Systematic palaeontology

Anura Fischer von Waldheim, 181325

Neobatrachia Reig, 195826

Australobatrachia Frost et al., 200627

Calyptocephalellidae Reig, 196028

Calyptocephalella Strand, 192829

Calyptocephalella sp.

Figures 2 and 3

Figure 2 Ilium (NRM-PZ B282) of Calyptocephalella sp. from Seymour Island, Antarctica. Ilium in lateral (a), medial (b), ventral (c) and dorsal (d) views. Magnified region of the dorsal protuberance in lateral (e – dashed rectangle in orange color) and dorsal (f – dashed rectangle in yellow color) view. The dashed line in black on (a) indicates the probable outline of the posterior extension of the ventral acetabular expansion. The asterisk (*) on (a,c) indicates the shallow and broad depression of the ventral acetabular expansion. The double asterisk (**) indicates the notch caudally from the dorsal protuberance. The dashed red lines on (e,f) outline the intact bone surface. Abbreviations: ac, acetabulum; ar, acetabular rim; dae, dorsal acetabular expansion; is, iliac shaft; paf, preacetabular fossa; spf, supraacetabular fossa; vae, ventral acetabular expansion; vd, ventral depression. Full size image

Figure 3 Skull bone fragment (NRM-PZ B281) of Calyptocephalella sp. from Seymour Island, Antarctica in dorsal (a), ventral (b) and lateral (c) views. Full size image

Referred specimens

Swedish Museum of Natural History NRM-PZ B282, right ilium (Fig. 2), NRM-PZ B281, skull bone (Fig. 3).

Locality, horizon and age

IAA 2/95, Marsupial site, Seymour Island, Antarctic Peninsula (64°13′58″S; 56°39′06″W). ‘Natica horizon’ within the Cucullaea I Allomember (Telm 5) of the La Meseta Formation, Bartonian (40 Ma), Eocene23,24 (Fig. 1).

Measurements

The preserved part of the ilium measures 3.9 mm in length, the distance from the tip of the dorsal acetabular expansion to the (preserved) tip of the ventral acetabular expansion measures 3.3 mm, the highest height of the acetabular fossa equals 2.5 mm. The skull bone measures 2.7 mm at its both broadest and longest parts.

Ilium

The fragmentary right ilium (NRM-PZ B282) lacks the caudal portion of the acetabulum and most of the iliac shaft. The dorsal acetabular expansion has a smooth lateral surface and is higher than the preserved part of the ventral acetabular expansion (Fig. 2a). A large and deep supraacetabular fossa is present at its base (Fig. 2a,d). The preserved portion of the acetabulum is concave and its shape allows concluding a (semi-)circular outline. The acetabular rim is most prominent at its anterior part (Fig. 2a,c). The barely-developed ventral acetabular expansion projects ventrally. The posterior-most portion of the ventral acetabular expansion is broken off. However, the anterior portion of the ventral acetabular expansion is higher than the preserved posterior portion. In ventral view (Fig. 2a,c), the lateral surface of the ventral acetabular expansion is convex. The ventral acetabular expansion possesses a shallow and broad depression. In the preacetabular zone, a small and shallow preacetabular fossa is present (Fig. 2c). The preserved portion of the iliac shaft is damaged and precludes a confident statement whether the dorsal protuberance is present or absent. A narrow and shallow longitudinal groove is observable in the lateral surface of the iliac shaft, which probably corresponds to the posterior extension of the ventral depression (sensu10) (Fig. 2a). However, intact parts of bone surface are preserved slightly ventral to the dorsal margin on both lateral and medial surfaces (Fig. 2e,f). The one on the lateral surface is a curved shallow groove and runs posteroventrally (Fig. 2e). This feature anteroventrally demarcates the slightly elevated roughened scar interpreted above as the dorsal protuberance. The area between the dorsal acetabular expansion and iliac shaft is slightly projected dorsally. This area corresponds to the position of the dorsal protuberance. In fact, no clear evidence of a dorsal protuberance can be found on the ilium, only a slightly roughened area with minimal elevation that corresponds to the dorsal protuberance and the scar for the insertion of the musculus gluteus magnus can be observed. At the caudal side of the dorsal protuberance, a distinct notch is visible (Fig. 2a) which we consider as a further evidence of our interpretation. The area corresponding to the dorsal protuberance is located anteriorly to the anterior margin of the acetabular rim. Medially, the entire surface opposing the acetabulum is lost and the area preserved more anteriorly is slightly convex medially and smooth. Anteriorly and dorsally, just adjacent to the anterior end of the dorsal protuberance a foramen is present (Fig. 2b).

The fragmentary right ilium can be referred to an anuran based on the following characters30 (the numbers before the characters correspond to the feature numbers of Appendix 1 in Gardner et al.30): 7. (semi-) circular acetabulum; 9. acetabulum with distinct margins; 10. acetabular surface concave; 13. at least dorsal acetabular expansion is strongly divergent; 18. the dorsal protuberance present. Thus, the ilium derives from a small-sized frog (3.8 ± 0.4 cm snout-vent length, see methods, Table 1). The specimen is partly eroded and rather poorly preserved; however, it can be compared with all South American and Australian frog families (Figs. 4, S1 and S2, Table S1). The families Ranidae, Bufonidae and Hylidae have not been illustrated in the present work, since their morphology is well known31 (Table S1). The comparison has been done at family level, since the ilia display diagnostic features characteristic for identification of the family (dimensions of the dorsal and ventral acetabular expansions; location of the dorsal protuberance relative to the anterior margin of the acetabular rim etc.32,33). The studied ilium (NRM-PZ B282) differs in: (1) Reduced anterior portion of the dorsal acetabular expansion from nearly all South American and Australian frog families and the genus Telmatobufo, which have moderately or strongly developed anterior portion of the dorsal acetabular expansion. Only the genus Calyptocephalella (Fig. 4c,e), the families Ranidae31, Pipidae (Fig. S1a), Rhinodermatidae (Fig. S1j), and Leptodactylidae (Fig. S2b) have similar state/morphology of this character. (2) Dorsal protuberance located either at the level of or anteriorly from the anterior margin of the acetabular rim from nearly all families, besides Brachycephalidae (Fig. S1a), Rhinodermatidae (Fig. S1j), Telmatobiidae (Fig. S1k), Hyloididae (Fig. S1m), Leptodactylidae (Fig. S2b) and the genera Calyptocephalella (Fig. 4b–e) and Telmatobufo (Fig. 4g,h). (3) Developed dorsal acetabular expansion from the families Ranidae31, Hylidae31, Bufonidae31, Myobatrachidae (Fig. 4i), Pipidae (Fig. S1i), Microhylidae (Fig. S1b), Telmatobiidae (Fig. S1k), Leptodactylidae (Fig. S2b), Allophrynidae (Fig. S2c), Centrolenidae (Fig. S2d) and the genus Telmatobufo (Fig. 4g,h). Other families have moderately or well-developed dorsal acetabular expansion, however, due to incomplete preservation of the Antarctic frog remain any further comparison is impossible. (4) Weakly developed dorsal protuberance and lack of dorsal tubercle from nearly all families (e.g. Limnodynastidae, Fig. 4j), besides Calyptocephalellidae (Fig. 4b–e,g,h), Myobatrachidae (Fig. 4i), Craugastoridae (Fig. S1e), and Dendrabatidae (Fig. S2e).

Table 1 Measurements of the snout-vent length (SVL) and height of the transition from the iliac shaft and ilial body (HT) of some Calyptocephalella spp. and Antarctic ilia (Fig. S3), with the value of the ratio between HT and SVL (RHS = HT/SVL*100%) with the value of the standard deviation (SD). Full size table

Figure 4 3D models of some skeletal elements of Australobatrachia. (a,f) skull and (b–e,g–j) ilia of Calyptocephalella gayi (a–e); Telmatobufo venustus (f–h); Myobatrachus gouldii (i); Limnodynastes convexiusculus (j). Collection numbers of each specimen are listed in Table S1. Full size image

Among the compared forms, only the South American endemic genus Calyptocephalella resembles all mentioned four characters. In addition to this, a shallow and broad depression on the anterior portion of the ventral acetabular expansion is a unique character observable on our ilium (NRM-PZ B282) and Recent Calyptocephalella gayi (Fig. 4). Further, the fossil ilium displays a ventral depression on its lateral surface anteriorly to the acetabulum (Fig. 2c), a comparable structure can be observed also in the fossil species Calyptocephalella canqueli9 but not in the Recent species C. gayi (Fig. 4).

Skull element

The second bone fragment (NRM-PZ B281) is flat and slightly curved. Both sides of the bone have different structures. One surface is covered by small to large circular or reniform in outline, rather deep pits, which sink in the planar surface of the bone (Fig. 3a). The diameters of pits vary from 0.1–0.7 mm and some of them are punctured by foramina. The opposite surface of the bone is in general smooth, slightly deepened and is pierced with some foramina, some of which are preceded by a groove (Fig. 3b). One side of the fragment preserves an unbroken margin of the original bone with a distinct process that is bent and that gives the bone a curved shape (Fig. 3c). The ornamented surface of the bone projects slightly over this process. Comparable ornamentation, build of pits of different size, is found on the dorsal surfaces of different cranial and postcranial bones of amphibians and reptiles34,35. Among them, the following groups can be excluded from consideration: (1) Albanerpetontidae (Allocaudata); albanerpetontids are a primary Laurasian lissamphibian group with a single occurrence in Northern Africa. So far no evidence of a Gondwanan radiation of albanerpetontids exists36. In addition to this, all their ornamented bones (e.g. frontal, premaxillae)37 do not resemble the bone described here. (2) Caudata; salamanders are also considered as a Laurasian group, with a number of occurrences in Africa which need critical revision38. In salamanders, ornamented bones are found both among skull bones and on vertebrae (on plates located on the tip of the neural arch)39. Bone ornamentation here (e.g. Tylototriton, Chelotriton, Echinotriton39,40) is represented by a network of pits, ridges and pointy spines that do not resemble the bone described here. (3) Crocodylia; in crocodyliforms, comparable patterns of ornamentation with well-developed pits appear only with growth during later ontogenetic stages41,42,43. On one hand, the bone dimensions indicate a small-sized animal (corresponding to a juvenile crocodilian without such developed ornamentation). On the other hand, crocodylian osteoderms are flat without any processes, unlike the studied bone. (4) Testudines; shell plates of several turtles, such as Trionyx, Allaeochelys etc.35, are also covered by ornamentation. The ornamentation is characterized by larger and closely arranged pits, which are not always clearly delimited from each other (see Scheyer35: Fig. 1a). (5) Lacertilia; lizards also have skull bones and osteoderms covered with ornamentation patterns44. They all are characterized by a network of spines, grooves, ridges45 and protuberances46, which differs from the morphology on NRM-PZ B281.

The ornamentation pattern found in NRM-PZ B281 is comparable to that of some frog genera, i.e. Thaumastosaurus47, Beelzebufo48, Calyptocephalella9 and Baurubatrachus49, but only the last three genera are Gondwanan forms and, thus, considered for comparison herein. Beelzebufo is a very large form and the ornamentation pattern is present both on skull bones and on vertebrae50. Calyptocephalella9 and Baurubatrachus49 have very similar ornamentation patterns on the surfaces of hyperossified skull bones, comparable to our specimen. A recent phylogenetic analysis49 placed the Late Cretaceous Baurubatrachus within both Recent calyptocephalellid genera Calyptocephalella and Telmatobufo. Though Muzzopappa and Báez10 mention that both Calyptocephalella and Telmatobufo are characterized by a heavily ossified neurocranium, we can confirm this only for the former genus (Fig. 4a,f). Within Calyptocephalella the ornamentation pattern on skull bones is variable. In C. conquella10, it is built either by network of pits in small individuals, or tuberculated ornamentation in adults. In C. satan9 and C. casamayorensis51, ornamented skull bones are slightly larger than NRM-PZ B281 but they have a similar pattern built of pits. C. pichifleufensis48 is known by larger individuals which show similar ornamentation patterns but with larger pits. In comparison to these species, the Antarctic frog displays an ornamentation most similar to that of C. satan9 and C. casamayorensis48. Taking into account our comparison, we conclude that the ornamented bone fragment NRM-PZ B281 represents a skull bone (most probably a nasal) of a small-sized Calyptocephalella or Baurubatrachus. Given the presence of a small Calyptocephalella as indicated by the ilium in the same, only few m2 measuring outcrop, it is most likely that specimen NRM-PZ B281 belongs to the same genus. A comparable record of an ilium and ornamented bones referable to the genus Calyptocephalella has been mentioned in Báez52.