Messelornis nearctica Hesse, 1992

Weidig, Ilka, 2010, New Birds from the Lower Eocene Green River Formation, North America, Records of the Australian Museum 62 (1), pp. 29-44 : 32-35

Messelornis nearctica Hesse, 1992

Holotype. SMF Av 406, complete skeleton.

Referred specimens. FMNH A 716, disarticulated partial skeleton lacking the skull and distal ends of the wings ( Fig. 2 ); SMNK-PAL 3810, nearly complete skeleton with feather remains ( Fig. 3 ); USNM 776273, partial skeleton lacking the skull and the left wing ( Fig. 4 ); USNM 336269, right leg with foot, parts of the pelvis and some vertebrae ( Fig. 5 ); USNM 336277, left leg with foot, slab and counterslab ( Fig. 6 ); BHI 1283, left leg, pelvis ( Fig. 7 ); BHI 1292, both legs, pelvis, some vertebrae; BHI 1294, parts of the sternum, wing ( Fig. 8 ); tentatively referred: USNM 776275, tip of bill, crushed skull, both wings, coracoid.

The new specimens exhibit the typical features of Messelornithidae ( Hesse, 1992) such as skull with nostrils extending far forward, compact, bowed ulna with short, broad, blunt olecranon, carpometacarpus with broad processus extensorius, tarsometatarsus with short, broad trochlea metatarsi III. Contrary to Hesse’s (1992) diagnosis, the trochlea metatarsi IV is longer than trochlea metatarsi II. According to Hesse (1992), M. nearctica differs from other members of the family mainly in details of the hypotarsus which are not recognizable in any of the new specimens. Additionally, the toes are longer and more slender compared to M. cristata ( Hesse, 1992) , a character which can also be seen in the new specimens. In addition to the features mentioned by Hesse, the os metacarpale minus is slightly longer than the os metacarpale majus in M. nearctica (same length in M. cristata ).

Dimensions. see Table 2 and 3.

Description. The new specimens differ considerably in size ( Table 1). This is in accordance with the type species of Messelornithidae , Messelornis cristata from Messel ( Hesse, 1988a; Hesse & Habersetzer, 1993). A detailed description of the holotype of M. nearctica is given by Hesse (1992). In the following, only new features visible on the referred specimens are mentioned.

The coracoid is short, but not as broad as in Itardiornis . The processus acrocoracoideus is hooked. The processus procoracoideus is very large as in Eurypyga . On the cranial surface two marked lines are visible. The angulus medialis is blunt and broad. A short but pronounced facies is visible dorsally of the facies articularis sternalis. The U-shaped furcula is small and rather wide; it seems to have a very small apophysis furculae. The scapula is short, but slender. It broadens in the curvature, the caudal end is pointed.

The sternum is moderately long and rather slender.A spina externa is not visible. Four incisurae intercostalis are present. The margo costalis reaches far caudad, it is nearly half as long as the whole sternum. The margo caudalis is not completely preserved in any of the specimens. Hesse (1992) stated that the taxon Eurypygoidea ( Eurypygidae + Messelornithidae ) is characterized by a “short, broad sternum, with two to three notches on each side” ( Hesse 1992:171). The new specimens show at least two incisurae, i.e. one per side. Since the middle portion of the margo caudalis is not preserved in any specimen, it is not possible to determine the exact number of incisurae. In relation to the total sternum length, the incisurae are deeper than in Eurypyga .

The humerus resembles that of Messelornis cristata . The tuberculum supracondylare ventrale is extremely pronounced and sharp. The humerus shaft is bowed. The condylus ventralis is proximo-distally narrow, and sharp. As in Itardiornis , the sulcus transversus is short, but well marked. The ulna is already described in detail in Hesse (1992). Hesse (1992: 173) stated that the “humerus (is) a little shorter than ulna”. This is not true for the new specimens, both in the FMNH PA 716 and the SMNK PAL 3810 specimen the ulna is shorter than the humerus ( Table 2). The os metacarpale minus of the carpometacarpus is straight; a very small processus intermetacarpalis is present as in Messelornis cristata and Itardiornis . In contrast to Messelornis cristata , the os metacarpale minus seems to be slightly longer than the os metacarpale majus. Hesse (1992) stated that the carpometacarpus is shorter in relation to the wing than in Messelornis cristata . However, the difference in length between the holotype of Messelornis nearctica and of Messelornis cristata is only very small (21% in Messelornis nearctica versus 22.4% in Messelornis cristata, Hesse, 1988a ). The new specimens show that this character is somewhat variable, they differ in their relative length of the carpometacarpus (up to 23.2% in the FMNH PA 716 specimen), which is not shorter than in Messelornis cristata .

The pelvis has a long processus terminalis ischii, which reaches the pubis. The foramen obturatum seems to be closed. The femur is strongly bowed. A trochlea fibularis is present with a high lateral edge. The tibiotarsus is the longest limb element. The proximal end of the tibiotarsus shows large cristae cnemiales; the crista fibularis is pronounced. A sulcus m. fibularis is not visible in any specimen, probably due to poor preservation. The trochlea cartilaginis tibialis is proximo-distally low, but caudo-cranially deep. The tarsometatarsus closely resembles that of Messelornis cristata . The hypotarsus is quite small, the number of cristae hypotarsi is not recognizable. The trochlea metatarsi III is the longest, followed by the trochlea metatarsi IV and then the trochlea metatarsi II; the difference in length between trochlea metatarsi IV and trochlea metatarsi II is less distinct than in Itardiornis . The trochlea metatarsi II is projected well plantad. The plantar surface of the trochlea metatarsi II is not flattened, but rounded as in Itardiornis ( Mourer-Chauviré, 1995) . A foramen vasculare distale is present. The toes have the usual number of phalanges. Compared to Messelornis cristata , the toes are relatively longer.

Only in the SMNK PAL 3810 specimen are several scattered feathers preserved. In all specimens, several ossified tendons are preserved. Most occur at tarsometatarsus, tibiotarsus, humerus, radius and ulna. Some tendons are scattered on the slabs, their origin is not discernible. The tendons at the tibiotarsus are split proximally.

Discussion. The fossil Messelornithidae were originally described by Hesse (1988a). Besides the type species Messelornis cristata from Messel, the Messelornithidae comprise Messelornis russelli and Itardiornis hessae from the Quercy in France ( Mourer-Chauviré, 1995), as well as Messelornis nearctica from the Green River Formation ( Hesse, 1992). Hesse (1988b) regarded the Eurypygidae to be the closest modern relative of the fossil Messelornithidae . This view was widely accepted (e.g., Cracraft, 2001; Peters, 1991) and supported by Livezey (1998) although Mourer- Chauviré (1995) noted some parallelism with Rallidae . Mayr (2004a) regarded the Messelornithidae as the sister-taxon to rails and finfoots and supported his classification with derived characters shared by the taxon ( Messelornithidae + ( Rallidae + Heliornithidae )).

The type species of the Messelornithidae , Messelornis cristata , was thoroughly described ( Hesse, 1988a,b, 1990); an extensive study of the osteology of this species was based on a total of 346 specimens. The description of the Green River species Messelornis nearctica , however, was based on a single, nearly complete, but crushed specimen ( Hesse, 1992). The new specimens described above reveal additional osteological features which have been unknown so far. In addition, they show that some details of the original description ( Hesse, 1992) are not wholly correct, because they were only based on one specimen; this mainly concerns size-related features.

Messelornis nearctica has as large a size-range as Messelornis cristata . Sexual dimorphism might be an explanation for the large size-range; it is possible to divide the ten specimens described above into two groups: four small specimens ( SMNK PAL 3810 , USNM 336273 , USNM 336275 and BHI 1294 ) and the remaining six large specimens ( Table 2). Hesse (1990) reported that, within the Gruiformes , males tend to have slightly longer limb bones than females, but the length of the limb bones of both sexes overlap considerably. Thus, the Green River specimens might represent four female individuals and six male individuals. However , in Messelornis cristata , which is known from a far larger sample, it is not possible to clearly distinguish two groups that represent the sexes ( Hesse, 1990) .

All Green River specimens seem to be adults, which is in accordance with the findings from Messel ( Hesse, 1990). As it was already proposed for Messelornis cristata ( Hesse, 1990) , this might indicate that the nests of Messelornis nearctica were not situated directly at the shore of the lake. The adult birds, however, probably spent a considerable amount of time close to the shore, considering the number of adult specimens.

Figures

Figure 2. Messelornis nearctica, FMNH A 716, scale bar 20 mm.

Figure 3. Messelornis nearctica, SMNK-PAL 3810, scale bar 20 mm.

Figure 4. Messelornis nearctica, USNM 776273, scale bar 20 mm.

Figure 5. Messelornis nearctica, USNM 336269, scale bar 20 mm.

Figure 6. Messelornis nearctica, USNM 336277, A = slab, B = counterslab, scale bar 20 mm.

Figure 7. Messelornis nearctica, BHI 1283, scale bar 20 mm.

Figure 8. Messelornis nearctica, BHI 1294, scale bar 20 mm.