Italosiren bellunensis ( De Zigno, 1875 ) Voss & Sorbi & Domning, 2017

Voss, Manja, Sorbi, Silvia & Domning, Daryl P., 2017, Morphological and systematic re-assessment of the late Oligocene “ Halitherium ” bellunense reveals a new crown group genus of Sirenia, Acta Palaeontologica Polonica 62 (1), pp. 163-172 : 166-169

Italosiren bellunensis ( De Zigno, 1875) comb. nov.

Fig. 2 .

1875 Halitherium bellunense sp. nov.; De Zigno 1875: 438, pls. 14–15. 1878 Halitherium bellunense De Zigno, 1875 ; De Zigno 1878: 68.

1882 Metaxytherium bellunense ( De Zigno, 1875) ; Lepsius 1882: 180. 1887 Halitherium bellunense De Zigno, 1875 ; De Zigno 1887: 728.

1905 Halitherium bellunense De Zigno, 1875 ; Abel 1905: 393, text-fig. 1.

1987 Halitherium bellunense De Zigno, 1875 ; Domning and Thomas 1987: 207.

1989a Halitherium bellunense De Zigno, 1875 ; Domning 1989a: 425, text-fig. 6.

Holotype: MGPD-18Z skullcap, MGPD-19Z left premaxilla with tusk, MGPD-20/21Z fragment of left maxilla with DP5–M2, MGPD-22Z left and MGPD-23Z right zygomatic processes of squamosals, MG- PD-7384Z fragment of left jugal, MGPD-7385/6Z fragment of right maxilla with DP5 and M1, MGPD-7387Z an isolated fragment of a molar, MGPD-7383Z a fragment of a lower molar, seven fragments of vertebrae (MGPD-7362Z, 7367Z, 7368Z, 7369Z, 7374Z, 7375Z, 7376Z), and six rib fragments (MGPD-7358/9Z, 7363Z, 7364Z, 7366Z, 7381Z) of a juvenile individual.

Type locality: Cavarzano, Valle delle Guglie, near Belluno, Italy.

Type horizon: Upper Chattian (latest late Oligocene), 27–25 Ma; basal portion of the Belluno Glauconitic Sandstone Formation.

Material.—Only the type material.

Emended diagnosis.—Dugongine sirenian having the following unique combination of synapomorphies: nasal process of premaxilla broadened and bulbous at posterior end, having more or less vertical joint surface in contact with frontal (shared with Dioplotherium , Xenosiren , and Rytiodus heali ); zygomatic-orbital bridge of maxilla shortened and thickened posteriorly, but not transformed into vertical wall shared with Dugong and Nanosiren ); ventral extremity of jugal lies ventral to orbit (shared with all dugongines except for Xenosiren , Crenatosiren , and Nanosiren ); processus retroversus of squamosal moderately inflected (shared with Crenatosiren , Callistosiren , Rytiodus , Kutchisiren , Dioplotherium allisoni , and Bharatisiren indica ); supraoccipital wider ventrally than dorsally (shared with Nanosiren , Dugong , Kutchisiren , and Bharatisiren kachchhensis ). Differs from various other Dugonginae in retaining the following unique combination of plesiomorphies: frontal roof flat without knoblike bosses medial to temporal crests (character combination shared with Bharatisiren indica ); squamosals do not reach temporal crests (shared with Corystosiren and Dioplotherium cf. allisoni from Brazil); ventral rim of orbit not overhanging (shared with Corystosiren and Callistosiren ); first upper incisor (I1) present with I1 crown having enamel on all sides (shared with Crenatosiren , Nanosiren sanchezi , and Dioplotherium manigaulti ). Differs further from the similar genus Rytiodus in lacking a distinct intertemporal constriction of the cranial roof, an overhanging orbital rim of the jugal, large and blade-like tusks, and differs at least from R. heali in having a supraoccipital that is only slightly wider than high.

Description.—This specimen was previously described and illustrated by De Zigno (1875) and Abel (1905). Here, the holotype material is re-described on the basis of a re-investigation contributing to new and more comprehensive indications of the taxon’s skeletal morphology. The anatomical terminology mentiond here follows Domning (1978).

Premaxilla: The left premaxilla ( Fig. 2A ) is almost completely preserved and measures 178 mm in total length with the symphysis reaching a length of 94 mm. The value of the symphysis length divided by the total length of the premaxilla, i.e., the ratio lSYM / lPM, is 0.53, indicating that the symphysis is longer than half the total length of the premaxilla. Taking into account the conditions in other dugongine taxa with similar ratios like Rytiodus capgrandi or Crenatosiren olseni , the symphysis is considered to be enlarged relative to the cranium. In lateral view ( Fig. 2A 1 ), the premaxillary symphysis is compressed to form a low middorsal ridge with a slight boss at the summit. The first incisor tusk (I1) is preserved in the symphysis, indicating that a medial dentiform process like that in Hydrodamalis is missing. In lateroposterior view, the maxillopremaxillary suture is indicated and perpendicular to the posterior end of the symphysis. Although the angle of deflection of the rostrum cannot be unambiguously determined, it is estimated to be strong, about 60°, as in most other dugongines. Anteromedial to the nasal process, the mesorostral fossa shows no indentations and extends, retracted and enlarged, in a dorsoposterior direction. Whether the external nares surpass the level of the anterior margin of the orbit cannot be determined due to the missing facial part of the skull. The posterior end of the nasal process is broadened and bulbous, having a posterolateral oblique joint surface in contact with the frontal and/ or nasal ( Fig. 2A 2 ).

Frontal: MGPD-18Z preserves the posterior part of the frontal roof, which is flat without knoblike bosses ( Fig. 2B 1 ). The dorsolateral edges that presumably bore the temporal crests are broken. The interior aspect of the preserved skullcap remains undescribed due to adhering sediment.

Parietal: The parietal roof ( Fig. 2B 1 ) is more or less flat and slightly concave centrally where the temporal crests reach their maximum height and constriction. Generally, the intertemporal constriction is weak, with the temporal plane of the parietal sloping as a flat wall laterally. The temporal crests form prominent and thickened lyriform keels that bulge medially at the centre of the parietal roof. They diverge and decrease in height anteriorly and posteriorly. The course of the temporal crests is not interrupted by the squamosals, so they reach the nuchal crest. Anteriorly, the frontal processes of the parietal are separated by an angle of about 90°, indicating the course of the frontoparietal suture, and presumably extended only a short distance on the frontal roof.

Supraoccipital: Small parts of the supraoccipital are missing on the left dorsolateral and dorsomedial sides, and its left and right thirds are separated from the mid-portion by fractures ( Fig. 2B 2 ). Nevertheless, it can be determined to form a compact wall, with a width/height ratio of about 1.36, which indicates that the supraoccipital is only slightly wider than high. It is wider ventrally than dorsally, and, in lateral view, it meets the parietal roof at an angle of about 115°. The nuchal crest is narrow and sharp-edged, and extends as a convex ridge dorsolaterad. Though partially eroded, the nuchal crest shows no notch in the median plane, but rather an external occipital protuberance that forms a prominent knob rising above the parietal roof. The muscle insertions for the attachment of the semispinalis capitis muscles are rounded. The external occipital protuberance extends ventrad as a distinct but low median ridge, somewhat shifted to the left side. It exceeds half the height of the supraoccipital and extends almost to its ventral tip. The ventral margin is pointed in the median plane, with the exoccipital sutures meeting at an angle of about 132°.

Maxilla: Two maxillary fragments associated with the cheek dentition are preserved. On the lateral side of the left fragment (MGPD-20/21Z; De Zigno 1875: pl. 15: 4), the broken base of the zygomatic-orbital bridge is observable, indicating that it lies slightly above the alveolar margin, elevated about 10 mm. Where broken, the bridge is dorsoventrally thicker posteriorly (17 mm) than anteriorly (12 mm). According to its length (about 34 mm) and height (17 mm), the zygomatic-orbital bridge is evaluated as being shortened thickness ≥ 0.40 × length) anteroposteriorly, but not transformed into a transverse vertical wall.

Squamosal: The cranial portion of the squamosal is not preserved, but its attachment areas posterolateral to the parietal show that it did not extend to the temporal crests ( Fig. 2B 1 ). Only the left and right zygomatic processes (MGPD-22Z and 23Z; Fig. 2C, D ) are present, with the left one having preserved the posterolateral part of the zygomatic root, indicating a distinct notch ( Fig. 2D ). The processus retroversus is moderately inflected without projecting below the jugosquamosal suture ( Fig. 2C ). In lateral view, the zygomatic process has a flat surface and is roughly triangular in shape, dorsoventrally high posteriorly and tapering anteriorly. Its posterodorsal end is convex in outline, the ventral margin is straight, except for a distinct convexity just in front of the processus retroversus. Medially, the zygomatic process is slightly concave with the dorsal margin inclined inward, forming a sigmoid margin.

Jugal: The middle part of the left jugal is preserved ( Fig. 2C ) with the preorbital and zygomatic processes present, but broken anteriorly and posteriorly, respectively. The zygomatic process may have been as long as the diameter of the orbit, judging from the corresponding attachment area on the ventral margin of the zygomatic process of the squamosal, which reaches as far posteriorly as the beginning of its ventral convexity. However, the orbital area of the skull is too incompletely preserved for an unambiguous conclusion on that character. Ventral to the orbit, the dorsal margin is thick, but not overhanging laterally. The ventralmost extremity is positioned under the orbit. Anterior to the tip of the squamosal zygomatic process, the postorbital process bulges upwards slightly, but without rising high enough to form a postorbital bar with the frontal.

Mandible: A bone fragment of about 40 mm in its maximum dimensions is referred to the lower jaw due to the presence of a two-rooted molar or molariform premolar (MGPD-7383Z). The tooth crown is only fragmentarily preserved, hampering any detailed descriptions.

Dentition: Besides two poorly preserved dental remains MGPD-7383Z and 7387Z) that do not provide any information, morphologically valuable records are the left premaxilla including the first incisor tusk and the left and right maxillary fragments. The upper dental formula is interpreted to be I1, C0, P0, DP5, M1–3.

De Zigno (1875: pl. 15: 1) illustrated the first incisor tusk I1) as being complete and unworn with a lateral exposure in the premaxilla of about 30 mm. Though the anteriormost tip of about 10 mm is broken and missing today ( Fig. 2A 1 ), the original state of preservation reveals this tusk to be most likely unerupted. In cross section, the crown is lens-shaped with sharp anterior and posterior edges, measuring 17 × 8 mm in diameters. The crown is covered by black enamel on all sides, but no conclusions can be drawn on the extent of the enamel, i.e., either along the entire length of the tusk or being absent from a distinct root. The slightly swollen lateral sides of the premaxillary symphysis indicate a length of the tusk alveolus that is not restricted to less than half the length of the premaxillary symphysis. However, whether the alveolus extends about half the length of the symphysis or even exceeds that level remains uncertain. The clarification of these characters must wait until more and better preserved material is known, particularly from adult specimens, because the extent of the I1 alveolus increases with the individual’s age, as can be observed in the living dugong ( Dugong dugon ) ( Marsh 1980; personal observations by MV, SS, DD). In any case, interpretation of an expanded nasal process of the premaxilla as indicating forceful use of large tusks ( Domning and Beatty 2007) argues in favour of the I1 being large in adult Italosiren .

The cheek dentition is qualitatively and quantitatively best preserved in the left maxillary fragment MGPD-20/21Z, and therefore this is the basis for the following description ( Fig. 2E ). Three cheek teeth are preserved, but their identification is uncertain; two possible interpretations are considered here:

(i) They may represent DP5, M1, and M2 (opinion of MV). Abel (1905) and Sorbi (2008) interpret a small rounded tooth fragment mesiolingual to the incomplete DP5 crown ( Fig. 2E : outlined area) as remains of the fifth premolar or the fourth deciduous premolar, respectively. Here, this tooth fragment, which is mesially slightly elevated above the level of the bone, is considered to be part of the irregularly broken DP5 crown. This observation is additionally supported by the original extent of the DP5 alveolus still discernible mesially, lingually and distally by imprints in the maxillary bone, and by De Zigno (1875: pl. 15: 3), who illustrates the anteriormost “molar” as a single tooth. As shown in Fig. 2D , the area that might have been covered by the complete crown includes that fragment, which additionally corresponds to the tight positioning of the cheek teeth that barely would have given space for a further premolar at this place. Accordingly, DP5 is only slightly smaller than the subsequent molars ( Table 2) and similar in having a slightly heart-shaped crown as also can be generally observed in related taxa. The fracture surfaces of the crown provide insights to the broken lingual and distolabial roots, identifying this tooth as a three-rooted molariform or submolariform premolar.

(ii) Alternatively, these three teeth may represent DP4, DP5, and M1 (possibility suggested by DD). If the fragment mesiolingual to the anterior tooth is part of its broken crown, then this crown was longer than either of the more posterior teeth, not smaller. However, Abel (1905) regards the fragment as part of a root; and De Zigno’s (1875: pl. 15: 4) shows an overhang of the crown at the front end, suggesting that the fragment is level with the bone and not high enough to be part of the crown. However, a molariform tooth has a single large root on the lingual side, not a small mesiolingual one (see also the cross section of this tooth in Zigno’s 1875: pl. 15: 7). Furthermore, the crown of the anterior tooth seems relatively long and narrowed toward the front, so perhaps not heart-shaped, even though it is so worn that its length is significantly diminished. It could therefore be the submolariform DP4, which should be proportionately longer than the fully molariform DP5 or M1. That would make the following two teeth DP5 and M1. Then the fragment might be a remnant (root) of DP3 (P5 of Abel 1905) that has been encroached on by mesial drift of DP4.

Along the small bony shelf anterior to the molar teeth, there are no traces of alveoli. Although this left maxillary fragment is incompletely preserved, and it is likely that additional deciduous premolars where present considering I. bellunensis is a juvenile, we therefore assume that permanent premolars were not developed in the animal’s lifetime, nor were likewise the canines or the second and third incisors. This would also be in agreement with other dugongid taxa showing similarly derived conditions such as Dioplotherium manigaulti ( Domning 1989a) and Callistosiren boriquensis ( Vélez-Juarbe and Domning 2015) .

The left M1 (according to MV; Fig. 2E ) is fully erupted and only slightly worn, with the uppermost tips of the main cusps flattened. Its cusp pattern does not differ from that of M2 and is summarised in the description below. The left M2 (according to MV; Fig. 2E ) is incompletely erupted, occupying a slightly lower position relative to M1. Its crown is entirely unworn. The precingulum transversally slopes from the lingual to the labial (or buccal) side and encloses a narrow anterior cingular valley that opens anterolabially. The protoloph is composed of the paracone and protocone, both bracketing the protoconule to form a transverse ridge. While the protocone forms a prominent vertically-rising cusp, the paracone and protoconule slope in the lingual direction. The transverse valley is deep and not obstructed by the metaconule, whereas the latter is nearly transversally aligned with the hypocone and metacone to form the metaloph. As on the protoloph, the metacone and metaconule are inclined lingually, but the hypocone slopes labially. A posterior cingulum is attached to the hypocone and encloses a posterior basin of moderate size that opens labially.

Vertebral column: Besides four vertebral fragments (MDPD-7362Z, 7374Z, 7375Z, 7376Z) that are not assignable to a specific region of the vertebral column, three centra (MGPD-7367Z, 7368Z, 7369Z) are identified as thoracics and one centrum (MGPD-7383Z) is allocated to the lumbars. All centra are lacking the transverse processes and neural arches. The thoracic centra show a slightly heart-shaped outline, while the lumbar centrum is oval. The proximal and distal extremities are flat and slightly concave medially.

Ribs: Six rib fragments are present, of which MGPD-7358/9Z is the largest. It is composed of originally two fragments that are now glued together, representing the distal part of the rib shaft. This element measures about 240 mm in maximum preserved length and has a more or less constant anteroposterior width that diminishes slightly in the last 70 mm of the somewhat tapered distal extremity. Cemented gravel and shells cover the rib fragment, but the fracture surface of its shaft reveals an elliptical cross section of 50 × 30 mm. According to the curvature of the distal end, the rib is allocated to the left side of the thorax and may have occupied an anterior position. Another noteworthy element is a 128 mm long proximal fragment, most likely belonging to the left first rib (MGPD-7366Z). The capitulum and tuberculum are separated by 42 mm, indicating a long collum typical for the anteriormost ribs. Ventral to the capitulum, a protuberance is not developed.

Figures

Fig. 2. Cranial and dental elements of the dugongine sirenian Italosiren bellunensis (De Zigno, 1875) comb. nov. from Cavarzano, Italy; upper Chattian, late Oligocene. A. Left premaxilla (MGPD-19Z) in lateral (A1) and dorsal (A2) views. B. Parietal-supraoccipital skullcap (MGPD-18Z) in dorsal (B1) and posterior (B2) views. C. Outline drawing of left zygomatic process of squamosal (MGPD-22Z) and jugal (MGPD-7384Z) in lateral view; shaded area indicates missing part. D. Outline drawing of left zygomatic process of squamosal (MGPD-22Z) in dorsal view. E. Left maxillary fragment (MGPD-20/21Z) in occlusal view; dashed lines indicate the estimated outline of DP5, outlined area indicates portion of controversial interpretation and is considered here as representing remnants of the DP5 crown.

Fig. 1. Geographic location of Cavarzano, Valle delle Guglie, near Belluno, Italy (modified after Bosellini et al. 1981 and Dallanave et al. 2009). Asterisk indicates estimated type locality of Italosiren bellunensis.