Order Xenarthra Cope, 1889

Suborder Tardigrada Latham and Davis in Forster, 1795 = Phyllophaga Owen, 1842

Family Megalonychidae Gervais, 1855

Genus Ahytherium gen. nov.

Synonymy: (?) Iporangabradyx Santos, 1991 Nomen nudum.

Generic diagnosis: as for type species by monotypy.

Species Ahytherium aureum, gen. and sp. nov. (Figs 2–4, Tables 1–3) = (?) Iporangabradyx collecti Santos, 1991 Nomen nudum.

Derivatio nominis: Ahy, from old Tupi-Guarani, a group of languages spoken by Brazilian indigenous peoples, meaning sloth; therium from the ancient Greek, meaning savage beast; and aureum, from the Latin, meaning golden.

In 2008, the Pontifícia Universidade Católica de Minas Gerais (PUC Minas) celebrates 50 years of dedicated activity and service to society. The specific epithet aureum pays homage to the University's golden jubilee and acknowledges the efforts of its professors, staff, and alumni in creating this rich history.

Holotype: nearly complete skull and mandible, MCL 22834 (Figs 2–3, Tables 1–3).

Referred material: MCL 22875, which includes several postcranial elements (e.g. a vertebrae, femur, tibia, fibula, and pes) (Fig. 4, Tables 2–3).

Type locality and age: Quaternary deposits (Lujanian SALMA, Poço Azul, municipality of Nova Redenção, Bahia State, Brazil, Fig. 1).

Diagnosis: more gracile and smaller than Megalonyx jeffersonii. Rostral part of skull markedly inflated dorsally and width between zygomatic processes of frontal greater than between paraoccipital processes. External choana wider than high. Lacrimals oriented anterolaterally and narrow. Maxillar zygomatic process located dorsal to M1. Length between C1 and M4 nearly equal to length between M4 and occipital condyles. Margin of dentary ventral to molariforms wide and markedly convex. Mandibular symphysis anteriorly short and narrow, and with concave ventral margin. C1s narrow, oval in section, and larger than similarly shaped c1s. Scapular spine subdivides lateral scapular surface into approximately equal halves. Humerus with prominent greater and lesser tubercles, the former more proximal than the latter and nearly reaching the level of the head. Humeral diaphysis approximately cylindrical, with prominent anterior crests and deltopectoral shelf, and transversely expanded at its distal third. Proximal epiphysis wider than distal articular surfaces. Ulna with short olecranon process, mediolateral narrow and longitudinally curved, producing concave anterior and lateral surfaces. Radius narrow and curved, anteriorly convex, and tapered lateral margin; width of anterior surface narrower at proximal third, and distal two-thirds of nearly constant width. Femur with semicircular head, with greater trochanter relatively distal in position, third trochanter relatively weak, and distal articular surfaces not contiguous. Tibia short with anterolateral margin strongly tapered. Astragalus with separated ectal and sustentacular surfaces. Calcaneum with tuber calcis strongly compressed. Pedal digits 1 and 5 rudimentary, 2–4 narrow and ungual bearing, and 3 larger than 2.

The well-preserved skull MCL 22834/01 is missing small parts of the mesial alveolar walls of the caniniforms, left zygomatic arch, and the pterygoid blades (Fig. 2A and C). The degree of sutural fusion indicates an adult individual. In dorsal view, the rugose, posteriorly slanting occiput is convex on either side of the prominent external occipital crest (Fig. 2B), which continues from the sagittal crest but does not reach the margin of the foramen magnum. The nucchal crest projects more markedly than the sagittal crest (Fig. 2B). The latter subdivides near the frontoparietal suture into the weakly developed temporal lines. Each curves anterolaterally to reach the postorbital process of the frontal, which bears a pronounced eminence (Fig. 2A). The width across this region is greater than between the paraoccipital processes. The rostrum is short and wide, with the preorbital constriction wider than the postorbital constriction (Fig. 2A–C, Table 1).

In lateral view, the dorsal border outlines a gentle arc between the nucchal crest and frontal eminence, but is depressed at the nasals (Fig. 2A). The brevity of the rostrum is clearly appreciated in this view. The large, nearly hemispherical zygomatic process of the frontal projects abruptly laterally, and the narrow, blade-like lacrimal is oriented anterolaterally.

The foramina in the sphenoidal region and temporal fossa are well preserved (Fig. 3). A small, possibly alveolar, foramen lies posteroventral to the posterior opening of the infraorbital canal. As in most xenarthrans, a single depression contains the openings of the optic canal and orbital fissure ventrally on the orbital wall. The foramen rotundum also lies in the depression, but is more ventrally located, as is the foramen ovale (Fig. 3). The distance between the orbital fissure and foramen rotundum is about equal to the distance between the latter and the foramen ovale.

The right zygomatic arch is well preserved (Fig. 2A–C). The short and robust zygomatic process of the squamosal is nearly parallel to the dorsal skull border (Fig. 2A–B). It is not fused anteriorly with the squamosal process of the jugal bone. The orbital portion is rounded and ventrally demarcates the orbit and bears two processes that are about equally developed, one projecting posterodorsally and the other posteroventrally.

In ventral view, the transverse plane through the distal molariforms divides the skull into approximately equal halves (Fig. 2C). The palate is narrow between the molariforms, but expands markedly anteriorly to the M1s towards the C1s, these two teeth being separated by a diastema that is just longer (40 mm) than half the length of the molariform series (72 mm) (Table 1). The anterior margin of the palate is semicircular, with a concavity oriented posteriorly. The internal choana is slightly higher (50 mm) than wide (40 mm). The basicranium, including the basioccipital, petrosal, tympanics, and mastoid, is illustrated and labeled in Fig. 3. The ectotympanic is oval and projects slightly more ventrally than the entotympanic (Fig. 3).

In anterior view the external choana is considerably wider (105 mm) than high (85 mm), and the maxillae extend farther anteriorly than the nasals. The lacrimals, already noted as narrow, expand anterolaterally (152 mm) to project farther laterally than the processes of the frontals, which bear at this point the marked eminence described above (145 mm).

In addition to the mandible of the holotype (MCL 22834, Fig. 2A and D–E, Table 1, Table 2 and Table 3) are five other specimens that are reasonably well preserved. The symphyseal region anterior to the caniniforms is preserved only in the holotype, which lacks, however, the coronoid process and a small part of the right angular process (Fig. 2D–E). The coronoids, though, are preserved almost entirely in MCL 22866. In dorsal view, the symphyseal region is a narrow trough, semicircular in section, that extends anteriorly to the caniniforms as a triangular spout and is approximately double the length of the diastema between c1 and m1 (Fig. 2D, Table 1). The posteroventral symphyseal margin lies at or slightly anterior to m2. The tooth rows between the molariforms are nearly parallel, with the caniniform on each side situated slightly vestibularly to the row (Fig. 2D). The angle formed between the horizontal rami of the dentaries at the symphysis is 45° (Fig. 2A and E). The distance between the condyles (95 mm) is nearly equal to the length of the tooth rows (c1–m3) (Fig. 2D and E, Table 1).

In lateral view, the symphyseal spout projects anterodorsally, with a concave ventral margin (Fig. 2A and E). The ventral surface of the horizontal ramus is transversely wide and convex. The base of the coronoid process bears a deep masseteric fossa. The pterygoid fossa, on the medial surface of the angular process, is well demarcated and rugose. The coronoid process extends farther dorsally than the mandibular condyle and is considerably raised relative to the occlusal surfaces of the lower teeth and of the symphysis (Fig. 2A and E).

The upper dentition is known from numerous implanted or isolated specimens that allow characterization of all the teeth and an appreciation of intraspecific variation (Table 1). Only the right C1 is missing from the holotype (Fig. 2C). C1 is curved in lateral view, oval in section, concave distally, with an obliquely worn apical surface, and occludes mesially to c1 (Fig. 2A). The trapezoidal M1 and subtriangular M2–M3 have longer lingual than vestibular surfaces. The suboval M4, the smallest molariform, has a longer vestibular than lingual surface (Fig. 2C). Implanted in the maxilla, the molariforms outline a gentle, lingually convex arch.

The lower teeth are also well known. Several mandibles (MCL 22869, MCL 22872, and MCL 22873) nearly or completely preserve the dentition. Isolated teeth are also preserved. The holotype MCL 22834 has only the right m2 and left m3 implanted (Fig. 2A and D–E, Table 1). Like its counterpart in the upper jaw, c1 is caniniform, but smaller and its oblique apical wear facet faces mesially. In section m1 is triangular, with a shorter lingual margin as base and apex vestibularly, m2 is rectangular, with a shorter lingual margin, and is the smallest molariform, and m3 is suboval (Fig. 2D and E, Table 1).

MCL 22875 preserves most of the vertebrae (the last few caudal vertebrae are missing) and the pelvic girdle. There are 16 thoracic, 4 sacral and, probably, 23–25 caudal (17 are actually preserved) vertebrae.

On the lateral surface of the scapula, the scapular spine is well developed and widens anteriorly into the acromial process. The acromial arch, the bony bridge between the acromial and coracoid processes typical of tardigrades, is not preserved in any of the five scapulae available for study. The posterior margin bears a well-developed crest that demarcates the subscapular fossa posteriorly. The glenoid fossa is anteroposteriorly suboval, with a more extensive posterior rather than anterior margin. The coracoscapular foramen lies midway between the proximal margin of the base of the scapular spine and the anterior margin of the glenoid fossa.

The humerus is represented by two complete elements and more than 13 specimens in various states of preservation (Fig. 4A). The diaphysis is elongated and tubular, except at its distal third, where it widens transversely and is flattened (Table 2). The greater tuberosity, larger and more proximally projecting than the lesser tuberosity, nearly reaches the level of the head. The anterior surface of the diaphysis bears a prominent deltopectoral shelf (De Iuliis [3] provides a detailed discussion of this structure in Megatheriinae ground sloths) that is widest at about the humeral midlength due mainly to the marked lateral development of the deltoid crest, which forms the shelf's lateral margin. The medial margin is formed by the pectoral crest, which arises proximally just distal to the bicipital groove, as occurs also in Megalonyx jeffersonii [26]. The pectoral crest extends distally and curves slightly medially, contributing to the width of the shelf (Fig. 4A). The crests, both rugose in their distal portions, converge and meet distally. A third, much shorter and nearly median, but markedly developed crest, probably for insertion of the brachioradialis or cephalohumeralis muscles, is present on the shelf where the pectoral and deltoid crests converge. A prominent notch on the lateral surface of the humerus, just distal to the most lateral excursion of the deltoid crest, continues distally into the gently rounded supinator ridge. Mediolaterally, the humerus bears a single entepicondylar foramen in seven of the eight preserved humeri. In the eighth, MCL 23004 (Fig. 4A), a partition divides the opening into separate foramina, an example of intraspecific variation, perhaps isolating the passages of the brachial artery and median nerve. Two humeri preserve the distal articular surface. The capitulum, for the radius, is anteroposteriorly more extensive than the trochlea, for the ulna.

In the radius, out of which three are preserved, (Fig. 4B, Table 2), the neck is relatively indistinct, with a weakly projecting bicipital tuberosity. From the tuberosity, the diaphysis expands slightly and deflects slightly medially, with the rest of the diaphysis being of nearly constant width. The lateral diaphyseal margin is thicker than the sharp-edged medial margin, which is slightly curved longitudinally and slightly concave posteriorly and medially.

Seven incomplete ulnae are preserved. The diaphysis of the ulna is markedly bowed laterally and of nearly constant thickness throughout most of its length, narrowing slightly at its distal quarter (Fig. 4C, Table 2). The olecranon process is short and the trochlear notch is widely open.

Preserved elements of the manus include three scaphoids, a lunar, trapezoid, magnum, unciform, two fused trapezoid-metacarpal I elements, two metacarpals II, III and IV, and a metacarpal V. The morphology and relationships of these elements are as in other sloths.

The femur, known from six specimens (Fig. 4D, Table 3), is relatively elongated and narrow, with a nearly parallelogram outline in anterior view, resembling in these aspects that of Megalonyx jeffersonii. The hemispherical head does not have a well-defined fovea. The neck is relatively elongated, but indistinct. The greater trochanter, while well developed and rugose, does not project proximally, in contrast to the condition typical of other sloths. Rather, the proximolateral margin of the femur is gently rounded. The lesser trochanter is distinct, but relatively small. The third trochanter is weakly developed, barely projecting laterally from the midlength of the diaphyseal surface (Fig. 4D). The ect- and entepicondyles are pointed and prominent. Distally, the medial and lateral articular facets are not contiguous with the patellar trochlea.

The patella (of which four are known) is flattened, extended distally, and narrow. The height and width of its articular facet for the femur are about equal and in this region the patella is thicker and wider.

The tibia, known from at least four well-preserved specimens, has a narrow-edged and strongly concave lateral border (Fig. 4E, Table 3). The medial border is thicker and not as markedly concave. Its proximal articular surface is about equal in width and depth. The anterior diaphyseal surface is wide and flattened and bears the anteriorly oriented tibial tuberosity. The distal articular surface for the astragalus is shallowly concave and contiguous laterally with that for the fibula.

The fibula is known from a single specimen, unfortunately missing its proximal articular portion. Its distal portion is robust. The diaphysis is more flattened proximally than distally, where the element expands anteroposteriorly and, more markedly, mediolaterally.

Nearly all the right and left pedal elements of one individual (MCL 22875, Table 3) are known, as well as isolated elements from other individuals.

The strongly compressed, axe-shaped calcaneal tuber is thinner plantarly than dorsally (Fig. 4F). Its articular facets, the cuboidal, ectal and sustentacular, are separated. The latter is the smallest and most distally projecting. A prominent, well-defined crest extends from the medial border of the cuboidal facet to the calcaneum's plantar margin.

The tibial facet of the astragalus is pulley-shaped, but with a relatively shallow groove so that the surface is flattened (Fig. 4G). The surface lateral to this portion is occupied by the fibular facet. An articular surface lies anteriorly and medioplantarly and consists of a circular, deeply concave portion medially and a rounded convex portion laterally, also present in most fossil sloths (Fig. 4G). The concavity and the medial part of the convexity articulate with the navicular; the lateral part of the convexity articulates with the cuboid. Distally the concave ectal and nearly circular and flat sustentacular facets are separated, with the former about three times the latter in area.

The cuboid, navicular, ectocuneiform, mesocuneiform, metatarsals and digits II–IV are known completely, and follow the general pattern present in other sloths, with digit V reduced and digits II–IV, at least, bearing three phalanges, the most distal being unguals. The ento- and mesocueiforms, and metatarsal I and its proximal phalanx are co-ossified. The ungual phalanges are narrow and strongly curved, as also occurs in the homologous elements of the manus.

During the 1970s, a reasonably complete but deformed megalonychid skull was discovered in a limestone cave in Iporanga (SP), Brazil. This specimen, MZ-USP 06, was analyzed by C. de Paula Couto in an article that was written in 1980, but never published [5]. In this work, the noted Brazilian paleontologist, following the work of Hoffstetter [8] outlined above, tentatively attributed this skull to the species ‘Ocnopus gracilis’ given that, again deferring to Hoffstetter's opinion, this species was based on postcranial remains, but its skull was not known. It was, therefore, not unreasonable to attribute the Iporanga skull to the headless ‘megalonychid’ skeleton.

Dos Santos [5] restudied the skull as part of his Masters dissertation. Doubtful about assigning it to the species of intertropical Pleistocene Megalonychidae then known, “X. cearensis” and “O. gracilis,” which he considered valid, Dos Santos [5] rejected Paula Couto's proposal and chose instead to erect a new species, “Iporangabradys collecti”. In the light of the above-noted conclusions [2] and [4] regarding the two purported megalonychid species, Dos Santos [5] bold decision has turned out to be correct, not because his doubt was justified, but because “X. cearensis” and “O. gracilis” turned out to be invalid.

A Masters dissertation is neither appropriate nor valid for formal erection of a new taxon [9]. Dos Santos’ [5] dissertation was produced only in fulfillment of his Masters degree and never formally published, so that the name “Iporangabradys collecti” is rendered invalid as a nomen nudum, in accordance with ICZN [9].

MZ-USP 06 is tentatively assigned to the new taxon described here, but there is doubt to this assignment, as the skull is not well preserved, particularly anteriorly and ventrally on the basicranium, two especially diagnostic regions. The present authors are in the process of analyzing another new megalonychid species from intertropical Brazil that differs mainly in these regions from that under consideration here. MZ-USP 06 may belong to this second new species.

The skull of Ahytherium aureum (Fig. 2A–C) clearly differs from that of Megalonyx jeffersonii (Desmarest, 1822), described by various authors [11], [12], [13] and [26]. The skull is relatively narrower and elongated, particularly in dorsal view, in Megalonyx. In A. aureum the rostrum is higher, wider and shorter, the ascending ramus of the jugal projects more prominently, the zygomatic process of the squamosal is more horizontally oriented, and the occiput is more oblique, so that the occipital condyles project more posteriorly. The temporal lines are more widely divergent and strongly curved, and the frontal eminences much more developed. The diastema between C1 and M1 is relatively shorter, so that the rostrum projects less markedly anteriorly to the molariforms. In the upper dentition, the caniniform of M. jeffersonii is relatively elongated mesiodistally and more nearly triangular, and the last two teeth are more rectangular. In M. jeffersonii the dentaries are more nearly parallel, the mandibular spout shorter and narrower, the condyles narrower (Fig. 2C–E, Table 1). The humeral diaphysis of A. aureum is markedly more gracile, the deltopectoral shelf less pronounced, and the distance across the distal epicondyles shorter. The femur of M. jeffersonii is more robust and quadrangular in outline, due mainly to a relatively greater proximal width and wider diaphysis, and the greater trochanter projects more proximally. The skull of Pliomorphus ameghinoi Kraglievich, 1923 is also lower and more elongated than that of A. aureum, including a longer, more slender rostrum. The occiput is oblique, but the external occipital crest reaches the margin of the foramen magnum. The femur of Pliomorphus mutilatus Ameghino, 1885 differs in having a proximally projecting greater trochanter, a more pronounced third trochanter, and the lateral tibial condyle and patellar trochlea connected by a narrow isthmus. Amphiocnus paranense Kraglievich, 1922 differs in having both tibial articular surfaces of the femur contiguous with the patellar trochlea.

The Caribbean megalonychids are also clearly distinct from A. aureum. In Megalocnus rodens Leidy, 1868, the skull is considerably lower, narrower, and more elongated, particularly in the rostrum, which is slender and drawn anteriorly with a notable diastema into a rodentiform morphology, a characterization reflected in a first tooth that is incisiform rather than caniniform. The humeral deltopectoral shelf is prominent and laterally projecting, and the femur is markedly expanded proximally. Similarly, the skull of Mesocnus browni Matthew, 1931 has a slender, elongated rostrum with a long diastema. Further, the base of the rostrum is strongly constricted, and the humerus has a markedly developed and laterally projecting deltopectoral shelf and nearly equally developed greater and lesser tubercles.

In describing the species Xenocnus cearensis, Paula Couto [21] created the new subfamily Xenocninae, which he considered necessary given the aberrant astragalus of X. cearensis. The species was based on isolated and fragmentary remains from Itapipoca (Ceará, Brazil), but diagnosed entirely on MNRJ 2851-V, the holotype, which Paula Couto [21] identified as an astragalus. Three incomplete molariforms, a highly worn symphyseal fragment, and a cuboid were assigned to X. cearensis. These specimens, included in the hypodigm, are not diagnostic.

Direct comparison of the holotype (MNRJ 2851-V) with several unciform specimens of the megatheriine Eremotherium laurillardi, the Panamerican giant ground sloth, leaves no doubt that MNRJ 2851-V is an unciform of this species rather than the astragalus of a new and aberrant megalonychid. The largely incomplete molariforms assigned to X. cearensis may be examples of M5 of E. laurillardi. The fragmentary symphysis (MNRJ 3383-V) is not megalonychid: the alveoli of the caniniform teeth converge basally to lie adjacent to one another, a feature that does not occur in megalonychids, and a symphyseal spout is not present. The size and morphology of the cuboid (which similarly lacks megalonychid features) suggest mylodontid affinities.

1.

Identification of the megalonychid skull MZ-USP 06 from Iporanga, São Paulo, as ‘Ocnopus gracilis’ by Paula Couto (in [24]) and Dos Santos as ‘Iporangabradyx collecti’ is incorrect, for reasons given in the above text.