As detailed above, only two supraspecific taxa of the geranoidids from the Willwood Formation are here considered to be well established, that is, Palaeophasianus (including Geranoides) and Paragrus (possibly including Eogeranoides). An assignment of both taxa to the Gruoidea is supported by derived features of the tibiotarsus, especially a marked notch in the distal rim of the condylus medialis and a tubercle on the cranial surface of the distal tibiotarsus, laterodistal of the pons supratendineus (Fig. 2j–t), which serves as a support for the eminentia intercotylaris of the tarsometatarsus (both features occur in various other extant taxa and the latter may be functionally correlated with a very long tarsometatarsus). At least Palaeophasianus furthermore agrees with taxa of the Gruoidea in a derived morphology of the proximal end of the tarsometatarsus, which bears a proximodistally long hypotarsus with a well-developed central crest that separates the tendon of musculus flexor hallucis longus from that of musculus flexor digitorum longus (Fig. 2a–i; the hypotarsus of Paragrus is unknown).
Palaeophasianus is well characterized by the derived morphology of the distal end of its tarsometatarsus, in which the trochlea metatarsi II is short, the incisura intertrochlearis medialis very wide, and the trochlea metatarsi III laterally deflected (Fig. 1). Although the distal end of the tarsometatarsus of Paragrus resembles that of Palaeophasianus in overall shape, it lacks the aforementioned derived characteristics. So far, no apomorphies were identified that are exclusively shared by both taxa, and it is, therefore, anything but certain that Palaeophasianus and Paragrus are sister taxa.
Within Gruoidea, geranoidids are considered to be closely related to the Eogruidae, the earliest representatives of which belong to Eogrus
aeola from the Middle and Late Eocene Eocene of China, Mongolia, and Kazakhstan (Fig. 3; Wetmore 1934; Mayr 2009). Later geranoidids, that is, the taxa Sonogrus, Amphipelargus, and Urmiornis, are characterized by a progressive reduction of the trochlea metatarsi II of the tarsometatarsus, which ultimately led to the complete loss of this trochlea and, hence, the second toe. Because of this remarkable case of didactyly, Olson (1985: 153) considered geranoidids to be stem group representatives of the palaeognathous Struthioniformes, the only other taxon in which the second toe is lost. The single phylogenetic analysis in which eogruids were included, by contrast, resulted in a sister group relationship between Eogruidae and a clade including Aramidae and Gruidae, and, therefore, established the traditional assignment of these birds to the Gruoidea (Clarke et al. 2005).
Whereas close affinities between Geranoididae and Eogruidae are very likely, the exact interrelationships between both taxa are not well established. Species of the Geranoididae were not considered in the analysis of Clarke et al. (2005), and although Cracraft (1973: 108) noted that “Geranoididae and Eogruidae shared numerous features that suggest their derivation from a common ancestor,” the evidence for a sister group relationship between both taxa is actually quite limited. Most characters listed by Cracraft (1973) refer to similar overall shapes of osteological structures, and among the more specific features are a parallel orientation of the condyles of the distal tibiotarsus and a lateral placement of the hypotarsus. Newly added here is a long and slender tarsometatarsus, which occurs in at least Palaeophasianus (as evidenced by the Geranoides jepseni holotype) and Eogeranoides (both the E. campivagus holotype and AMNH 5127).
Currently, most of our knowledge about the osteology of eogruids and geranoidids is restricted to features of the major leg bones. However, in the collection of the AMNH, there is a left coracoid from the Middle Eocene type locality of Eogrus aeola, which was initially assigned to the Falconiformes by Wetmore (1934). Olson (1985: 154) was the first to propose that the specimen (AMNH 2941) actually belongs to Eogrus aeola, and this identification is followed here. As noted by Olson (1985) and Mayr (2009), this coracoid differs in several features from the corresponding bone of extant Gruoidea. In particular, the extremitas omalis is more medially inflected, the processus procoracoideus shorter, and the cotyla scapularis more excavated than in extant Gruoidea, and unlike in the latter, there is a deep fossa in the sulcus supracoracoideus, just below the cotyla scapularis (Fig. 5b). In further contrast to extant Gruoidea, the dorsal surface of the extremitas sternalis is smooth and does not exhibit marked ridge-like scars (Fig. 5). Some of these differences were taken by Olson (1985) as evidence for a possible flightlessness of Eogrus. Alternatively, however, they may be genuinely plesiomorphic, and especially the absence of ridge-like scars on the extremitas sternalis of the coracoid, the cup-like cotyla scapularis, and the less pronounced tubercle on the distal tibiotarsus support a position of Eogrus outside crown group Gruoidea. Clarke et al. (2005) identified one character that suggests a sister group relationship between Eogruidae and the clade (Aramidae + Gruidae), that is, “trochlea metatarsi II subequal to IV in distal projection, distal end reaching approximately middle of trochlea metatarsi IV” (Clarke et al. 2005: 10). Actually, however, the trochlea metatarsi II is shorter than the trochlea metatarsi IV in Eogruidae, Aramidae, and Gruidae (see Figs. 1h, i, 3h), but the polarity of this feature is uncertain, because a short trochlea metatarsi II also occurs in many Ralloidea, which are the sister taxon of the Gruoidea (e.g., Ericson et al. 2006; Prum et al. 2015), as well as in the extinct Parvigruidae, which are another group of stem group Gruoidea (Mayr 2013 and below).
The morphology of Palaeophasianus meleagroides (“Geranoides jepseni”) likewise does not support its position within crown group Gruoidea. The crista medialis of the hypotarsus of Palaeophasianus is less developed in extant Gruoidea, and unlike in the latter, the femur of geranoidids, as exemplified by AMNH 5156 (cf. Palaeophasianus) and AMNH 5127 (cf. “Eogeranoides”), lacks a well-developed crista trochanteris (Fig. 4). These plesiomorphic features suggest a position of at least Palaeophasianus outside crown group Gruoidea. The hypotarsus of Palaeophasianus meleagroides differs from that of all extant Gruoidea, with the most notable characteristics being its marked plantar prominence and the presence of a distinct sulcus for the tendon of musculus flexor hallucis longus (Fig. 2). This peculiar morphology may be an autapomorphy of P.
meleagroides related to a specialized use of the hindlimbs of this species. Indeed, the hypotarsus of AMNH 5156, the specimen that was referred to P. meleagroides by Cracraft (1969), better conforms with that of Eogrus in that it is less plantarly prominent, and the sulcus for the tendon of musculus flexor hallucis longus is much less marked (Fig. 2e, f).
The presence of a tubercle on the distal end of the tibiotarsus may indicate that geranoidids are more closely related to crown group Gruoidea than are the Parvigruidae from the Early Oligocene of Europe, which were also considered to be stem group representatives of the Gruoidea (Mayr 2013). However, before a meaningful analysis of the interrelationships of these and other Paleogene Gruoidea can be performed, it would be desirable to revisit collections of bird fossils from the Willwood Formation and to identify further skeletal elements of geranoidids. In this regard, it may be particularly rewarding to restudy the holotype of the alleged heron Calcardea junnei from Clark’s Fork Basin of the Willwood Formation (Gingerich 1987). The specimen consists of partial coracoids, fragmentary tarsometatarsi, as well as a sternum fragment, and two incomplete vertebrae. Unfortunately, no photographs of these fossils were published, but at least judging from the published drawing, the coracoid resembles the just mentioned bone that is likely to belong to Eogrus. A reexamination of the Calcardea junnei holotype may, therefore, be worthwhile and may shed further light on the osteology and affinities of geranoidids.
I would like to conclude this revision of some North American geranoidids with some notes on the possible occurrence of these birds in the early Paleogene of Europe. The avifaunas of Europe and North America were very similar in the Early Eocene, and intermittent land connections allowed even the dispersal of flightless birds (Mayr 2009). It would therefore not be unexpected to find geranoidids in the Early Eocene of Europe. Indeed, it was recently suggested that fossils from the Early Eocene of France may represent such remains (Bourdon et al. 2016). These specimens were described as Galligeranoides boriensis Bourdon et al., 2016, and the referred material includes a tibiotarsus and a tarsometatarsus. However, some differences to the North American Geranoididae were already noted by Bourdon et al., (2016), and especially in distal view, the tarsometatarsus of Galligeranoides is clearly distinguished from that of Palaeophasianus (“Geranoides”), with the mediolateral width of the trochlea metatarsi III of G. boriensis exceeding its dorsoplantar depth, but being much deeper than wide in Palaeophasianus (including “Geranoides”) and Paragrus (compare Bourdon et al. 2016: fig. 6B4 with Fig. 1j). The hypotarsus of Galligeranoides furthermore consists of a single large crest, and there are no marked sulci for the tendons of either musculus flexor digitorum longus or musculus flexor hallucis longus. The distal end of the tibiotarsus is distinguished from that of Palaeophasianus and Paragrus in that the condylus medialis does not bear a notch in its distal rim, and the pons supratendineus is much wider. Accordingly, it is here considered doubtful that Galligeranoides belongs to the Geranoididae, and it was already previously suggested that the taxon is more likely to be a representative of the palaeognathous Palaeotididae (Mayr 2015). Still, however, eogruids may well have occurred in the Early Eocene of Europe, and, at least judging from the published figures, the distal end of the tibiotarsus of Paragrus prentici is remarkably similar to that of Palaeogrus princeps from the Middle Eocene of Italy. This latter species was described by Portis (1884) and is only known from the holotype distal tibiotarsus. P. princeps is currently assigned to the Gruidae (Cracraft 1973; Mlíkovský 2002), but has not yet been compared with geranoidids. Because of its comparable age and morphological alikeness, I consider an assignment to the Geranoididae more likely than its current referral to the Gruidae.