Fossil Sirenia of the West Atlantic and Caribbean region. Ix. Metaxytherium albifontanum, sp. nov.

ABSTRACT We describe a new species of the halitheriine dugongid genus Metaxytherium from the late Oligocene of Florida and South Carolina. The new species is represented by cranial and postcranial material, including parts of the axial and appendicular skeleton. Metaxytherium albifontanum, sp. nov., differs from other species of Metaxytherium by the following unique combination of plesiomorphic and derived characters: posterior end of nasal process of premaxilla broad and flat relative to what is observed in most other members of the genus (somewhat resembling M. subapenninum); ventral extremity of jugal under posterior edge of orbit (character 85[1]) (shared with M. krahuletzi); exoccipitals separated in dorsal midline (character 66[1]) (shared with all other species in the genus, except some M. krahuletzi); and innominate with acetabulum (nearly lost or lost in M. crataegense, M. floridanum, M. serresii). This new species was sympatric with two dugongines, Crenatosiren olseni and Dioplotherium manigaulti. The small tusks and cranial morphology of M. albifontanum, sp. nov., indicate that it was likely a consumer of small seagrasses. Our phylogenetic analysis is consistent with previous ones in placing Hydrodamalinae within a paraphyletic Metaxytherium spp. and placing the Metaxytherium spp. + Hydrodamalinae clade as the sister group to Dugonginae. Metaxytherium albifontanum, sp. nov., is the oldest known member of its genus; this might indicate that the group originated in the West Atlantic and Caribbean region and later dispersed to the Old World Tethys region. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP.


INTRODUCTION
Dugongid sirenians of the genus Metaxytherium are commonly found in Neogene shallow marine deposits in the western Atlantic, Caribbean, eastern Pacific, and Mediterranean regions (Domning, 1996;2010, and references therein). The geochronologically oldest and most plesiomorphic species currently referred to this genus is Metaxytherium krahuletzi Depéret, 1895, from the early Miocene of the north Tethys and western and central Paratethys regions (Domning, 1994;Domning and Pervesler, 2001;Sorbi, 2008). This has led authors to the conclusion that the genus originated in the Tethys region (Domning and Pervesler, 2001;Sorbi, 2008). The subsequent appearance of members of this genus in the late early Miocene of the Americas (Simpson, 1932;Kellogg, 1966;Varona, 1972;Muizon and Domning, 1985;Toledo and Domning, 1991) has seemed to support this view as well as call for an east-west trans-Atlantic dispersal of the group (Domning, 1988).
New material from the Oligocene of the West Atlantic and Caribbean region challenges these ideas. It is the aim of this paper to describe some of this material, which represents a new species of Metaxytherium from the late Oligocene of the east coast of North America. The material described includes wellpreserved cranial and postcranial material. This new species was sympatric, at least in part of its range, with two other dugongids, Crenatosiren olseni (Reinhart, 1976) and Dioplotherium manigaulti Cope, 1883 (Domning, 1989a(Domning, , 1989b(Domning, , 1997Velez-Juarbe et al., 2012).
Etymology-Latin albus = white + fontanus = of a spring; in reference to the type locality near White Springs, Florida.

Skull
Description of the skull is based on the holotype (UF 49051) 7; Table 1), which is a subadult individual (based on recently erupted M3 and unfused basioccipital/basisphenoid suture).
Each alveolus creates a slight bulge in the lateral surface of the bone. The dorsal and palatal sides of the rostrum are parallel in lateral view except anteriorly, where the dorsal outline abruptly becomes concave and then again convex at the tip of the snout (Fig. 3). The dorsal surface of the symphysis is slightly convex posteriorly, broad and flat more anteriorly, and narrows forward to a keel less than 7 mm thick above the tusks (c. 10[1]). The incisive foramen lacks a well-defined anterior end. The nasopalatine canal is slightly flattened dorsoventrally. The masticating surface of the rostrum is trapezoidal in outline and deeply concave (Fig. 2). Fractures and displacement of the bones prevent exact measurement of the rostral deflection, but it was approximately 60 • . The anterior part of the mesorostral fossa is minimally constricted by a pair of small (∼1 cm in length) bumps about 3 cm posterior to its front end. The nasal process of the premaxilla is mediolaterally broad and relatively flat posteriorly (Fig. 1), a condition similar to that in Metaxytherium subapenninum (Sorbi et al., 2012), but scored as (c. 6[0]) because it does not approach the more derived conditions observed in dugongines (e.g., Dioplotherium manigaulti [Domning, 1989]). The part of it in contact with the lacrimal, nasal, and frontal extends less than half the length of the mesorostral fossa. Nasal-The nasals are reduced (c. 32[1]); their dorsal exposure is about 35 mm long and 20 mm wide, broad anteriorly and narrowing posteriorly (Fig. 1). The nasals are separated in the midline by thin processes of the frontals (c. 31[1]), and together with these processes they form a dorsally convex arch. Anterolaterally, the nasals are overlapped by the premaxillae. . Abbreviations: as, alisphenoid; bo, basioccipital; bs, basisphenoid; dP, deciduous upper premolar; eo, exoccipital; fm, foramen magnum; hf, hypoglossal foramen/canal; if, incisive foramen; ioc, infraorbital canal; j, jugal; M, upper molars; mx, maxilla; oc, occipital condyle; pgp, postglenoid process; pl, palatine; pmx, premaxilla; pp, paroccipital process; pr, processus retroversus; pt, pterygoid; ptp, posttympanic process; sq, squamosal; tc, temporal condyle; zpj, zygomatic process of the jugal.
Ethmoidal Region-A large (of about the same height as the nasal cavity) ethmoturbinal forms a thick vertical plate on either side of the dorsal part of the nasal cavity. Its rugose anterior end is about 2 cm high and 4 mm thick, and curves backward and inward ventrally as if to form a short, blunt hook. The remainder of the nasal cavity is still filled with matrix.
Lacrimal-A large (extending for more than half of the length of the anterior orbital margin), somewhat crescentic bone measuring 30 mm dorsoventrally and 15 mm anteroposteriorly; a nasolacrimal foramen is absent (c. 91[1]). It is in contact with the premaxilla anterodorsally (c. 93[1]), the maxilla anteriorly and medially, and the jugal anteroventrally (Figs. 1, 3). Its ventral end projects laterad as a low knob.
Frontal-The supraorbital process has a blunt, rounded anterior end, a poorly developed posterolateral corner (c. 36[1]), no division in its lateral margin (c. 44[0]), and no orbicular apophysis (Figs. 1, 3). Its dorsal surface slopes laterally rather steeply, and is separated from the nearly horizontal frontal roof by a low but distinct temporal crest that extends straight forward across the base of the supraorbital process. The edges of the frontal roof do not overhang. The central portion of the roof, including the internasal processes, is gently convex (c. 42[0]) without knob-like bosses (c. 45[0]). The anterior border between the nasals is thin and broken but seems to have extended nearly the full length of the nasals, the nasal incisure thus being small or absent (c. 37[0]). The anterior extremity of the frontoparietal suture lies about 6 cm abaft the nasals. The temporal wall of the frontal is recessed ventral to the crista orbitotemporalis, which is a smooth ridge, indistinct anteriorly, that extends posteroventrad from the supraorbital process. The lamina orbitalis of the frontal, which forms this recessed part of the wall, is thin (c. 38[0]) and extends forward as a thin (<1 cm) plate to a level about 2 cm abaft the posterolateral corner of the supraorbital process; its anterior border is falciform.
Parietal-The cranial vault is nearly rectangular in frontal section anteriorly. The roof is 13 mm thick in the anterior midline, and nearly flat; no sagittal crest is present (c. 51[1]) (Fig.  1). The posterolateral corner of the roof is slightly indented by FIGURE 4. Posterior view of the holotype skull of Metaxytherium albifontanum, sp. nov. (UF 49051). Abbreviations: eo, exoccipital; eop, external occipital protuberance; fm, foramen magnum; oc, occipital condyle; pmx, premaxilla; pp, paroccipital process; scf, supracondylar fossa; so, supraoccipital; sq, squamosal; sr, sigmoid ridge; zps, zygomatic process of squamosal. the squamosal. The temporal crests are low, barely rising above the level of the roof and confined to its lateral edges (temporal crest type A [Domning, 1988]). Endocranially, the internal occipital protuberance is conical and extends well below the rest of the internal surface. The tentorium and transverse sulcus are distinct, but the latter is shallow and lacks deep lateral pits. The anterior end of the bony falx is flattened and indistinct.
Exoccipital-The exoccipitals are separated in the dorsal midline by about 7 mm (c. 66[1]) (careful examination of the type by both of us confirms that this region has been restored erroneously in the holotype, giving the false impression that they were joined); the foramen magnum has an acute dorsal peak and reaches the supraoccipital (Fig. 1). The dorsolateral border of the exoccipital is rounded and about 10 mm thick, without any posteriorly projecting flange (c. 70[0]). The supracondylar fossa is fairly deep and wide (c. 67[2]). The hypoglossal foramen is single and surrounded by bone (c. 72[0]) (Fig. 2). The occipital condyle extends below the paramastoid process; its articular surface subtends an arc of about 120 • .
Basioccipital-Bears a pair of convex rugosities for the longus capitis muscles. It had not yet fused with the basisphenoid (Fig. 2).
Basisphenoid-The sella turcica is deeper than the surrounding dorsal surface of the bone, with a strong, posteriorly overhanging tuberculum sellae.
Orbitosphenoid-The optic foramen lies at the level of the dorsal side of the sphenorbital foramen.
Alisphenoid-The lateral side of the pterygoid process is ridged ventrally (Fig. 3). A smooth horizontal ridge continues Pterygoid-The pterygoid fossa is deep and broad, extending above the roof of the internal nares (c. 102[1]); its lateral and medial edges converge dorsally and intersect at a small protuberance (Figs. 2, 3). The medial edge projects farther aft than the lateral edge. The ventromedial tip is formed by a thickened, smooth trochlea (hamulus) for the M. tensor veli palatini tendon.
Palatine-Forms the rounded anteromedial side and ventral tip of the pterygoid process. Dorsally it forms part of the lateral wall of the internal naris, and anteriorly it reaches forward to the level of the rear edge of the zygomatic-orbital bridge (c. 99[0]). The palatal incisure is very deep and narrow (c. 97[1]) (Fig. 2). The palatines appear to join anteriorly in a suture, which is at most 1.5 cm in length.
Maxilla-The edges of the palatal surface are lyriform, or more precisely, 'mandolin-shaped' sensu Kobayashi et al. (1995); i.e., not pushed anterolaterad by a large tooth at the front of the cheektooth row (Fig. 2). The edges border a moderately deep palatal gutter; each consists of a pair of ridges separated by a slight groove. Two canals open anteriorly into the palatal gutter at the level of the front of the tooth row. The palatal and rostral surfaces meet in a smooth curve. The posterior opening of the premaxillary canal on the side of the maxilla is about 6 mm in diameter. The zygomatic-orbital bridge was probably elevated 1 cm or less above the alveolar margin (11[0?]). The bridge is badly fragmented, but was some 4 cm long fore-and-aft (c. 14[0]), with a thin, sharp anterior and a thicker, rounded posterior edge (c. 22[0]) (Fig. 2). The infraorbital foramen is poorly preserved, but seems to have been large (c. 13[1]). The infraorbital canal is unobstructed (c. 20[0]). A thin (<5 mm) plate ascends from the dorsal side of the massive alveolar process, helping to form the wall of the temporal fossa; where it meets the lamina orbitalis of the frontal, a prominent, thin, horizontal shelf is formed. The edge of this shelf bears a groove for the optic nerve.
Squamosal-The sigmoid ridge is prominent (projecting laterally ∼1 cm), with a rugose surface, and almost semicircular in outline in posterior view (c. 74[2]) (Figs. 3,4). The mastoid indentation is deep. The surface of the cranial portion dorsal to the zygomatic root is not inflated or bulging. The postglenoid process and postarticular fossa have well-defined edges. The temporal condyle is oval, with well-defined edges. The processus retroversus is not strongly turned in ventrally (c. 77[1]); it lacks distinct dorsal and ventral posterior terminations, in contrast to what is often observed in Metaxytherium floridanum (Domning, 1988). The zygomatic process is dorsoventrally narrow anteriorly and posteriorly, and broad in the middle; its posterodorsal edge is irregularly concave in outline and convex laterad; its medial surface is slightly concave (c. 84[0]). Its anterior portion has a sharper lateral than medial edge. The external auditory meatus is mediolaterally short (c. 75 [1]) and about as high as it is wide (c. ). This ventral tip is formed by a rounded process somewhat distinct from, and displaced slightly lateral from, the border behind it. The latter is somewhat irregularly ridged and beveled. The zygomatic process is long (c. 89[0]), and tapers to a dorsoventrally flattened tongue that reaches the level of the middle of the temporal condyle.
Periotic-The lateral surfaces are smooth; no clear boundary exists between the pars temporalis (= tegmen tympani) and pars mastoidea; the anteroventral notch at the intersection of these two parts is semicircular, as in Metaxytherium floridanum . The anteromedial end of the pars temporalis is prolonged, gradually tapering, and terminates in an abruptly downturned flange that appears to be curled laterally at its ventral edge. The raised area (processus fonticulus) on the posterolateral surface of the pars mastoidea forms less of a triangular protuberance than a sharp, thick, vertical ridge that is inserted into the mastoid foramen. This ridge has a roughly semicircular outline in posterior view. The endolymphatic foramen is covered with sediment but seems to be narrow and slit-like. The cavity above the aquaeductus vestibuli extends somewhat laterad over the medial shelf of the pars temporalis. The medial outline of the pars petrosa bears two convexities, the posterior much larger; its edges are fairly thin and sharp. The dorsal side of the pars petrosa is more or less flat; the ventral end of the promontory is bluntly pointed. The fossa muscularis minor for the origin of M. stapedius is distinct.
Tympanic-Bulbous, with anterior and posterior borders curving somewhat irregularly towards a sharp ventral point (Fig. 5E, F). The sides are smooth, without marked irregularities except for a rough protuberance on the anteromedial surface where the M. tensor veli palatini probably took its origin. A faint, laterally positioned ridge may indicate the attachment of the posterior part of the tympanic membrane; no distinct sulcus tympanicus is present.
Malleus-Anteroposterior length = 13 mm. The posterior end (orbicular apophysis) is slightly convex. The processus muscularis is moderately large, and forms a distinct nipple-like protrusion on the medial side of the bone. The dorsolateral surface bears a rounded longitudinal ridge that does not connect with the external edge of the manubrium; they are instead separated by a broad, 'U'-shaped valley. The external edge of the manubrium is 15 mm long, delicate, and strongly convex. The articulations with the incus are not prepared.
Incus-Not prepared. Stapes-10 mm long, with a prominent stapedial foramen 1 mm in diameter located 4 mm above its base and close to the anterior side of the bone (Fig. 5G). The head end is flattened anteroposteriorly, the basal end dorsoventrally. The basal end is offset backward slightly; the basal facet is curved, its anterior part facing mediad, its posterior part more ventromediad. A groove for the facial nerve is present.
Mandible-In the holotype (Fig. 6A, B; Table 2), the alveolar portion, coronoid process, and anterodorsal part of the symphysis are missing. The distance between its anterior and posterior ventral extremities ( Table 2). The symphyseal masticating surface is incompletely preserved in SC 89.115, but it seems to have been broad with a nearly rectangular outline, as it is in the other referred specimen (ChM PV4757) (c. 121[3]). In contrast to the holotype, the ventral border of the horizontal ramus of SC 89.115 is moderately and evenly concave (c. 122[2]) ( Fig. 6C-F); the degree of concavity thus may vary individually and/or increase with life age (cf. M. krahuletzi; Domning and Pervesler, 2001:pl. 5). The region of the mental foramen is mostly broken in both referred specimens, but it appears that there were no accessory foramina (c. 123[1?]). In the holotype and SC 89.115, the mandibular condyle is elliptical in outline, its surface being convex to flat. In UF 49051, the pterygoid fossa has a rugose surface and well-delineated posteroventral border. The posterior border of the dentary descends smoothly from the condyle, becoming broadly convex beginning at the level of the tooth row (c. 125[2]). The anterior border of the ascending ramus is broken at the base in all specimens. The coronoid canal is oriented anterodorsally-posteroventrally, and the dental capsule is exposed posteroventrally (c. 127[1]). In the referred specimens, the horizontal ramus is dorsoventrally broad (c. 128[1]).

Dentition
UF 49051 (Figs. 2, 3, 7), a subadult individual, has a single empty alveolus, presumably for a root of DP4, at the front of the upper cheek tooth row. Four upper molariform teeth (DP5-M3), each three-rooted, are still present on each side, although the first two are worn out and the last has not quite come into wear. The adult upper dental formula would be I 1, C 0, DP 1 or 0, M 3 (c. and a lightly worn m3 are present, as well as empty alveoli for m1; hence, the adult mandibular dental formula is presumed to be i 0, c 0, dp 1 or 0, m 3. The alveolar region in ChM PV4757 is incompletely preserved. I1-The overall length of the tusk is 42 mm. The root is mediolaterally compressed, 13 × 8 mm in diameter, closed, and tapers toward the end. The root is intermediate in shape between the two specimens of Metaxytherium floridanum illustrated by Domning (1988:fig . 7); the posteroventral margin is uniformly convex; the apical end of the root curves slightly anterodorsad; closer to the crown, the anterodorsal margin of the root bears a distinct hump. The enameled crown is distinct from the root (c. 137[0]), virtually unworn, smooth, subconical, 11 mm high,  (Domning and Beatty, 2007). M2-Moderately worn; displays the cusp pattern typical of Metaxytherium in its most basic form, without accessory cuspules. A smooth precingulum is connected lingually to the protocone but also closely appressed buccally to the paracone, thereby constricting or blocking the buccal end of the anterior cingular valley. The protoloph is straight. The transverse valley is open, impinged on only slightly by the centrally located metaconule. The metaconule and hypocone have become confluent through wear, and are continuous with a smooth postcingular ridge that descends buccally from the hypocone. The posterior cingular valley is open buccally. Length = 21.7 mm; anterior width = 19.2 mm; posterior width = 16.7 mm.
M3-Unworn; differs from M2 only in that the postcingulum consists of two small cuspules rather than a smooth ridge, and the posterior cingular valley is occupied by a large cuspule. Length = 21.3 mm; anterior width = 17.6 mm; posterior width = 14.3 mm.
Atlas-The upper arch has no articular surface for the axis. The spine is low and blunt; on its anterodorsal surface, there are two depressions separated by a midline ridge, likely for insertion of M. rectus capitis dorsalis minor. The lower arch has a small posteroventrally directed triangular process. The articular surface for the odontoid process is round and well developed, but its lateral and posterior borders are not well defined. The anterior cotyles are irregularly semilunar to kidney-shaped in outline, with flaring dorsal, lateral, and ventral edges; two small rugosities are present on the medial borders in UF 49051. The processes for the transverse ligament are located on the lateral borders of the neural canal at about the midlevel of the anterior cotyles. Dorsal to the anterior cotyles the notches for the passage of the first cervical nerve are open anteriorly. The transverse processes are thick and blunt, with rough tips, and directed posterolaterally; in UF 49051, there is a low protuberance on the ventromedial surface on both sides, and together with the transverse processes it forms a shallow notch that is absent in SC 89.115; transverse processes of SC 89.115 are dorsoventrally thicker. There are no vertebrarterial canals. The posterior cotyles are similar in outline to the anterior ones, but are smaller and less concave; in SC 89.115, the dorsal borders of the posterior cotyles have a squaredoff outline. Abbreviations: l, left; r, right; +, measurement taken on incomplete specimen. Axis-The odontoid process is circular in outline, ∼2 cm long, with an irregularly pointed tip in UF 49051. On the ventral surface of the odontoid process, near its base, there is an oval, flat surface for articulation with the corresponding facet on the atlas. The cotyles are triangular (SC 89.115) to kidney-shaped (UF 49051) in outline, with well-demarcated borders; their articular surfaces are convex to nearly flat. The vertebrarterial canals are open laterally and are bounded dorsally and ventrally by short, laterally directed processes. The posterior surface of the centrum is concave and kidney-shaped in outline. The pedicles are triangular in cross-section, with their lateral surfaces oriented anteromedial-posterolateral. The neural canal is nearly square in outline. The postzygapophyses are short and stout, with articular surfaces oriented ventrolaterad. Medial to each postzygapophysis is a small posteroventrad-oriented process, likely for attachment of the ligamentum flavum. The neural spine is incompletely preserved on UF 49051; in SC 89.115, it is robust, rounded, with convex anterior and dorsal surfaces and a flat posterior surface; no articular surface for the atlas is present on the neural spine; the midline suture is unfused in SC 89.115.
Cervicals 3-7-The vertebrarterial canals on these cervicals are small, about 5 mm in diameter. The transverse process of ?C6 projects ventral to the centrum.
Thoracics-Only two anterior thoracics are preserved in UF 49051, of which only one is sufficiently preserved. The neural spine slopes strongly aft; the outline of its summit forms a straight line parallel to the body axis. Anterior and posterior costal demifacets are present; the anterior demifacets face anterolaterally, the posterior demifacets face posterolaterally and are larger. The transverse processes are robust and have ventrolaterally directed tubercular facets. Anterior to the postzygapophyses there are two mediolaterally elongate depressions that serve as attachment sites for the ligamentum flavum. The postzygapophyses are not separated from the laminae; therefore, they do not project farther aft than the transverse processes. The epiphyses of the centra are not fused.
At least 18 thoracic vertebrae are present in SC 89.115 (Table 5). With the exception of the first three and the last vertebrae, we are uncertain of their exact positions within the vertebral column; we have arranged them in as precise an order as possible based on comparisons with Metaxytherium krahuletzi (Domning and Pervesler, 2001) and Dusisiren jordani (Domning, 1978), and have assigned them letters a-l.
Sacral-The centrum has a hexagonal outline, its ventral surface being concave as in L3. The transverse processes extend laterally and ventrally, with distal ends lower than the ventral surface of the centrum. The anterior edges of the transverse processes are straight, the posterior edges are convex; the distal ends are rugose and expanded dorsoventrally, more so than in the lumbars. The expanded ends of the transverse processes likely served for ligamentous attachment to the ilia.
Caudals-The neural spines are vertical in more anterior caudals (Ca2), becoming more posteriorly oriented distally (Ca4, Ca6). The transverse processes are oriented laterally on the first three caudals, and progressively become shorter and more posteriorly oriented caudally; by Ca9 the transverse processes extend beyond the posterior surface of the centrum. The transverse processes are dorsoventrally flattened up to Ca6, then become more triangular and almost rod-like in some of the more distal vertebrae. The centra are hexagonal in outline, with their ventral surfaces bearing two pairs, one anterior and one posterior, of demifacets for the hemal arches; on each side the anterior and posterior demifacets are connected by a keel, with the surface between the keels being concave. The transverse processes on Ca12 and 13 seem to be more laterally oriented than the preceding ones, indicating the presence of a dugong-like fluke (Brandt, 1868:pl. 8, fig. 4;Domning, 1978;Zalmout and Gingerich, 2012) (Fig. 10P).
Ribs-Only the distal ends of 19 ribs are preserved in UF 49051. They are relatively slender, mediolaterally flattened, and pachyosteosclerotic; the largest has cross-sectional dimension of about 41 × 22 mm. In SC 89.115, there are about 15 right and 16 left ribs preserved; however, except for R1, we are uncertain of their individual positions within the ribcage. R1-The capitulum and tuberculum lie dorsal to the neck (Fig. 10). On the ventral surface of the neck there is a prominent anteroventrally oriented process for M. longus capitis. The shaft is subtriangular in cross-section, becoming flatter distal to the angle. A low ridge extends along most of the length of the posterior surface of the shaft. The shaft is narrower at about midlength; anteriorly, just distal to this point, there is a protuberance for M. sternocostalis. The shaft tapers distally and turns inward; its end forms an oval rugose surface for attachment of cartilage.
The ribs are pachyosteosclerotic; only a small amount of cancellous bone is present at proximal and distal ends. Two distinct facets can be observed on the capitulum in more anterior ribs, whereas only one facet is present in posterior ones. The distance between the capitulum and the tuberculum diminishes posteriorly, but these never merge. The shafts are elliptical in cross-section; their distal ends taper and are turned inward and posteriorly. Nearly all ribs have a large, oval depression for M. iliocostalis on the lateral surface distal to the angle.
Sternum-Not preserved in the holotype; the description is based on the sterna of SC 89.115 and ChM PV4757 (Figs. 11, 14C; Table 6). The sternum in SC 89.115 comprises three parts: manubrium, an intermediate sternebra, and xiphisternum, whereas in ChM PV4757 only the xiphisternum is preserved. The anterior end of the manubrium is not preserved. The manubrium is flat, dorsally concave with a low, inconspicuous ventral keel. The anterolateral surface bears a rugose, flat, elongate surface for the sternal cartilage. The costal facets for rib 1 are oval and oriented dorsolaterally; they are asymmetrically located on the manubrium, with the left one more anterior than the right. The posterior surface of the manubrium is subrectangular in outline with a rugose surface. The single intermediate sternebra is irregularly hexagonal in outline, dorsoventrally flat, and concave dorsally. The xiphisternum is straight overall, with convex dorsal and ventral surfaces. Anteriorly the xiphisternum is wide, becoming narrower posteriorly, then widening again where it bifurcates and becomes dorsoventrally flat. Only the left posterior extremity is completely preserved in SC 89.115; it is long and bowed laterally, with a length of ∼6.5 cm; a foramen is present at about its midlength with a posteromedial-anterolateral orientation; a shallow groove on the dorsal surface is continuous with the foramen. Both posterior extremities are incomplete in ChM PV4757, but it seems to have been similar to the other referred specimen. Scapula-Not preserved in the holotype and poorly preserved in ChM PV4757; the description is based on the left scapula of SC 89.115 ( Fig. 12; Table 7). The scapula is somewhat sickle-shaped, with a broad supraspinous fossa and shallower infraspinous fossa. Its medial surface is poorly preserved, anteroposteriorly convex and dorsoventrally concave. The vertebral border is mostly broken, but where preserved it is rugose for attachment of a scapular cartilage. The spine of the scapula is not preserved. The anterior border of the blade is thinner than the posterior border. Posterodorsally on the lateral surface of the blade, a low crest marks the origin of M. teres major. The neck of the scapula is long and narrow with a broad suprascapular notch. The surface on the posterior edge of the neck is rugose, marking the origin of the long head of M. triceps brachii. The coracoid process is medially inflected; its anteromedial surface is rugose, marking the origin of M. coracobrachialis. The glenoid is oval in outline, with its anterior end narrower than the posterior; the articular surface is concave, more so anteroposteriorly.
Humerus-Not preserved in the holotype; the description is based on the left humeri of SC 89.115 and ChM PV4757 (Figs. 12, 14D; Table 8). The proximal epiphysis is detached, indicating a young age for the specimens. The humerus is robust and dumbbell-shaped; the shaft is triangular in cross-section. The greater tubercle extends farther proximal than the humeral head; Abbreviations: e, estimated measurement; +, measurement based on incomplete specimen.
facets for the infra-and supraspinatus mm. on the greater tubercle are poorly defined. A flange of the greater tubercle extends anteromedially over the bicipital groove, which is deep and narrow. The head is elliptical and oriented obliquely to the long axis of the shaft. A suboval rugose area on the posterolateral surface of the head marks the origin for the lateral head of M. triceps brachii. The lesser tubercle does not rise above the humeral head. The deltoid crest is prominent and recurved; distally, it is continuous with the deltopectoral crest, ending in an elongated, shallow, pitted depression, which marks the insertion of M. pectoralis major. Distally, the trochlea is canted obliquely to the long axis of the shaft; a shallow circular notch subdivides the anteroproximal surface. A shallow groove on the posterolateral corner of the trochlea marks the insertion of a humeroulnar ligament. The olecranon fossa is deep, with well-defined borders; the coronoid fossa is shallow. The entepicondyle extends distally as far as the trochlea. Radius-Ulna-Not preserved in the holotype; the description is based on the left radii-ulnae of SC 89.115 and ChM PV4757 (Figs. 12, 14E; Table 9). The tip of the olecranon and the distal epiphyses of the ulna and radius are not preserved in SC 89.115. The radius and ulna are unfused proximally in SC 89.115, but contact each other over a flat and rugose surface, which would have prevented pronation or supination; in ChM PV 4757, they are fused proximally and unfused distally. The shafts are both nearly straight, only slightly bowed anteriorly, with little torsion between them. The olecranon is deflected posteriorly ∼50 • from the long axis of the shaft; the tip is oval in outline and rugose. An elongate shallow depression, ∼2.5 cm long, on the medial surface of the olecranon posterior to the semilunar notch, possibly marks the insertion of the long head of M. triceps brachii. An indentation on the lateral portion of the ulnar surface of the semilunar notch marks the insertion of a humeroulnar ligament. The crosssection of the ulnar shaft is triangular, distally becoming more oval and elongated anteroposteriorly. A shallow groove for the tendon of M. extensor digiti quinti is located distally on the lateral surface of the ulna. The proximal end of the radius has a concave, mediolaterally elongate surface that forms part of the semilunar notch. The radial tuberosity, located on the medial surface about 2 cm distal to the proximal end, marks the insertion of M. brachialis. The shaft of the radius is oval in cross-section, be- coming triangular distally. Distally, there are two grooves on the lateral surface of the shaft. The first, on the posterolateral surface, is a shallow groove for the M. extensor pollicis tendon; the second, on the anterolateral surface, is shallow, but broader than the other, and was likely for the M. extensor carpi radialis tendon. The distal epiphysis is partially unfused in ChM PV4757; the distal articular surfaces do not differ from those of other species of Metaxytherium.

Manus
Not preserved in the holotype; the description is based on preserved elements of SC 89.115 ( Fig. 12; Table 10), consisting of the left scaphoid-lunar-centrale and metacarpals ?III and IV.
Scaphoid-Lunar-Centrale-The proximal articular surface is roughly hourglass-shaped; the posteromedial one-fourth of its surface is concave, the rest being flat to convex. The anterior surface is convex. The posterior surface of the bone has a shallow median convexity. The medial half of the posterior surface has a suboval facet and the lateral half has a smaller mediolaterally elongated facet; these two facets are for articulation with the cuneiform-pisiform. The distal articular surface has two facets for the trapezium-trapezoid-magnum; the anterior of these two facets is mediolaterally elongated, oval, and flat; the posterior one is concave, triangular in outline, with its apex pointed anteriorly.
Metacarpal ?III-The proximal articular surface is nearly rectangular in outline; it has two articular surfaces, one canted anteriorly and the other posteriorly, which are separated by a mediolaterally oriented ridge; the surfaces on both facets are flat to slightly convex. Proximally, the anterior, medial, and posterior surfaces of the shaft are rugose; the posteromedial corner has a raised knob. The lateral surface of the shaft is flat, whereas its medial surface has a low ridge that extends proximodistally, beginning from the anteromedial corner and going to the midline. The distal end of the shaft is oval in outline, with its long axis oriented anteroposteriorly. The distal epiphysis is not preserved.
the anterior, medial, and posterior surfaces of the shaft are rugose. The lateral surface of the shaft is flat and its medial surface is rounded. The distal end of the shaft is anteroposteriorly expanded, subtriangular in outline with its apex posterior. The distal epiphysis is not preserved. Innominate-Not preserved in the holotype; the description is based on the left and right innominates of SC 89.115 ( Fig. 13; Table 11). The ilium is long and rod-like; the cross-section of the shaft is oval. The proximal end of the ilium is flat mediolaterally and expanded dorsoventrally, with its dorsomedial surface rugose but not forming an auricular surface. This rugose surface was likely in contact with or, more likely, connected by ligaments to the expanded end of the transverse process of the sacral vertebra. An iliac spine is absent. The acetabulum is shallow, oriented posteroventrad, and oval in outline with no acetabular notch. The surface medial to the acetabulum is concave. A short pubis seems to have been present, but this area is broken on both innominates. The obturator foramen is absent. The ischium is mediolaterally flattened, with its distal end (ischial tuberosity) rugose and thicker posteroventrally, giving it a triangular cross-section. The ischium forms an angle of ∼129 • with the long axis of the ilium. Based on the morphology of the innominate, it seems this specimen was a male (cf. Domning, 1991;Zalmout and Gingerich, 2012).

PHYLOGENETIC ANALYSIS
The choice of taxa used in the phylogenetic analysis follows that of Velez-Juarbe et al. (2012), with the addition of Halitherium christolii Fitzinger, 1842, andMetaxytherium subapenninum Bruno, 1839, and using Phosphatherium escuilliei Gheerbrant et al., 1996, andCornwallius sookensis (Cornwall, 1922) as outgroup taxa representing Proboscidea and Desmostylia, respectively, for a total of 38 taxa. We chose Phosphatherium over Moeritherium (used in previous analyses, e.g., Domning, 1994) because it is now the basal-most proboscidean for which good comparative cranial material is known; character states were scored based on the description by Gheerbrant et al. (2005). We used Cornwallius as a representative of Desmostylia because it is one of the oldest members (late Oligocene) of the group that is known from good cranial material (see Beatty, 2009). Character states for Cornwallius were scored using Beatty (2009) and direct observations of the specimens referred to in that publication. Character states for sirenians were scored from direct observation of specimens by one or both of us. The characters and states used follow Domning (1994), Domning et al. (1994), Bajpai and Domning (1997), Domning and Aguilera (2008), Bajpai et al. (2010), and Velez-Juarbe et al. (2012), with the addition of characters 21, 23, 76, 81, and 118 and modification of others for a total of 69 characters (Supplemental Data S1, S2). All characters were left as unordered. The matrix was analyzed with TNT (Goloboff et al., 2008a) by doing a traditional search using the tree bisection-reconnection (TBR) algorithm with the following parameters: 1000 replicates and keeping 10 trees per replicate. This was followed by an analysis using the implied weight method (Goloboff, 1993;Goloboff et al., 2008b) set to K = 3.00. Results-The phylogenetic analysis resulted in 2205 most parsimonious trees (MPTs), 185 steps long with consistency index of (CI) = 0.5205 and retention index (RI) = 0.7578. The strict consensus was 271 steps but had very poor resolution. Analyzing those results using the implied weighting method resulted in one MPT, 187 steps long with CI = 0.5348 and RI = 0.7521 (Fig. 15). The topology of the tree is largely consistent with that of Vélez-Juarbe et al. (2012). Some of the main differences are that (1) the polytomies between an undescribed halitheriine (USNM 541417) and Caribosiren turneri as well as that among the different species of Metaxytherium have been resolved; and (2) Halitherium christolii is no longer the sister taxon to the Metaxytherium spp. + Hydrodamalinae + Dugonginae clade (as in Domning, 1994). The position of Metaxytherium albifontanum among other species of Metaxytherium confirms its generic affinities.

Comparisons
Among species of Metaxytherium, UF 49051 is most similar, in size and overall morphology, to M. crataegense. However, M. albifontanum retains the plesiomorphic condition of having the ventral extremity of the jugal under the posterior edge of the orbit (c. 85[1]), a characteristic that it shares with M. krahuletzi, more basal halitheriines, and some basal dugongines (Fig. 3). M. albifontanum differs from some M. krahuletzi by having exoccipitals that do not meet in a suture dorsal to the foramen magnum (c. 66[1]) ( Fig. 4; this area has been partially reconstructed in the holotype, giving the false impression that they did), a derived condition that is polymorphic in M. krahuletzi and present in all other Metaxytherium, hydrodamalines, and some dugongines. In this respect, UF 49051 seems to be a morphological intermediate between the more basal M. krahuletzi and other species within the genus, despite its earlier geological age.
The sternum gives a similar impression. M. krahuletzi and M. albifontanum (as well as M. arctodites) retain the tripartite condition of the sternum seen in Halitherium schinzii (Lepsius, 1882:pl. 6, figs. 73-75;cf. Domning and Pervesler, 2001:pl. 14), whereas no other species of Metaxytherium for which the sternum is sufficiently known (M. floridanum, M. medium, M. serresii, M. subapenninum), nor any hydrodamaline, has been found to exhibit more than two separate elements, termed the manubrium and xiphisternum. A notable difference between M. krahuletzi and M. albifontanum, however, is the latter's greater width of the manubrium relative to the posterior parts of the sternum. Three sterna of M. krahuletzi (a young adult and two mature adults) are quite uniform in their long, narrow shapes, with the manubrium not appreciably wider than the intermediate sternebra or the anterior part of the xiphisternum. None of these has the proportions seen in M. albifontanum. In the holotype of M. arctodites, the manubrium (not yet published) is intermediate in shape, being long relative to the rest of the sternum as in M. krahuletzi but expanded at its anterior end as in M. albifontanum.
Given the wide range of variation in sirenian sterna generally, and the inadequate sampling of individual and ontogenetic variability of this element in the taxa at issue, it would be premature to conclude that these characters of the sternum are (or are not) of phylogenetic value. The evidence suggests patterns that are interesting, but too complex to decipher from the available data.
tusks was most likely less well suited for uprooting larger seagrasses than the other two species. A much more detailed description of the paleoecology of Metaxytherium albifontanum was recently published by Vélez-Juarbe et al. (2012).

Discussion
The more basal position of Metaxytherium krahuletzi with respect to M. albifontanum (Fig. 15) indicates that the divergence between the two occurred prior to the late Oligocene. Indeed, the sister taxa to the Metaxytherium + Hydrodamalinae + Dugonginae clade include two early Oligocene taxa from Puerto Rico, an undescribed genus and species of halitheriine (USNM 542417), and Caribosiren turneri. Of these, USNM 542417 is the most fitting, morphologically and chronologically, to be the ancestor of the Metaxytherium + Hydrodamalinae clade. Cari-bosiren, although the right age, shows derived features not seen in any of the later 'halitheriines,' such as extreme rostral deflection and possible loss of tusks. Previous assumptions for the origins of Metaxytherium have been in favor of either a European origin from Halitherium christolii or a New World origin (Domning, 1994). Based on our results, we argue in favor of a West Atlantic-Caribbean origin for Metaxytherium around the late early Oligocene. However, the more basal position of M. krahuletzi likely points to a more complicated history in this group.