded along its ventral surface, and the basicranium is absent except for a small portion of the right parietal and right alisphenoid (Fig. 4). The skull preserves most of the dorsal aspect of the supraoccipital, including small portions that articulate with the vertex and nuchal and sigmoidal crests (Figs. 3A–3C). Overall, the profile of the skull is dominated by the rostrum (Fig. 5), which is complete and comprises approximately 75% of the length of the preserved skull (the rostrum length is 36.6 cm; Table 1). The anterior portion of the rostrum is slightly displaced by both an oblique and transverse fractures, likely from geologic compaction or other diagenetic factors, which displace the elements approximately 1–2 mm from their life positions. Most of the upper dentition is missing from the skull, except for the anterior teeth, some of which are complete; other more posterior teeth are incomplete, while three isolated teeth were recovered from the quarry at the type locality. Despite the heavy erosion that removed most of the left portion of this skull, sufficient anatomical details are preserved on the right side of the cranium, and along the rostrum to provide insights into the morphology of Isthminia.

Figure 4: Skull in ventral view. Isthminia panamensis (USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See Ventral views of the type skull of(USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See http://3d.si.edu/explorer?s=iEpExr to measure, modify, or download this model.

Premaxilla. In dorsal view, the premaxilla dominates the visible part of the rostrum, comprising the entirety of the rostrum from its anterior end to about 75% of the length of the rostrum. In this view, the premaxilla occupies a width greater than that of the maxilla until the level of the maxillary flange (sensu Mead & Fordyce, 2009:62), where the width of the premaxilla begins to taper relative to the expansion of the maxilla overlying the cranium, in dorsal view (Fig. 3). Along the rostrum, anterior of the premaxilla-maxilla suture, there are several shallow longitudinal canals that terminate in small oval foramina (∼5 mm long by ∼2 mm wide). These canals are similar to those observed in adult specimens of Inia, but markedly different from the singular, deep groove that separates the posterior connection of the premaxilla and maxilla in Pontoporia, Ischyrorhynchus, immature specimens of Inia, and Lipotes. In Isthminia, adult Inia and Lipotes, these canals disappear posteriorly, as the premaxilla-maxilla suture becomes seamless along the length of the rostrum (Figs. 3 and 5).

Figure 5: Skull in lateral view. Isthminia panamensis (USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See Right lateral views of the type skull of(USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See http://3d.si.edu/explorer?s=jn4ynp to measure, modify, or download this model.

The paired right and left premaxillae are unfused for 4 cm at their anterior tip (Figs. 3A, 3B and 3D), presenting a slight gap, which is likely homologous in other odontocete taxa with the mesorostral groove (sensu Mead & Fordyce, 2009:16). This gap is then obscured posteriorly by full sutural fusion between the premaxillae for 24 cm along the midline of the rostrum until an elongate (6.9 cm-long) window is exposed between the overarching right and left premaxillae, just anterior of the level of the antorbital notches (Figs. 3A and 3B). Near the anterior origin of this window, the anteromedial sulcus appears, approximately at the transverse level of the last upper tooth alveolus (Fig. 4). This latter sulcus extends subparallel to the latter window until it terminates posteriorly in the premaxillary foramen. In Inia, the anteromedial sulcus extends farther anteriorly, and the portion of the premaxilla medial to the sulcus is more bulbous, while in Pontoporia the anteromedial sulcus is deeper, and nearly enclosed dorsally by overhanging flanges of the premaxilla. Fossil pontoporiids are broadly similar to Pontoporia, whereas in Pan-Inia, such as Ischyrorhynchus and Meherrinia, this area is not well preserved. At the level of the premaxillary foramen, the right and left premaxillae diverge from their midline fusion in separate paths around the external bony naris (Fig. 6). This divergence produces a V-shaped gap, 32 mm in anteroposterior length and 9 mm in lateral width, which is narrowed and longer than fossil pontoporiids, such as Auroracetus; this gap is small and variable in Inia, and broad and triangular in Ischyrorhynchus and Meherrinia.

Figure 6: Close-up on vertex of skull. Isthminia panamensis (USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See Close-up views of the vertex in the type skull of(USNM 546125) from (A) photographs and (B) orthogonal digital three-dimensional polygon model prepared from CT data, with lighting and color modifications using the Smithsonian X 3D browser. See http://3d.si.edu/explorer?s=cGDc1L to measure, modify, or download this model.

The premaxillary foramen itself is thinly ovate, 11 mm anteroposterior length, and 4 mm wide (Fig. 6), unlike the small, subcircular foramina in other fossil inioids. (The left side of the cranium, from this level posteriorly is not preserved, and thus the remainder of the description necessarily uses the right side of the cranium). The posterolateral sulcus is shallow, and extends slightly laterally from its deepest portion at its origin, the premaxillary foramen. The posterolateral sulcus terminates posteriorly in a faint way at the level of the anterior margin of the external naris. This condition is similar to Meherrinia and Brachydelphis, while it is different from Pontoporia, Auroracetus, Pliopontos, Pontistes and Inia, which present a deeply excavated sulcus along the posterolateral edge of the premaxilla. This portion of the premaxilla is not well preserved in Ischyrorhynchus. Medially, the posteromedial sulcus is unusual in originating 9 mm posterior of the premaxillary foramen and bifurcating into lateral and medial tracts that delineate the borders of the premaxillary sac fossa. Along with the posterolateral sulcus, these bifurcating tracts create a Z-shaped sulci pattern that is shallow laterally and deep (>3 mm) anteromedially (Fig. 6B). The path of medial tract of the posteromedial sulcus extends along the lateral margin of the anterior half of the external naris, but it is not confluent with the border of the naris. This morphology is completely new, and not observed in any inioid nor delphinidan. The bifurcating tracts enclose a low, but convex premaxillary sac fossa located lateral to the external naris and dipping medially, whereas the premaxillary sac fossa in all other inioids is located anterolateral of the external naris and is strongly convex, except for Meherrinia and Pliopontos. This portion is not preserved in Ischyrorhynchus. The premaxillary sac fossa in Lipotes is flat, with elevated margins.

The patent posterior termination of the entire premaxilla is spatulate, flat, and it appears at the level of the posterior half of the external bony naris, as in Meherrinia. There is an 8 mm separation between the posteomedial termination of the premaxilla and the anterolateral-most point of the nasal. In contrast, the posterior termination of the premaxillae of Pontoporia reaches the level of the posterior edge of the external nares, while in adult Brachydelphis spp., Pliopontos, Pontistes, Inia, and Lipotes, it extends even farther posteriorly; in young specimens of Brachydelphis and Pontoporia, it is in an intermediate position. Although there is slight erosion of the bony surface along the immediate margin of the external naris, the gap between the premaxilla and nasal is patent.

Maxilla. Throughout most of the anterior two thirds of the rostrum, the maxillae and premaxillae have a cylindrical outline (Fig. 3). Dorsally, the maxilla is exposed slightly on the lateral margin of the rostrum that is otherwise dominated by the premaxilla until about the proximal third of the rostrum where the maxilla becomes flatter along the maxillary flange. (As with the premaxilla, nearly all of the facial portion of the left maxilla has been lost to erosion, and the description is based on the right side). The antorbital notch is widely open, U-shaped, and oriented anteriorly. Posterior to the antorbital notch, the maxilla is expanded to cover most of the supraorbital process of the frontal, with the exception of the posterior-most and posteromedial edge, where the frontal is exposed. This posteromedial exposure of the frontal is similar to the condition observed in Ischyrorhynchus and Inia (mainly in juveniles), and differs from Pontoporia, Pontistes, Pliopontos, Meherrinia, Brachydelphis spp., and Lipotes, where the maxillae reaches the nuchal crest, and the lateral edges of the vertex. Posterolateral to the antorbital notch, the maxilla form a low maxillary crest (sensu Mead & Fordyce, 2009:51), which extends from the preorbital process, continues along the length of the supraorbital process of the frontal, but terminates at the postorbital process, unlike in Inia, where the crest continues well posterior of the postorbital process and join the temporal crest. In Isthminia, the maxillary crest is mediolaterally thicker (2–6 mm), but lower (∼5 mm), than the thinner, but higher (>5 mm) crest observed in Inia; in Pontoporia and Pliopontos this crest extends only the length of the supraorbital process.

Dorsally, the right maxilla shows a large diameter (∼10 mm) anterior dorsal infraorbital foramen, located at the level of the antorbital notch (Figs. 3A, 3B, 3D and 6). A second, anterior dorsal infraorbital foramen is posterolateral to the first one, and it is smaller in diameter (∼7 mm), and oriented posterolaterally. A single, posterior dorsal infraorbital foramen is located posterolateral to the external nares, it has a diameter of about 9 mm and its orientation is posterodorsal. The posterior dorsal infraorbital foramen of Isthminia is absolutely larger and located farther posteriorly than the corresponding foramen in Inia, Ischyrorhynchus, Meherrinia, Brachydelphis, Pontistes, Pliopontos, Pontoporia, and Lipotes.

In ventral view, the rostral portion of the maxilla bears alveoli for at least 14 maxillary teeth, with thin interalveolar septa (Fig. 4). At the ventral midline contact between the maxillae, there is a longitudinal groove that extends from anteriorly to about the level of the fifth maxillary tooth; a similar sulcus is also observed in Inia and Pontoporia, whereas this groove reveals a palatal exposure of premaxilla and/or vomer in Ischyrorhynchus and Brachydelphis mazeasi. Along the ventral surface and anteromedial to the jugal, there is a shallow (∼2 mm) oval (∼17 mm long by 10 mm wide) fossa; a similar fossa is also present in some specimens of Inia, Ischyrorhynchus, Brachydelphis spp. and very slightly Pontoporia. Medial to this shallow fossa, which we term the ventral maxillary fossa, there is an elongated fossa that continues anteriorly parasagittally for about 60 mm, and 5 mm in width and depth. The location and morphology of this latter fossa corresponds to the anterior sinus of Inia (Fraser & Purves, 1960), and it is exposed in Isthminia because its overlying maxilla and palatine were eroded. An anterior sinus is also found in Ischyrorynchus, however it is shorter than that in Inia and Isthminia. The rostral portion is not preserved in the other genera of inioids, preventing any comparison.

Lacrimal and Jugal. The lacrimal appears to be ankylosed with the anterior margin on the supraorbital process of the frontal, forming its anterior surface, a condition common to all adult inioid specimens (Figs. 3–5). Ventrally, the lacrimal extends medially to join the jugal, which together forms the anteroventral surface of the antorbital notch. The preserved part of the jugal is a thin strut that is subcylindrical in outline (∼4 mm wide; 17 mm long; ∼2 mm thick) and oriented posteroventrally. Overall, it is very similar in morphology to the jugal of Inia.

Frontal. Dorsally, the frontal is mostly covered by the maxilla, but it is exposed along the posterior and posteromedial edges of the skull roof (Figs. 3 and 5–7). In Isthminia, the right and left frontals form the highest part of the vertex, and together form a pair of rounded, rectangular knobs with a slight midline cleft (Figs. 3A–3C, 5 and 6). This topographic high for the frontals at the vertex is similar in Inia, Ischyrorhynchus or Meherrinia, and even Pontoporia and Lipotes, although the frontals in Isthminia are small and low by comparison with Pan-Inia. Unlike Inia and Meherrinia, the midline cleft between the right and the left frontals at the vertex does not show participation of an anterior supraoccipital (or possibly interparietal) wedge externally nor in internal CT scan data (Fig. 7 and Video S1). The dorsal surface of the vertex is lightly rugose, but not as strongly as in adult specimens of Inia.

Figure 7: Transverse CT slices through the skull. Isthminia panamensis (USNM 546125) across four slightly sub-transverse planes that pass anterior to posterior (A–D). CT slices (A–D) represent respective CT slices numbers 20566, 20625, 20655, and 20708, available for download on the Smithsonian X 3D browser ( Computed tomography (CT) slices through the vertex of(USNM 546125) across four slightly sub-transverse planes that pass anterior to posterior (A–D). CT slices (A–D) represent respective CT slices numbers 20566, 20625, 20655, and 20708, available for download on the Smithsonian X 3D browser ( http://3d.si.edu ). Numbers 1 and 2 denote facial and endocranial sagittal midlines, respectively, showing the sinistral displacement of the facial bones typical in many odontocetes ( Geisler & Sanders, 2003 Mead & Fordyce, 2009 ).

The supraorbital process is dorsoventrally thin (∼5 mm) with a blunt preorbital process; in contrast, the postorbital process is more elongated with a triangular cross section through its longitudinal axis, similar to the general condition of the other inioids. Nevertheless the distance between this two processes (52 mm), reflecting the size of the orbit, is about twice that of adult specimens of Inia, but in Isthminia it is proportionally similar to the other fossil inioids (all known specimens of Ischyrorhynchus lack this feature; see Table 3). In dorsal view, the lateral edge of the supraorbital process is relatively straight and oriented parasagitally, unlike Inia and Pontoporia where this border is laterally concave and oriented anterolaterally, or the nearly straight but anterolaterally oriented borders of Pliopontos and Brachydelphis. Additionally, the postorbital process is shorter than the length of the orbit, contrasting with the much longer process and smaller orbit in Inia. The ventral surface of the supraorbital processes is gently concave with a low, but distinct postorbital ridge. Medially and posterior to the frontal groove there is a shallow (<1 cm) round (∼1.5 cm diameter) fossa for the postorbital lobe of the pterygoid sinus. This same fossa varies tremendously in adult specimens of Inia, where it can either be shallow and slit-like (e.g., USNM 49582) or form a deep pit (e.g., USNM 239667). By contrast, this fossa in Pontoporia is deep, rounded and floored posteroventrally by the alisphenoid; in Brachydelphis spp., this fossa is shallow, as it is in Lipotes.

In the ventral view of the endocranium (Fig. 4), the right and left frontals surround the anterior aspect of the endocranium, where the extensive cerebellar juga are preserved on the ventral surface (Mead & Fordyce, 2009:18). Medially, the posteromedial margins of the frontals inside the endocranial region enclose a deep dorsal sagittal sinus sulcus along the midline. Such a structure is not visible in intact, extant skulls of Inia and Pontoporia, available for observation, nor is it preserved in most fossil inioids. Incomplete crania of Brachydelphis referable to B. jahuayensis (Gutstein et al., 2009: Fig. 7B) show no such sinus, but instead a low, bony ridge. Finally, a small wedge of the supraoccipital directly ventral to the vertex separates the fan-like posterior-most margins of the right and left frontals, which eventually contact the anterodorsal margins of the parietals along the frontoparietal sutures.

Nasal. The right and left nasals are paired at the vertex, sloping away from the topographic high of the paired frontals (Figs. 3, 5 and 6). Overall, the nasal is large (width = ∼12 mm; length = 41 mm), dominating the anterodorsal surface of the vertex, and occupying the entire posterodorsal margin of the external bony naris. The anterior margin of nasal is concave. Together, the right and left nasals are anteroposteriorly elongate with some tapering posteriorly, as in Pontoporia, Brachydelphis, Pontistes, Auroracetus, Pliopontos. However, the nasal in Isthminia is dorsoventrally more massive than these latter genera, and it is not as anterodorsally inclined as in Meherrinia not as anterior-facing as in Ischyrorhynchus, Inia, and Lipotes.

The anterior margin of the nasal displays a low sigmoidal crest that extends transversely with a small protuberance that rises in the middle of the nasal, about 10 mm from its anterior margin; with the paired right and left nasals, these small crests and the base of these protuberances outline a wide, but shallow V-shaped concavity, pointing posteriorly (Figs. 3A, 3B and 3D). The posterior margin of the nasal is difficult to resolve without close inspection because the sutural distinction between the nasal and the frontal in this part of the vertex is overlapping and thin (see also Fig. 6). The posterior termination of the nasal overlaps with the frontal by passing in a broadly posteromedial path, terminating anterior of the level of the posteriormost margin of the maxilla. Together, the posterior termination of the right and left nasals show an anteriorly-pointed V-shaped margin. This condition is similar to Pontoporia and Brachydelphis, where the contact between the nasal and frontal shows a similar V-shaped margin; in Auroracetus and Meherrinia, a small wedge of the frontals insert medially between the nasals.

Vomer and Ethmoid. The vomer is poorly preserved ventrally, but a small portion is patent along the midline palatal surface adjacent to the medial margin of the highly eroded right maxilla, approximately extending 45 mm, with an anterior extent to the transverse level of the 8th maxillary tooth alveolus (Fig. 4). The ethmoid is incompletely preserved; the crista galli is shallow with very small (<1 mm) foramina in its surface. The ethmoid forms the bony nasal septum, rising dorsally to the same horizontal level as the premaxillae, but not quite reaching the level of the nasals. The lateral wings form the posterior and posterolateral walls of the external nares, which are cleanly separated from the anterior margin of the nasals by a continuous gap 5–8 mm wide (Fig. 6).

Parietal. The parietal is exposed broadly on the posterior margin of the temporal fossa, along with the frontal and squamosal (Figs. 3C, 3D, 5 and 7). The lateral surface of the parietal is smooth and convex; in posterior view, the temporal crest of the parietal is posterolaterally oriented, as opposed to the ventrally oriented crests in Inia and Pontoporia. The anterior extent of the parietal is unclear because the parieto-frontal suture is not patent, similar to adult specimens of Inia.

Supraoccipital. Only the dorsal half of the supraoccipital can be reliably determined for Isthminia. Dorsally, the supraoccipital does not participate in the vertex, but participates in the temporal and nuchal crests (Figs. 3A–3C); the nuchal crest is transversely straight, about 10 mm thick, and unlike the more anteromedially oriented crest in Inia and the posteriorly concave crest of Pontoporia. Posteriorly, there is a midline external occipital crest that is bounded laterally by deep (9 mm) semilunar fossae; such fossae are also patent in adult specimens of Inia and Pontoporia. The external surface is smooth and convex. The temporal crests are nearly vertical, and dorsally they join the nuchal and orbitotemporal crests (sensu Fordyce, 2002:194), forming a tabular, triangular surface at the triple junction. When viewed posteriorly, the supraoccipital has a square outline, unlike the more sub-triangular outline in Inia, or the general pentagonal outlines of Pontoporia and Lipotes.

Squamosal. The right squamosal is nearly completely preserved. The zygomatic process of the squamosal is relatively long, mediolaterally thin, laterally convex, and medially concave. Overall, its main corpus is rectilineal in lateral view, in contrast to the gently tapering profile of Pontoporia and acute tapering in Inia. In Isthminia, the anterior tip of the zygomatic process is expanded, with a squared-off anterior margin, more like Inia, and to a lesser degree Brachydelphis mazeasi, rather than the rounded, tapering tip of Pontoporia and Pliopontos. The dorsal surface of the root of the zygomatic process in Isthminia is concave, while its lateral edge flares outward about 10 mm farther laterally than the anterior part of the process (Figs. 3–5). In lateral and ventrolateral views, the postglenoid process is not patent, but it shows no indication of supporting elaboration, such as the bulbous postglenoid process in both Inia and Pontoporia, and acute and thin in Brachydelphis spp. (Gutstein et al., 2009; Lambert & de Muizon, 2013). Ventrally, the outline of the glenoid fossa in Isthminia is elongate, shallowly convex, and faces ventromedially. The tympanosquamosal recess extends as a deep (∼5 mm) sulcus medial to the glenoid fossa, as it does in other inioids. The posterolateral surface of the squamosal has a broad and relatively deep concave sternomastoid fossa, deeper than Inia.

The squamosal plate is relatively low, occupying only about the lower quarter of the surface of the temporal fossa, which is dominated by the parietal (Fig. 5). This configuration is similar to the condition seen in Pontoporia and Brachydelphis, but contrasts with Inia, where the squamous portion is much higher, a condition also visible in Lipotes. The anterior extent of the squamosal plate is ankylosed with the posteroventral edge of the temporal wall exposure of the alisphenoid in the type specimen of Isthminia.

Alisphenoid. Only the dorsal portion of the alisphenoid is preserved in the type specimen of Isthminia above the horizontal level the squamosal fossa (Fig. 5). In lateral view, the parieto-alisphenoid suture extends in a path from the squamosal plate at the posterior margin of the temporal fossa dorsally to a level in line with the nuchal crests; in this way, this sigmoidal suture partitions the parietal (dorsally) and the alisphenoid (ventrally) in the middle of the temporal fossa. The anterior margin of the alisphenoid extends at least to the level of the postorbital processes of the frontal, although the actual sutures are not patent at the anterior end (see also Fig. 7). In lateral view, the dorsal extent of the alisphenoid on the temporal wall is much greater than that seen in Inia, but we note a degree of variability in Inia.