Stilotherium dissimile, AMEGHINO, 1887

Abello, María Alejandra, Martin, Gabriel M. & Cardoso, Yamila, 2021, Review of the extinct ‘ shrew-opossums’ (Marsupialia: Caenolestidae), with descriptions of two new genera and three new species from the Early Miocene of southern South America, Zoological Journal of the Linnean Society 193 : -

STILOTHERIUM DISSIMILE AMEGHINO, 1887

( FIG. 6)

Stilotherium dissimile Ameghino, 1887, p. 7 .

Garzonia typica Ameghino, 1891, p. 307 .

Pichipilus exilis Ameghino, 1891, p. 307 .

Garzonia minima Ameghino, 1891, p. 308 .

Garzonia captiva Ameghino, 1891, p. 308 .

Garzonia tipica Ameghino, 1903, 167, fig. 91.

Garzonia patagonica Sinclair, 1906, p. 424, plate LXIII, figs 8, 8a, 10–13.

Stilotherium typica Reig, 1955, p. 62 .

Stilotherium typicum Reig, 1955, p. 63 .

Garzonia annectens (partim, lectotype) Ameghino, 1891, p. 307.

Halmarhiphus didelpoides (sic) (partim, paralectotype) Ameghino, 1891, p. 308.

Halmarhiphus didelphoides (partim, paralectotype) Ameghino, 1894, p. 357.

Halmariphus didelphoides (partim, paralectotype) Ameghino, 1902a, p. 424, fig. 4.

Parhalmarhiphus annectens (partim, lectotype) Ameghino, 1894, p. 357.

Phonocdromus patagonicus (partim, paralectotype) Ameghino, 1894, p. 355.

Parhalmarhiphus didelphoides (partim, paralectotype) Ameghino, 1903, p. 157 and 163, fig. 86.

Neotype of Stilotherium dissimile ( Reig, 1955, p. 62, fig. 1) MACN-A 8464, right mandible with i2, alveolus of i3, three uniradiculate teeth, and complete p2-m4 ( Reig, 1955, fig. 1; Marshall, 1980, p. 38, fig. 6; 1982, p. 68, fig. 31; Pascual & Herrera, 1975, fig. 3; see Fig. 6A, B in this work).

Holotype of Garzonia typica MACN-A 5700 , right mandible with basal i2, one a.u., one uniradiculate tooth, alveolus of?p1, complete p2-m2, and alveolus for anterior root of m3 (figured by Ameghino, 1893, fig. 5; 1894, fig. 41; 1898, fig. 51; currently the type specimen lacks the mandibular portion corresponding to the m3).

Holotype of Garzonia captiva MACN-A 5706 , right mandible with p3-m2.

Holotype of Garzonia minima MACN-A 5709 , right mandible with m2-4.

Holotype of Pichipilus exilis MACN-A 5698 , left mandible with m3, posterior alveolus of m1, and alveoli of m2 and m4.

Lectotype of Garzonia annectens MACN-A 5703 , left mandible with m1-4.

Referred material: The type specimens and the following referred material: MACN-A 5701 , left mandible with alveolus of i2, four a.u., complete p2, roots of p3, and fragmentary m1-3 ; MACN-A 5702 , right mandible with alveolus of i2, one a.u., complete?p1-2, roots of p3, complete m1-2, and alveoli of m3-4 ; MACN-A 5710 , right mandible with complete m1-3 (figured by Ameghino, 1903: figs 81, 121; 1904, fig. 31; 1906, fig. 197) ; MACN-A 5717 , left mandible with alveolus of i2, 2 a.u., alveoli of p2, complete p3, alveoli of m1, complete m2-3 and alveolus for m4 (paralectotype of Halmarhiphus didelphoides Ameghino, 1891 ) ; MACN-A 5723 , right mandible with fragmentary i2, one uniradiculate tooth, two a.u., complete?p1-2, fragmentary p3-m1, and anterior root for m2 ; MACN-A 8426 , right maxilla with M2-3 (figured by Marshall, 1980: 37, fig. 5; 1982: 66, fig. 30; Fig. 6C, E this work) ; MACN-A 8427 , anterior cranial fragment with right C, P2-3 and alveoli of P1, and left complete P2-3 (figured by Marshall, 1980: 36, fig.4; 1982: 65, fig.29) ; MACN-A 8429 , left mandible with i2, one uniradiculate tooth, alveoli of p3-m1, and complete m2-4 ; MACN-A 8431 , right isolated M1 (figured by Marshall, 1980: 37, fig.5; Fig. 6D, F this work) ; MACN-A 8447 , right mandible with m3; lot MACN-A 8449–8455 , isolated right M1 ;

O, MPEF-PV 4861, isolated left M 1 in occlusal (I), lingual (J), posterior (K) and anterior (O) views; L–N, MPEF-PV 4801, isolated right M 2 in occlusal (L), lingual (M) and posterior (N). P–R, Stilotherium cf. dissimile , isolated right m 1 in occlusal (P), labial (Q) and lingual (R). Scale bar, 1 mm.

MACN-A 8456 , left mandible with m2-4 (paralectotype of Phonocdromus patagonicus ); lot MACN-A 8458–8460 , left mandible with m1-2 ; MACN-A 8463 , left mandible with p3-m2 ; MACN-A 8465 , right mandible with p3-m4 ; MACN-A 8466 , right mandible with i2, four a.u., alveoli of p2-3, complete m1-3 and alveolus of m4 ( Fig. 6G) ; MACN-A 8467 , left mandible with i2, five a.u., alveolus of p2 and complete p3-m4 ; YPM-VPPU 15238 , left mandible with i2-3, two a.u., two uniradiculate teeth, anterior root for p2 and p3-m4, fragment of scapula, left and right humeri, left radius and left ulna ( Sinclair, 1906, plate LXIII, figs 8, 8a, 10–13; Fig. 6H) .

Te m p o r a l d i s t r i b u t i o n: L a t e E a r l y M i o c e n e, Santacrucian SALMA.

Stratigraphic and geographic distribution: Santa Cruz Formation, Santa Cruz province, Argentina. Localities and specimens: La Cueva (MACN-A 8464, MACN-A 8465, MACN-A 8466, MACN-A 8467, MACN-A 8447, lot MACN-A 8449–8455, lot MACN-A 8458–8460, MACN-A 8426, MACN-A 8427, MACN-A 8429, and MACN-A 8431); Cerro Observatorio (= Monte Observación) (MACN-A 5723, MACN-A 5698, MACN-A 5709, MACN-A 5710, MACN-A 5703, MACN-A 5701, MACN-A 5702, and MACN-A 8456); Río Chalia (= Sehuen) (MACN-A 5700 and MACN-A 5717); San Jorge (MACN-A 8463). Specimen YPM-VPPU 15238, was recovered by O. A. Peterson in 1986, five miles South of Coy Inlet.

Measurements: Supporting Information, File S6, Tables S1a, S 2.

New diagnosis: Stilotherium dissimile is distinguished from Stilotherium parvum by (1) its larger size, (2) smaller retromolar foramen, (3) p2 longer than p3, (4) absence of postcingulum on m2-3 and (5) absence of paracone on M2.

Remarks: Stilotherium dissimile was the first extinct caenolestid described, but it was originally placed as a member of the marsupial family Microbiotheriidae ( Ameghino, 1887) . The original name-bearing specimen of S. dissimile was lost ( Chornogubsky et al., 2019). As noted by Marshall (1980: 36–38; see also Fernicola, 2011), some uncertainty exists about the final repository of the fossils collected by Carlos Ameghino from the Santa Cruz Formation, which included the type specimen of S. dissimile , which were the basis for the publication made by Florentino Ameghino the same year (i.e. Ameghino, 1887). In the review of caenolestines by Reig (1955), he was unable to locate the holotype and designated a neotype for this species (MACN-A 8464). In the catalogue of the ‘Colección Ameghino’ at MACN, specimen MACN-A 5723 is labelled as the type of S. dissimile , but this material does not coincide with the original description of this species ( Ameghino, 1887: 7) and was recovered from a different locality in a fieldtrip posterior to 1887 ( Chornogubsky et al., 2019). Since there are doubts concerning MACN-A 5723 as the original type (or a neotype selected by Ameghino himself), we follow previous authors ( Reig, 1955; Marshall, 1976, 1980, 1982) in considering MACN-A 8464 as the neotype of S. dissimile .

We mostly agree with the concept of S. dissimile of Marshall (1980, 1982), but we exclude Halmarhiphus nanus Ameghino, 1891 (lectotype MACN-A 5720) as synonym of S. dissimile , because it is currently considered as Gaimanlestes ? nanus (see below). During this revision, we could not corroborate the taxonomic status of Garzonia captiva and Garzonia minima , since we were unable to locate the holotypes of each species. In the case of G. minima , the right mandible deposited under number MACN-A 5709 (type material) does not agree with the information recorded in the collection catalogue (a left mandible) nor that figured by Ameghino (1903: figs 81, 121). However, we follow Reig (1955) and Marshall (1982) who studied the type specimens of G. captiva and G. minima firsthand, and consider these species synonyms of S. dissimile . Regarding Garzonia typica, Reig (1955) proposed it should be treated as a valid species of Stilotherium , Stilotherium typicum ( Reig, 1955: 63) , with Parhalmarhiphus annectens as a synonym of the latter. These taxonomic decisions were based on differences in mandible height, deeper in G.typica than in S. dissimile . In contrast, Marshall (1980) considered there was no justification for the specific separation of these species. We agree with Marshall’s (1980) conclusions, since we observed that mandibular height is a variable character in paucituberculatan species (see also: Engelman & Croft, 2016), and because G. typica and S. dissimile are similar in their dental morphology. Specimens referred to S. dissimile show not only variation in mandibular height, but also in other characters (see below) that, as a whole, we interpret as part of intraspecific variation. Mandible height varies from shallow to deep (c.f. measurements of MACN-A 8464, MACN-A 8465, MACN-A 8466 and MACN-A 5700, MACN-A 5702 in Supporting Information, File S6, Table 1a). Deeper mandibles are additionally stouter, and have a thickened and more posteriorly developed symphysis. The number of antemolar teeth are also variable among specimens of S. dissimile . Mandibles can have four or five teeth (or alveoli for uniradiculate teeth), between the procumbent incisor and p2. In several specimens with well-preserved anterior mandibular portions (YPM-VPPU 15238, MACN-A 8464, MACN-A 8466 and MACN-A 8467; Fig. 6G, H), we could verify the staggered condition of the i3 ( Hershkovitz, 1995). In this material the tooth (or its alveolus) posterior to the first, hypertrophied incisor (i2; see: Abello, 2013) has its root lingually oriented and wedged between the i2 and the next posterior tooth (or its alveolus), which we take as evidence of its homology with the i3 of the scheme proposed by Hershkovitz (1995; Fig. 6G, H). Between i3 and p2 there are one a.u. and three uniradiculate teeth in the holotype, two a.u. and two uniradiculate teeth in YPM-VPPU 15238 and four a.u. in MACN-A 8467. Thus, and similarly to Caenolestes species ( Osgood, 1924; Martin, 2007, 2008), the antemolar dentition of S. dissimile could include seven or eight teeth. Although this variation is documented, four incisor-like teeth seem to be the most common number in this species. Other variable characters are: (1) presence (MACN-A 8464, MACN-A 8467 and MACN-A 8465) or absence (MACN-A 8429, MACN-A 8456, MACN-A 5703 and MACN-A 5709) of a talonid in m4, which when present could be uni- or bi-cuspidate; (2) length of p2, shorter in MACN-A 5701 and MACN-A 5702 than, for example, MACN-A 5700 and MACN-A 8464 (see Supporting Information, File S6, Table 1a).

Stilotherium dissimile is mostly known by dental and cranial remains, although various postcranial materials were also assigned to this species ( Sinclair, 1906; Marshall, 1980; see referred material). We could not evaluate these specimens firsthand, but included them as referred materials following Marshall (1980).

According to the revised concept of S. dissimile , and the recognition of its sister-species, S. parvum , we propose here a new diagnosis based on characters of the upper and lower dentition. In contrast to S. parvum , S. dissimile is better known as it is represented by numerous, relatively more complete remains. Current known distribution of S. dissimile includes several localities of the Late Early Miocene of Santa Cruz province, but ongoing studies on metatherian assemblages from Pinturas Formation (Late Early Miocene, Santa Cruz province) could indicate a wider geographic range for this species ( Barmak, 2019).