Interrelationships of the Threskiornithidae and the phylogenetic position of the Miocene ibis ‘Plegadis’ paganus from the Saint-Gérand-le-Puy area in central France

Gerandibis gen. nov

Type species

Gerandibis pagana (Milne-Edwards 1868); type species by monotypy.

Derivation of name

From Saint-Gérand-le-Puy, genus name is feminine.

Diagnosis

Gerandibis gen. nov. is a monotypic genus for the small ibis from the Saint-Gérand-le-Puy area, characterized by a combination of the following features (see also Cheneval 1984): (1) notched interorbital region of frontal, (2) relatively large fonticuli occipitales, (3) short rostrocaudal fusion of ossa palatina in ventral view, (4) small fonticuli orbitocraniales, (5) prominent tuberculum ancorae ligamentosa on margo medialis of extremitas sternalis of coracoid, (6) dorsal facet of crista articularis sternalis of coracoid overhanging ventral facet, (7) fossa m. brachialis of humerus moderately deep, (8) marked tubercle on crista lateralis sulci patellaris of femur, (9) poorly developed epicondylus medialis of tibiotarsus, (10) trochlea metatarsi IV of tarsometatarsus extending distally past trochlea metatarsi II, (11) crista medialis hypotarsi projecting plantarly beyond crista lateralis hypotarsi, (12) prominent crista lateralis hypotarsi separated from cristae hypotarsi intermediae by well-marked sulcus for musculus flexor hallucis longus and (13) conspicuous dorsal opening of canalis interosseus distalis.

Of the above mentioned characters, (1), (2), (3), (4), (6), (7) and (9) are shared with E. ruber and species of Plegadis and (3), (6) and (7) are additionally present in T. melanopis. Features (11) and (12) distinguish this taxon from E. ruber and species of Plegadis. Gerandibis agrees with species of Plegadis and B. hagedash in character (13) and with B. hagedash, G. eremita and P. papillosa in character (5). Character (8) is autapomorphic for the fossil genus.

Gerandibis pagana (Milne-Edwards 1868)

Figure 1a, e, j, m; Fig. 2a, d, f–j, k, n–o, r.

Ibis pagana Milne-Edwards 1868: 450, pl. 69, figs 2–8; pl. 57, figs 12–90; pl. 71, figs 1–12.

Milnea gracilis Lydekker 1891: p. 72.

Eudocimus paganus (Milne-Edwards 1868): Brodkorb 1963: p. 278.

Plegadis paganus (Milne-Edwards 1868): Olson 1981: pp. 165–166, fig. 1.

Lectotype

MNHN 8693 (right tarsometatarsus), chosen by Cheneval (1984).

Diagnosis

Same as for genus; see also Cheneval (1984).

Locality and age

Saint-Gérand-le-Puy area, Early Miocene, Aquitanian, MN1–MN2 (c. 23–20.5 Ma), but see De Pietri et al. (2011a,b).

Newly referred specimens

(L, left; R, right): MNHN SG 10043 (complete cranium with caudal portion of maxilla); NMB S.G.15819, NMB S.G.20538 (R scapulae missing caudal end), NMB Au.94, NMB S.G.12684 (L scapulae missing caudal end), FSL 8331272 (R coracoid), NMB M.A.618 (R coracoid), NMB M.A.627 (L coracoid), NMB M.A.671 (R coracoid), NMB M.A.1090 (R coracoid), NMB M.A.1412 (R coracoid), NMB S.G.1217 (R coracoid), NMB S.G.17722 (R coracoid), NMB S.G.19327 (R coracoid), NMB S.G.21628 (L coracoid), NMB S.G.21889 (L coracoid), NMB S.G.6233 (partial L coracoid), NMB S.G.6236 (partial R coracoid), NMB S.G.7462 (R coracoid), NMB S.G.7727 (R coracoid), NMB Au.29 (distal R humerus), NMB M.A.1571 (L humerus), NMB M.A.1574 (proximal L humerus), NMB M.A.1587 (distal L humerus), NMB Ph.2765 (L humerus), NMB Sau.2064 (distal L humerus), NMB S.G.6622 (L humerus), NMB S.G.6623 (distal L humerus), NMB S.G.6228 (proximal L humerus), NMB S.G.20625 (distal L humerus), NMB S.G.21926 (R humerus), NMB S.G.21948 (proximal R humerus), NMB M.A.1596 (L ulna), NMB M.A.2531 (L ulna), NMB Ph.3516 (L ulna), NMB S.G.4018 (R ulna), NMB S.G.6221 (proximal R ulna), NMB S.G.7247 (L ulna), NMB S.G.17304 (L ulna), NMB S.G.19340 (L ulna), NMB Au.1488 (distal L carpometacarpus), NMB Chr. 421 (L carpometacarpus), NMB S.G.23395 (R carpometacarpus), NMB S.G.12019 (R femur), NMB S.G.17461 (L femur), NMB S.G.17196 (L femur), NMB Au.173 (R tibiotarsus), NMB S.G.4016 (R tibiotarsus), NMB S.G.6698 (L tibiotarsus), NMB M.A.1688 (R tibiotarsus), NMB S.G.6222 (distal R tibiotarsus), NMB Au.35 (proximal L tarsometatarsus), NMB Au.84 (proximal R tarsometatarsus), FSL 442811 (R tarsometatarsus), FSL 442813 (R tarsometatarsus), NMB Chr. 694 (distal L tarsometatarsus), NMB S.G.7397 (R tarsometatarsus), NMB S.G.17770 (L tarsometatarsus), NMB S.G.7732 (juvenile, R tarsometatarsus), NMB S.G.7395 (juvenile, L tarsometatarsus), NMB S.G.19380 (L tarsometatarsus).

Measurements

Measurements for G. pagana were provided by Milne-Edwards (I. pagana, 1867–1868, p. 459) and Cheneval (P. paganus, 1984, p. 83). For size comparisons with some extant threskiornithids, see Table 1.

Remarks

The type material of G. pagana was listed by Cheneval (1984) and Mlíkovský (2002) under P. paganus. Tibiotarsi NMB S.G.4016 and NMB S.G.6698 were incorrectly referred to the Early Miocene scolopacid bird Elorius limosoides by De Pietri and Mayr (2012), and should therefore not be considered in any future discussion regarding this taxon.

Description and comparisons

The overall appearance of the skull (Fig. 1a) is most similar to that of E. ruber (Fig. 1b). As in all threskiornithids, the interorbital region of the os frontale is wide (Fig. 1a–d), but contrary to the condition observed in most ibises (e.g. P. papillosa, Fig. 2d), the orbital rims are strongly notched in Gerandibis, Eudocimus and species of Plegadis (Fig. 1a–c). The lateral rim of the interorbital and supraorbital portions of the os frontale (Fig. 1a) are wide in G. pagana and E. ruber, whereas they are dorsoventrally compressed in species of Plegadis and most other ibises. The fonticuli orbitocraniales (Fig. 1j) are large but, unlike members of Ardeidae, the orbital foramen does not occupy most of the interorbital septum.

Only the caudal portion of the beak is preserved in MNHN SG 10043. As in E. ruber, there is a depression where the frontal meets the os nasale, a condition also absent in species of Plegadis.

Contrary to the condition present in other ibises such as Geronticus and Pseudibis, the fossae temporales are moderately deep, reaching towards the midline of the cranium (Fig. 1e). The processus postorbitales (Fig. 1e) are proportionally longer than in E. ruber and species of Plegadis (Fig. 1f,g). Unlike in these taxa, the processus paraoccipitales (Fig. 1j) are relatively short, resembling the condition in most other ibises (e.g. T. melanopis, P. papillosa).

In the occipital region, the prominentia cerebellaris does not bulge caudally as prominently as in members of Plegadis (Fig. 1c). The fonticuli occipitales are relatively large (Fig. 1e). There are well marked depressions dorsal to the crista nuchalis transversa as in E. ruber. The paired sulci venarum occipitalis externae, lateral to the foramen magnum, are wide and deep as in most threskiornithids, most closely resembling those of E. ruber.

The maxillopalatine complex (Fig. 1j) is similar to that of species of Plegadis and, contrary to the condition observed in several threskiornithids (e.g. Geronticus, Pseudibis, Lophotibis), the processus maxillopalatinus is not fused with the os palatinum (Fig. 1l). Also, the palatine bears a conspicuous concavitas palatis, which is pierced as in several threskiornithids (Fig. 1m); this condition is absent in species of Plegadis (Fig. 1n). In ventral view, the ossa palatina are fused in the midline and, unlike in several threskiornithids (e.g. Pseudibis, Geronticus, Bostrychia), this fusion is rostrocaudally short (Fig. 1m–o).

The coracoid of G. pagana (Fig. 2a) closely resembles the corresponding bone of extant members of Plegadis, Eudocimus and Theristicus. As in all threskiornithids, a foramen nervi supracoracoidei is present. In dorsal view, the shaft of the coracoid of G. pagana bears a well-defined scar that traverses the shaft diagonally in a mediolateral direction, delimiting the omal outline of the impressio m. sternocoracoidei (Fig. 2a). In some ibises (e.g. Bostrychia, Lophotibis) this scar is situated at mid-shaft, which is also the case in Gerandibis; by contrast, it is situated closer to the sternal end of the coracoid in Geronticus and Pseudibis (Fig. 2c). The impressio m. sternocoracoidei is somewhat shallow. At the sternal end, the dorsal facet of the crista articularis sternalis overhangs the ventral facet, a condition that is also present in E. ruber, species of Plegadis and T. melanopis (Fig. 2b). In all other examined threskiornithids, the ventral facet of the crista articularis sternalis extends further sternally than the dorsal facet (Fig. 2c). Tuberculum ancorae ligamentosa (sensu Livezey & Zusi 2006) is well developed and angular in G. pagana (Fig. 2a); as such, it is also present in several other ibises such as Bostrychia, Pseudibis and Geronticus (Fig. 2c).

The scapula of G. pagana (Fig. 2d) does not differ markedly from the corresponding bone of other threskiornithids. There is a relatively deep depression on the medial side of the acromion, as in Eudocimus and species of Plegadis, which is absent in members of Platalea (Fig. 2e) and T. aethiopicus.

The proximal end of the humerus shows little variation within Threskiornithidae; the pneumatic foramen is relatively small as in most threskiornithids (Fig. 2g), although there are some exceptions (e.g. G. eremita). The distal end is a very close match to that of E. ruber, bearing a deeper fossa m. brachialis (Fig. 2f) compared with that of some other ibises (e.g. Plegadis).

As with the humerus, the overall appearance of the ulna (Fig. 2h) does not seem to vary much within threskiornithids. Unlike in T. aethiopicus and species of Platalea, a noticeable, albeit shallow, round depressio radialis, proximal to the sulcus intercondylaris, is present. The carpometacarpus of members of Threskiornithidae differs primarily in proportions among taxa. Unlike the carpometacarpus of Gerandibis, the corresponding bone of species of Plegadis is very long and slender; in Gerandibis and most other threskiornithids the os metacarpale majus is proportionally short.

The femur of Gerandibis (Fig 2i–j) is similar to that of other ibises. There is, however, a well-marked tubercle on the crista lateralis sulci patellaris, proximal to the sulcus patellaris, which is also present in A. cinerea. As in all other ibises, the tuberculum m. gastrocnemialis is well developed.

The proportions of the tibiotarsus and tarsometatarsus of G. pagana closely resemble those observed in E. ruber and T. melanopis, with the tarsometatarsus being about two-thirds the length of the tibiotarsus. These elements are more slender in Gerandibis. Although leg elements are also very long and slender in species of Plegadis (Fig. 2m, p), the ratio of the tarsometatarsus to tibiotarsus is much higher. As observed by Roux (2002), a proportionally much shorter tarsometatarsus is present in Geronticus, Pseudibis, Lophotibis and Bostrychia, a condition that may well have been plesiomorphic for Threskiornithidae, as evidenced in the stem ibis Rhynchaeites (e.g. Mayr 2002a, 2009, Mayr & Bertelli 2011). A record of a putative stem ibis from the Early Eocene of the Green River formation suggests that proportionally longer limb elements than in Rhynchaeites may have been present in the specimen (Smith et al. 2013). Within Threskiornithidae, a proportionally short tarsometatarsus is also present in some New World taxa not available for this study, such as in the Green Ibis Mesembrinibis cayennensis and in the Bare-faced Ibis Phimosus infuscatus (Miller & Bowman 1956).

As already indicated, tibiotarsi NMB S.G.4016 and NMB S.G.698 were erroneously attributed to the scolopacid bird E. limosoides from the Early Miocene of Saint-Gérand-le-Puy by De Pietri and Mayr (2012). Although superficially similar to the corresponding bone of the Black-tailed Godwit Limosa limosa, contrary to the condition in most charadriiforms, the medial and lateral condyles are approximately the same size. Additionally, a relatively well-marked intercondylar prominence is usually present in Threskiornithidae (Fig. 2m) but is absent in the scolopacid L. limosa. This feature was not visible in the tibiotarsi described by De Pietri and Mayr (2012), but the examination of further material has corroborated that these tibiotarsi can only be attributed to G. pagana. As in extant Threskiornithidae, the sulcus extensorius is situated medially (Fig. 2k). Gerandibis further resembles several other ibises in the absence of a well-marked epicondylus medialis, a condition present in P. papillosa and G. eremita (Fig. 2l).

The proportions of the tarsometatarsus of G. pagana (Fig. 2n–o), with a proportionally long shaft, are similar to those of most threskiornithids, although it is not as slender as the tarsometatarsus of species of Plegadis. In other ibises such as Pseudibis and Geronticus, the tarsometatarsus is proportionally short and stout (Fig. 2q).

The overall morphology of the hypotarsus of Gerandibis (Fig. 2r) is similar to that of other ibises (except Platalea and Threskiornis, in which there is a single wide and fully open sulcus hypotarsi). Contrary to the condition in Plegadis and Eudocimus, the crista medialis hypotarsi protrudes farther plantarly than the crista lateralis hypotarsi (Fig. 2r–v). This feature was also noticed by Cheneval (1984), and this configuration is also found in Threskiornis, Geronticus, Theristicus and Pseudibis, among other taxa. The crista lateralis hypotarsi does not extend plantarly past the cristae hypotarsi intermediae and, as in most threskiornithids, the sulcus for musculus flexor hallucis longus is apparent (Fig. 2u). Contrary to this condition, the hypotarsus of Eudocimus and species of Plegadis is much narrower than in all other ibises, and the crista lateralis hypotarsi has been greatly reduced, resulting in a very shallow sulcus for musculus flexor hallucis longus (Fig. 2s, u). As in all Threskiornithidae, there is a well-marked tubercle lateral to the cotyla lateralis (Fig. 2t), and the tuberculum m. fibularis brevis is well developed.

At the distal end, there is a conspicuous dorsal opening of canalis interosseus distalis (intertrochlear foramen), distal to the foramen vasculare distale (Fig. 2n), which had already been reported by Olson (1981) and Cheneval (1984). This condition appears to be variable within specimens of G. pagana, as it was not apparent in all specimens. Contrary to what was reported by Olson (1981), the presence of a conspicuous dorsal opening of canalis interosseus distalis is not, within Threskiornithidae, only present in the extant genus Plegadis (Fig. 2p), but a similar configuration to that of P. ridgwayi was found in the examined specimen of B. hagedash, and in some individuals of P. leucorodia (see Harrison 1986 for a thorough discussion of this character).

Fossa metatarsi I (Fig. 2o) is well marked in G. pagana. In several ibises there is a distinct tubercle distal to the fossa; this is also the case in the fossil but not in species of Plegadis. Trochlea metatarsi II (Fig. 2n) is more medially orientated than in species of Plegadis (Fig. 2p); and trochlea metatarsi IV projects farther distally than trochlea metatarsi II. Judging by published photographs (e.g. Mayr & Bertelli 2011, p. 234) in the Eocene stem ibis Rhynchaeites trochlea metatarsi II projects farther distally than trochlea metatarsi IV, which is also the case in all examined ibises with the exception of Gerandibis, Eudocimus and species of Plegadis.