Prepoplanops boleadorensis, Carlini, Alfredo A., Brandoni, Diego & Dal Molin, Carlos N., 2013

Carlini, Alfredo A., Brandoni, Diego & Dal Molin, Carlos N., 2013, A new genus and species of Planopinae (Xenarthra: Tardigrada) from the Miocene of Santa Cruz Province, Argentina, Zootaxa 3694 (6), pp. 565-578 : 568-575

Prepoplanops boleadorensis new sp.

( Figs. 3 , 4 , 5 , 6 , 7 E)

Referred species: only the type species.

Derivation of name: from Cerro Boleadoras, its geographical provenance.

Holotype: MLP 97-XI-3-1, nearly complete specimen with, skull and mandible with almost all teeth, dorsal vertebra (cervicals, thoracics, lumbars) and ribs, almost complete left scapula, partial right scapula, partial left and right humeri, partial left and right ulna, partial radius, carpals, damaged pelvic girdle, partial left (without the head) and partial right femora (distal third), left and right tibia, partial right fibula, left and right astragali, left and right calcanei, left navicular, left cuboid, left metarsal IV and V, phalanges.

Geographic and stratigraphic occurrence: 60 km S from Los Antiguos town on Provincial Route 41, NW of Santa Cruz Province, south from Lago Buenos Aires, Argentina (46º 50’ 04” S 71º 49 29” W). Río Zeballos Group, Cerro Boleadoras Formation (Miocene).

Diagnosis: As the genus by monotypy.

Description: Similar in size to Planops magnus . Although with several cranial sutures (naso-frontal, frontoparietal) persisting, the specimen corresponds to an adult, because the intermaxillary, interpalatal, maxillopalatal sutures, and those of the postcranial long bones are closed.

The skull of Prepoplanops boleadorensis ( Fig. 3 ), although somewhat modified by postmortem compression, is nearly complete, but lacks the premaxillae, the descending lamina of both pterygoids, the right condylus, right jugal, right Mf2 and left Cf1. The skull is elongated, slender, tubular in shape, and presents its greater width at the level of the preorbital processes and mastoid area.

In dorsal view ( Fig. 3 A), as in Planops magnus the sagittal crest is almost absent. The frontals are elongated, nearly twice the length of the parietals or nasals. The rostrum (pre-orbital area forward the maxillary zygomatic root) is long and its lateral walls are slightly distally divergent, whereas in Planops longirostratus the lateral walls of the rostrum tend to converge distally. The infraorbital passage for the trigeminal-fascial nerves is divided into two foramina as in Planops magnus and Planops longirostratus , and is not a single foramen as in Planops martini .

In lateral view ( Fig. 3 B), the skull roof is slightly convex along its anteroposterior axis; with its highest point located behind the contact between frontals and parietals, whereas in Planops magnus the highest point lies at the level of the midpoint of the frontals. The maxillo-jugal junction lies on the plane of the middle part of Mf1 and clearly over the level of the alveolar plane (similar to Planops longirostratus and Planops magnus ). The squamosal presents a long subtriangular zygomatic process pointed anteriorly and with dorsal and ventral convex edges. As in Planops magnus , the ventral margin of the pterygoid seems to be below the ventral plane of the occipital condyles. The occipital condyles are more prominent than in Planops magnus , and their ventral margins lie below the level of the occlusal plane. Despite Hoffstetter’s (1961) statement, the lacrimals appear to be located similarly to those of Planops martini , Planops magnus , and Planops longirostratus .

In occlusal view ( Fig. 3 C), the predentary portion is very long. In Prepoplanops boleadorensis , the lateral margins of the maxillae converge anteriorly up to the premaxillae notch, but diverge forward. In Planops magnus and Planops martini , these margins are subparallel, and in Planops longirostratus they are nearly convex but not diverging anteriorly. At the diastema level, between the caniniform and the molariform tooth-row, the edges of the maxillae are straight; whereas in Planops magnus , Planops longirostratus , and Planops martini , these margins are relatively concave ( Fig. 7 A to D). As in Planops magnus , Planops longirostratus , and Planops martini , at the level of the molariform tooth-row, the margin of the maxillae is convex on its anteroposterior axis ( Fig. 7 ). In Prepoplanops boleadorensis the palate is flat at its anteriormost portion (from the Mf1 level up to its end) although it presents two keels, and is nearly convex at the Mf3-5 level, whereas in Planops species the entire surface of the palate is slightly convex. The posterior margin of the palate is behind the Mf5; whereas in Planops longirostratus this margin is at the midpoint plane of the Mf5 ( Fig. 7 C, D). In the new species the maxillo-palatal suture lies between the Mf2-3, whereas in Planops magnus is at the level of the midpoint of Mf3. As in Planops magnus , the glenoid fossa extends over the squamosal zygomatic process, and is slightly concave and wide. The margins of the occipital condyles are nearly straight, showing subtriangular shape; whereas in Planops magnus the margins bear two opposite notches (one medio-distal and one latero-proximal) that result in an eight-shaped outline in lateral view.

The alveoli of the caniniforms are oval in cross-section and are separated from the molariform tooth-row by a very short diastema (similar in length to the anteroposterior length of the caniniform). In Planops longirostratus the diastema is longer, and is even longer in Planops magnus and Planops martini (almost twice the length of the anteroposterior Cf1) ( Fig. 7 ). The caniniform is elliptic in cross-section and its occlusal surface is separated into two parts (not as in Planops or even in Prepotherium ), one larger surface facing backward and a shorter one facing rearward. Except for the right Mf2, all the upper molariforms are preserved in Prepoplanops boleadorensis . As in Planops magnus , Planops longirostratus , and Planops martini , the Mf1-Mf4 are subquadragular in outline with rounded edges and a labiolingual major axis ( Fig. 7 A to D). As in Planops magnus and Planops longirostratus the Mf5 are oval and present a posterior notch, which is absent in Planops martini (see Hoffstetter 1961). The occlusal surface of the molariforms resembles that of Megalonychidae (see Bargo et al. 2009) having two transverse crests separated by a valley.

The mandible ( Fig. 4 ) is nearly complete, but only the right cf1, right mf2, and left mf3 are preserved.

In lateral view ( Fig. 4 A), as in Prepotherium , the anterior margin of the coronoid process lies posterior to the posterior edge of the mf3, and this molariform is entirely visible in lateral view. The abovementioned margin forms a 100º angle with the occlusal plane. The posterolateral opening of the mandibular canal lies on the base of the coronoid process slightly below the occlusal plane, entirely posterior to the mf3, and is visible in lateral view. In Prepotherium filholi the posterolateral opening of the mandibular canal lies on the lateral side of the coronoid process. In Prepoplanops boleadorensis the mandibular condylar process lies over the plane of the molariform tooth-row. The ventral margin of the angular process is convex along its major axis. The symphyseal part of the mandible is subtriangular in shape and extended as a spout, with an anterior apex. Its dorsal edge lies at the molariform tooth-row plane and its walls are slightly concave and converge at the symphyseal plane. The anterior opening of the mandibular canal lies at the midpoint of the predentary length. The ventral margin of the dentary, at the tooth row level, is nearly flat being convex in Prepotherium filholi (see Scott 1903-04).

In occlusal view ( Fig. 4 B), the lateral margins of the predental area are nearly convex and converge to the tip of the mandible. The predentary length is greater than the tooth-row cf1-mf3 length, whereas it is shorter in Prepotherium filholi . The posteroventral edge of the mandibular symphysis lies anterior to the caniniform plane, but is slightly posterior in Prepotherium . The mandibular condylar process is oval in outline, with its major axis oriented transversally. The molariform tooth-rows are parallel, as in Planops . Scott (1903-04) and Hoffstetter (1961) indicated that in Prepotherium the molariform tooth-rows converge posteriorly (but see Discussion). A very short diastema (shorter than the anteroposterior length of cf1) occurs between the caniniform and the molariform tooth-row, whereas Prepotherium filholi has a longer diastema (greater than the anteroposterior length of cf1). The cf1 is subcircular in cross-section, and bears a forward-facing wear surface. The mf2 is oval in cross-section and the mf3 is subcircular (as in Prepotherium filholi ).

In ventral view, the distal half of the symphysis bears a keel-like structure, whereas the proximal half is almost flat.

The humerus ( Fig. 5 A, B) is slender and distally broad and flat; the humeral head is hemispheric to pyriform; the tuberosities are well developed and separated by a distinct groove from the articular surface of the head. Its general morphology is similar to that of Prepotherium potens and Planops martini . However, as in Planops martini , both tuberosities lie at the same level and their proximal margins do not reach the proximal level of the humeral head. As in Prepotherium potens (see Scott 1903-04: 338), the greater tuberosity is slightly more robust than the lesser tuberosity; whereas in Planops martini both tuberosities are similar in size (Hoffstetter 1961: 75). The deltopectoral crest (formed by the merge of the deltoid and pectoral crests) ( Fig. 5 A) is well developed and extends from the greater tuberosity along the anterior surface of the diaphysis to its distal third. The deltoid crest is not well preserved, but seems to be a thin lamina that extends medially and reaches the pectoral crest; whereas in Planops martini the deltoid crest is well developed and laterally projected. At the distal end of the deltopectoral crest, the diaphysis has its greatest anteroposterior depth.

The entepicondylar foramen ( Fig. 5 A) is 22 mm long at its major axis, and its anterior edge is formed by a thin bone bridge, 13 mm wide. The epitrochlea is relatively more projected than the epicondyle.

As in Planops martini , the capitulum is hemispheric, and its articular surface is pyriform in outline and pointing backward. In anterior view, the distal outline of the trochlea is angled with respect to the distal plane (i.e. the plane perpendicular to the vertical diaphyseal axis); but in Planops martini this trochlear margin is more angled with respect to the abovementioned plane and ventromedially projected. In addition, in Prepoplanops the distal margin of the capitulum is distally extended relative to the distal margin of the trochlea; whereas Planops martini presents the opposite condition.

Posteriorly ( Fig. 5 B), the olecraneal fossa is 25 mm in diameter (along its major axis).

The ulna of Prepoplanops boleadorensis ( Fig. 5 C, D) is more slender than that of Planops martini .

Proximally, as in other Tardigrada, the ulna has an articular facet for the humerus divided into two successive areas (one for the capitulum and another one for the trochlea), and a third proximal facet for the radius. In the new species the olecranon projects posteromedially, and the angle of the trochlear facet (with respect to the anteroposterior vertical plane) is approximately 80º, whereas in Planops martini the olecranon is not projected, and the angle is greater.

In the femur ( Fig. 5 E, F) the femoral diaphysis is anteroposteriorly compressed, with the greater anteroposterior thickness on the medial margin. The third trochanter ( Fig. 5 E) is thin, subtriangular in section, and forming a lamina as in Planops martini . The patellar facet is broad and slightly concave latero-medially; it is connected with both articular condyles by an isthmus as in Santacrucian sloths, mylodontids, several Megalonychidae , and some Megatheriinae . In Planops martini the isthmuses have nearly the same width, whereas in Prepoplanops boleadorensis and Prepotherium potens the external isthmus is wider than the internal one. As in Prepotherium potens , the angle between the medial margin of the patellar trochlea and the medial margin of the medial condyle is almost 170º ( Fig. 5 F), but in Planops martini both medial margins form a 130º angle between them.

The articular condyles are prominent and the intercondylar fossa is deep and narrow. As in other Planopinae, in Prepoplanops boleadorensis the articular surface of the medial condyle is pyriform in outline ( Fig. 5 F); however, the medial margin of the medial condyle is not as rounded as in Prepotherium potens or Planops martini . The articular surface of the lateral condyle is subpentagonal in outline and latero-medially concave. Both epicondyles are well developed, but not as projected as in Prepotherium potens or Planops martini .

The general morphology of the tibia ( Fig. 5 G, H) resembles that of Planops martini and Prepotherium potens but seems to be more slender than those. Proximally, the facets for the femoral condyles are similar in size; the medial facet is concave and trapezoidal in outline, and the lateral one is slightly convex and quadrangular. The proximal facet for the fibula is flat and located slightly posterior to the diaphyseal shaft. Distally, there are three contiguous facets; two for the astragalus, and one for the distal end of the fibula. The canals (two) for the tarsal extensor tendons are very laterally placed, and the laminae that delimit them are extended and strongly project postero-ventro-medially.

In the fibula ( Fig. 5 G, H), the proximal epiphysis bears a main bilobated facet for articulation with the tibia and, posteriorly, a smaller facet (for a sesamoid?). The diaphysis is slender and subtriangular in section, with the medial surface slightly concave on its anteroposterior axis. Distally the external malleolus is very massive as in Planops martini and Prepotherium potens . Distally, the facet for the tibia is not well preserved but seems to be oval in outline.

The pes ( Fig. 6 ) is almost complete proximally. The fibular facet of the astragalus is subtriangular in outline with posteriorly directed apex, and almost flat; dorsomedially it is limited by and contacts with the discoid facet, at a 90º angle. Its antero-ventrolateral portion contacts the ectal facet forming a 92º angle. Lateromedially and distoproximally the discoid facet is nearly convex, and contacts the odontoid facet forming a 130º angle. The navicular facet is sessile and located on the anteriormost part of the astragalus, it is subtriangular to oval in section, with its major axis oriented dorsolateral-ventromedially. This facet presents two portions: one (dorsolateral) is concave and the other (ventromedial) convex. Medially with respect to the navicular facet, the cuboidal facet is almost flat, with a subtriangular to pyriform outline. The ectal facet is oval to subtriangular and antero-posteriorly concave; the sustentacular facet is smaller, trapezoidal, and nearly flat. Both facets are separated by a deep sulcus calcanei.

The calcaneum is large, its diaphysis is dorso-ventrally compressed and its proximal and distal ends are expanded. The central part of the diaphysis is subtriangular in section with dorsolateral apex; ventrolaterally the diaphysis is concave along its anteroposterior axis. The tuber calcaneus is well developed, being particularly prominent on its medial portion. The ectal facet is broad, subtriangular in section, and slightly convex on its lateromedial axis. Proximo-medially, the ectal facet is very close to the sustentacular facet; but latero-distally, both facets are separated by a broad sulcus calcanei. The sustentacular facet is subtriangular in section, and it lies next to the cuboidal facet, forming a continuous sigmoid articular area. There is no evidence of ligament fossettes in Prepoplanops , whereas they were described by Hoffstetter (1961) for the calcaneum and astragalus of Planops martini .

The navicular is anteroposteriorly compressed and oval to subtriangular in shape, with its major axis dorsolaterally to ventromedially oriented. The astragalar facet, located on its proximal surface, is divided into a dorsolateral half that is nearly circular and forms a condylar prominence, and a ventromedial half that is concave along its major axes. A small subtriangular facet that articulates with the cuboid is present ventrolateral to the concave ventromedial part.

Distally, three facets are developed and clearly recognizable; they articulate with the mesocuneiform, entocuneiform, and ectocuneiform respectively. The facet for the mesocuneiform (dorsally and laterally placed) is oval and nearly flat; the facet for the entocuneiform (medially placed) is subtriangular and convex; the facet for the ectocuneiform (ventrally and laterally placed) is rhomboidal and convex.

The cuboid is irregular in shape, more or less cubic, massive, and has two important and complex articular surfaces separated by non-articulating bone. The posteromedial surface bears facets for the calcaneum, astragalus, navicular and ectocuneiform. The laterodistal surface bears facets for metatarsals III, IV, and V. The dorsal and ventral surfaces of the cuboid are rugose, relatively flat, and without articular facets. The calcaneal facet is posterolateral in position, trapezoidal in section, and nearly flat. The astragalar facet is posteromedial, approximately subtriangular in section, and contacts the calcaneal facet at straight angle. The navicular facet, which forms a straight angle with the astragalar facet, is flat and suboval in section. The facet for the ectocuneiform is dorsolaterally placed, just over the navicular facet and continuous with it.

The facets for the Mt IV and Mt V form a single surface that is subtrapezoidal in shape. The portion that corresponds to the Mt IV is relatively flat and subrectangular in outline. The portion that corresponds to the Mt V is square and slightly convex. Medially with respect to the facet for Mt IV, and forming a 90º angle with it, there is a very small facet that probably articulates with Mt III.

The proximal end of the Mt IV bears three facets that contacts with the Mt III, cuboid, and Mt V, respectively. The anteromedial facet for the Mt III is almost trapezoidal in shape, with its anterior half anteroposteriorly convex and its posterior half concave. The facet for the cuboid is dorsoventrally rectangular. The facet for Mt V is located laterally; it is flat and pyriform with the apex pointed ventro-medially. The distal epiphysis is subtriangular in cross-section with laterodorsal apex. The facet for the proximal phalanx of digit IV, located at the distal end of the Mt IV, is keel shaped. A very small subtriangular and convex facet lies ventromedially to this latter facet; it probably contacts with a sesamoid.

The Mt V is slightly longer than Mt IV. Its proximal half is depressed dorso-plantarly, and laterally expanded. It bears two contiguous facets on its proximo-medial surface, which are articulated with the cuboid and Mt IV. The cuboid facet is concave and suboval in shape. The facet for Mt IV, located distally with respect to the cuboid facet, is flat and pentagonal in outline. MtV is expanded distally, and does not show well-defined articular surfaces.

Measurements (in mm): Skull, preserved total length, 247; predentary length 46; c1-m4 length, 61; m1-m4 length, 44; diastema length, 4.8; maximum predentary width, 38.7; maximum palatal width on M2, 48. Mandible, total length, 222; predentary length, 79.5; c1-m3 length, 52,4; m1-m3 length, 37.9; diastema length, 4.9; mandibular depth at m2, 49.1. Humerus, total length, 298; max proximal width, 80; max distal width, 101; distal facet width, 63.5. Femur, preserved total length, 268; max distal width, 113; internal condyle width, 47; external condyle width, 43; trochlear width, 46.5. Tibia, total length, 235; proximal width, 101; distal width, 81.2. Fibula, total length, 219.

Comparisons. The morphology described for MLP 97-XI-3-1 justifies the recognition of a new genus of Planopinae. Prepoplanops differs from Planops and Prepotherium in: the shape of the lateral walls of the rostrum (slightly distally divergent in Prepoplanops , tending to converge distally in Planops ); highest point of the skull roof located behind the contact between frontals and parietals in Prepoplanops , at the level of the midpoint of the frontals in Planops ; the occipital condyles of Prepoplanops are more prominent than in Planops magnus ; the margins of the occipital condyles are nearly straight, showing a subtriangular shape in Prepoplanops , bearing two opposite notches (one medio-distal and one latero-proximal) that result in an eight-shaped outline in Planops magnus ; the lateral margins of the maxillae converge anteriorly up to the premaxillae notch in all taxa, but diverge forward in Prepoplanops , are subparallel in Planops magnus and Planops martini , and are nearly convex not diverging anteriorly in Planops longirostratus ; at the diastema level, between the caniniform and the molariform tooth-row, the edges of the maxillae are straight in Prepoplanops and relatively concave in Planops ; the palate is flat at its anteriormost portion (from the Mf1 level up to its end) although it presents two keels, and is nearly convex at the Mf3-5 level in Prepoplanops , the entire surface of the palate is slightly convex in Planops , the posterior margin of the palate is behind the Mf 5 in Prepoplanops , and is at the midpoint plane of the Mf 5 in Planops longirostratus ; the maxillo-palatal suture lies between the Mf 2-3 in Prepoplanops , is at the level of the midpoint of Mf 3 in Planops magnus ; a very short diastema (similar in length to the anteroposterior length of the caniniform) is in Prepoplanops , is longer in Planops longirostratus and even longer in Planops magnus and Planops martini ; the posterolateral opening of the mandibular canal lies on the base of the coronoid process slightly below the occlusal plane, entirely posterior to the mf 3 in Prepoplanops , and lies on the lateral side of the coronoid process in Prepotherium filholi ; the ventral margin of the dentary, at the tooth row level, is nearly flat in Prepoplanops and is convex in Prepotherium filholi ; the predentary length is greater than the tooth-row cf1-mf3 length in Prepoplanops , and is shorter in Prepotherium ; the posteroventral edge of the mandibular symphysis lies anterior to the caniniform plane in Prepoplanops , and is slightly posterior in Prepotherium ; a very short diastema (shorter than the anteroposterior length of cf1) occurs in Prepoplanops , whereas a longer diastema (greater than the anteroposterior length of cf1) occurs in Prepotherium filholi ; the deltoid crest of the humerus seems to be a thin lamina that extends medially and reaches the pectoral crest in Prepoplanops , and is well developed and laterally projected in Planops martini ; in the humerus, the distal outline of the trochlea is angled with respect to the distal plane in Prepoplanops , is more angled with respect to the abovementioned plane and ventromedially projected in Planops ; the distal margin of the capitulum is distally extended relative to the distal margin of the trochlea in Prepoplanops , whereas Planops martini presents the opposite condition; the ulna of Prepoplanops boleadorensis is more slender than that of Planops martini ; in the femur, both epicondyles are well developed, but not as projected as in Prepotherium potens or Planops martini ; in Prepoplanops the medial margin of the medial condyle of the femur is not as rounded as in Prepotherium potens or Planops martini ; the general morphology of the tibia resembles that of Planops martini and Prepotherium potens but seems to be more slender than those; there is no evidence of ligament fossettes in Prepoplanops , but they were described by Hoffstetter (1961) for the calcaneum and astragalus of Planops martini .

Figures

FIGURE 3. Skull of Prepoplanops boleadorensis (MLP 97 - XI- 3 - 1). (A) dorsal view, (B) left lateral view, and (C) palatal view.

FIGURE 4. Mandible of Prepoplanops boleadorensis (MLP 97 - XI- 3 - 1) (A) left lateral view and (B) occlusal view.

FIGURE 5. Pos-cranial elements of Prepoplanops boleadorensis (MLP 97 - XI- 3 - 1). Right humerus in (A) anterior view and (B) posterior view; right ulna (proximal half) in (C) anterior view and (D) medial view; right femur (without its head) in (E) anterior view and (F) distal view; right tibia and fibula (articulated) in (G) posterior view and (H) anterior view.

FIGURE 6. Left articulated partial pes of Prepoplanops boleadorensis (MLP 97 - XI- 3 - 1).

FIGURE 7. Comparative palatal views of the best known Planopinae. (A) Holotype of Planops martini (BM (NH) M 9204); (B) holotype of Planops magnus (AMNH PV 15346); (C) holotype of Planops longirostratus (no collection number); (D) Planops longirostratus (AMNH 9302); (E) Holotype of Prepoplanops boleadorensis (MLP 97 - XI- 3 - 1); (F) Holoype of Prepotherium filholi (no collection number).