Diomedeoides brodkorbi ( Cheneval, 1995 )

De Pietri, Vanesa L., Berger, Jean-Pierre, Pirkenseer, Claudius, Scherler, Laureline & Mayr, Gerald, 2010, New skeleton from the early Oligocene of Germany indicates a stem-group position of diomedeoidid birds, Acta Palaeontologica Polonica 55 (1), pp. 23-34 : 26-32

Diomedeoides brodkorbi ( Cheneval, 1995)

Fig. 4.

Type material: Froidefontaine specimen, three slabs; NP 23−24, Rupelian, early Oligocene; Froidefontaine, Territoire de Belfort , France ( Cheneval 1995) .

Locality: Rheinweiler near Bad Bellingen, Baden−Württemberg, Germany. This location is situated at the eastern shoulder of the Upper Rhine Graben, which stretches approximately 300 km in a north−south axis and represents the central part of the European Continental Rift system.

Horizon: Fischschiefer, Rupelian, early Oligocene (NP 23, D14).

Material.— MHNF 30877, disarticulated partial skeleton on three slabs, lacking sternum, most wing elements from the left side, and left leg.

Measurements (unless indicated otherwise, maximum length in mm).—Skull (as preserved), 73.6; mandible, 76.6; main body of hyoid, 8.3; left coracoid; 26.8; left scapula, 32.8 (broken); right humerus, 66.8; left humerus, 66.2; right ulna, 65.5; right carpometacarpus, 38.2; right phalanx proximalis digiti majoris, 22.1; right phalanx distalis digiti majoris, 25.5; right phalanx digiti minoris, 9.8; right femur, 35.4; left second pedal phalanx of third digit, 9.5; left proximal phalanx of fourth digit, 28; left second pedal phalanx of fourth digit, 11;?left third phalanx of fourth digit, 7.5.

Description and comparisons.—In the new specimen, the skull of Diomedeoides is for the first time visible in dorsal view and presents a number of previously unknown osteological details (Fig. 5). In dorsal view, the overall shape of the skull resembles the Recent genus Pelagodroma the most.

Fig. 4. Partial disarticulated skeleton of the diomedeoidid bird Diomede− Ą oides brodkorbi ( Cheneval, 1995) , MHNF 30877 from the early Oligocene of Rheinweiler, Germany (A); partial counterslab of MHNF 30877 (B); slab showing the proximal phalanx of the fourth digit and some ribs (C).

doi:10.4202/app.2009.0069

The nasofrontal hinge area is nonetheless shorter, the proportions being like those of Puffinus . As noted by Mayr et al. (2002), the fossae glandularum nasales are narrow and shallow, showing no evident projections at their caudal end. The central part of the os frontale is thus wide and shows a shallow medial furrow. The beak, whose tip is broken in the specimen, is less curved than that of all extant procellariiforms. This can be appreciated in side view, and is also known from other diomedeoidid specimens (see figures in Cheneval 1995; Mayr et al. 2002). Both ossa lacrimalia have been lost, indicating that, in contrast to some Recent procellariids, they were not fused with the os frontale. Likewise, unfused lachrymals are present in members of the Hydrobatinae , Oceanitinae , and Diomedeidae . The processus postorbitales are large and distinct; their tips project rostro−laterally. Large postorbital processes are known for several procellariid species, albeit in these they are usually related to broader fossae glandularum nasales.

The left os pterygoideum is situated between the rami of the lower jaw and is visible in dorsal view. This bone is not preserved in any of the previously known diomedeoidid specimens and most closely resembles the pterygoid of taxa in the Hydrobatinae and Oceanitinae . The medial wing of the fossil, however, is broader than in these two taxa. Within procellariiforms, the procellariids and the pelecanoidids possess ossa pterygoidea with a rostral wing that articulates with the basipterygoid process only caudally (these processes are often vestigial; Pycraft 1899). The ossa pterygoidea are rodshaped in species in Oceanitinae , Hydrobatinae , and Diomedeidae . In these taxa they lack an articulation facet for the basipterygoid processes, which are absent in the Diomedeidae and absent or vestigial in species in Oceanitinae and Hydrobatinae ( Pycraft 1899) . The pterygoid of Diomedeoides likewise does not seem to bear any basipterygoidal facets, so that basipterygoid processes either were absent or vestigial. It should be noted that the pterygoid of the fossil specimen shows an awkward rostro−lateral projection. Upon closer inspection there is a clear separation between the main body of the pterygoid and this protuberance, which also shows rugged edges, and this protuberance may possibly be an artefact of preservation.

The quadrate (Figs. 4 and 5) does not significantly differ from that of extant procellariiforms, which is quite uniform throughout the group. The tip of the processus orbitalis is broken and most features have been poorly preserved. The caudal margin of the bone between the processus oticus and the condylus caudalis is more markedly concave in the fossil than in the examined extant species.

For the first time, the lower jaw can be fully appreciated in ventral view (Fig. 5). The rami mandibulae do not diverge as strongly as in other procellariiforms; the symphysis is of similar relative length to that of Pelagodroma , being longer than in all other extant procellariiforms examined. In lateral view, the rostrum mandibulae is straight. The anterior parts of the rami mandibulae are unusual in that they are more closely aligned than in most of the Recent procellariiforms we have examined, the exception being some Puffinus species. This condition can also be observed in the Froidefontaine specimen of D. brodkorbi described by Cheneval (1995), and a similar morphology occurs in Phaethontidae (tropicbirds; Fig. 5E) and the early Eocene Prophaethon shrubsolei Andrews, 1899 ( Prophaethontidae ; see Harrison and Walker 1977: pl. 5), as well as in some members of other avian orders (e.g., some “pelecaniforms” [pelicans and allies] and some “gruiforms” [cranes, rails, and allies]). The processus mandibulae medialis resembles that of extant procellariiforms in size, shape and orientation. The processus mandibulae lateralis likewise does not differ from that of extant species. The fossae caudales are deep and very well defined, so that there is a deep incision between the processus mandibulae lateralis and medialis. This incision is quite shallow in Oceanodroma and tends to be deeper in the procellariids. A small ossicle next to the extremitas ventralis of the scapula may represent the basihyale, but this identification needs further verification.

Ten presacral vertebrae can be counted on the slab, at least two of which are thoracic ones. Overall, they resemble those of extant procellariiforms, although the poor preservation of these elements does not allow for any sensible interpretation. On the other hand, three out of five caudal vertebrae have been nicely preserved, and do not differ from those of extant procellariiforms.

The morphology of the coracoid of the Diomedeoididae has so far been only poorly known. In the new specimen the bone is completely exposed and well preserved (Fig. 6). In overall proportions it most closely resembles that of Pterodroma . The facies articularis clavicularis is short and does not protrude far medially as in the Diomedeidae and in some procellariids (e.g., Puffinus ). Diomedeoides further differs from these two families in having a less developed processus procoracoideus. As in species of Hydrobatinae and Oceanitinae , the processus acrocoracoideus does not protrude medially beyond the processus procoracoideus. Most notably and in contrast to all Recent procellariiforms, the cotyla scapularis is cup−shaped, circular and deeply excavated. The circular outline is most similar to Pelagodroma , and to a lesser extent to Oceanodroma , although it is much shallower in these two genera. The processus lateralis is broken from the main part of the coracoid, but preserved on the counter slab. It has a similar overall shape to that of extant procellariiforms, but its tip is less pointed and upwardly curved than in extant tubenoses. The impressio musculi sternocoracoidei is well marked.

Fig. 5. Skull and mandible of Diomedeoides brodkorbi from the early Oligocene of Rheinweiler, Germany, in comparison to extant Procellariiformes . Ą A. Diomedeoides brodkorbi ( MHNF 30877), with caudal portion of skull missing: skull (A 1), lower jaw (A 2). B. Pelagodroma marina Latham, 1790 ( SMF 8312): skull (B 1), lower jaw (B 2). C. Pterodroma incerta Schlegel, 1863 ( SMF 4138) skull (C 1), lower jaw (C 2). D. Skull of Macronectes giganteus Gmelin, 1789 ( SMF 7265). E. Lower jaw of Phaethon rubricauda Boddaert, 1783 ( SMF 7287).

doi:10.4202/app.2009.0069

The furcula (seen as right clavicle, caudal view) is widely ventral direction rather than pointing slightly cranially as in

U−shaped; Mayr et al. (2002) mentioned that it is wider than other tubenoses. To a lesser extent, this feature can be ob−

in all Recent procellariiforms investigated by them. The served in Pterodroma . The proportions of the carpometa−

scapus claviculae has the same width throughout. The facies carpus resemble those of extant procellariids; although the articularis acrocoracoidea has a smooth angle. The processus bone is slightly longer in the fossil than in the members of this acromialis does not appear to be as pointed as in Puffinus , be− family (see also Cheneval 1995). The ventral rim of the carpal ing rather like in other procellariids (e.g., Pterodroma ). trochlea is less rounded and lower compared to other pro−

The short acromion of the scapula agrees with that of Re− cellariiforms. The os metacarpale minus is straight, whereas it cent procellariiforms. The facies articularis humeralis does is more bent in Pelagodroma (Fig. 6). The sulcus tendineus is not project as far ventrally as in Puffinus . The overall propor− very well marked, as in Oceanodroma and Pelagodroma , be−

tions are like those of procellariids (e.g., Procellaria ), and ing less so in most procellariids (e.g., Puffinus , Bulweria ,

not like Pelagodroma , whose scapula is much shorter. The Procellaria , Daption ). Mayr et al. (2002) mentioned that thin sheet of bone at the dorsal end has broken off, and there− Murunkus , a fossil from the Eocene of Kazakhstan which is fore its total length is unknown. known from a carpometacarpus, could be a member of the

The morphology of the ulna of the diomedeoidids has so Diomedeoididae . The processus extensorius of Murunkus ,

far only been incompletely known. The bone resembles that however, has a very different shape from that of Diomedeo−

of some extant procellariids (Fig. 6), but markedly differs ides, making a position of Murunkus within the Diomedeo−

from the much stouter and proportionally shorter ulna of the ididae unlikely. Furthermore, the cranially directing tuberosity oceanitines. The proportions of the bone resemble those of on the distal end of the os metacarpale majus of Murunkus

Recent procellariids. Unlike in species of Oceanitinae , the seems less protruding, unlike that of Diomedeoides . The ulna is slightly longer than the humerus. As in most extant carpometacarpus of Murunkus is also smaller (34.4 mm) than procellariiforms, the olecranon is low. The proximal end is that of MHNF 30877.

very much like that of Fulmarus , the main differences resid− The shape of the phalanx proximalis digiti majoris closely ing in the shape of the tuberculum ligamenti collateralis resembles that of Pelagodroma , whereas it is narrower and ventralis, which is narrower in MHNF 30877, and in the more elongated, with a less curved cranial margin in other ex−

depth of the impressio brachialis, which is deeper in the fos− amined procellariiforms (Fig. 6). The similar shape of this sil. The processus cotylaris dorsalis of Diomedeoides is less bone in Diomedeoides and Pelagodroma could be related to protruding than in procellariids, a condition also present in more rounded wings (see Mayr 2009b concerning presumed

Pelagodroma and Oceanodroma . Nevertheless, the ulna of flight and foraging strategies of these birds). As in all Pro−

Pelagodroma is highly derived and very different from that cellariiformes, the processus internus indicis is well−devel−

of the Rheinweiler specimen (Fig. 6). The tuberculum liga− oped, but seems to point slightly more ventrally in Diome−

menti collateralis ventralis of Diomedeoides is well devel− deoides.

oped and, as in most procellariids (e.g., Daption , Procellaria ), situated farther distally than that of Pelagodroma . On the distal end of the bone, the tuberculum carpale appears to Discussion

be proportionally smaller than in most procellariids examined, its shape being like that of Pelagodroma and Oceano− As noted in the introduction, the affinities of Diomedeoides droma , where it is quite small. The depressio radialis is very within the Procellariiformes have remained uncertain.

well marked in the fossil specimen. Both the condylus Cheneval (1995) confidently assigned Diomedeoides to the ventralis ulnaris and the dorsal edge of the condylus dorsalis Procellariidae although he did not present derived characters ulnaris of Diomedeoides resemble those of Fulmarus . supporting this classification, which was mainly based on

The left os carpi radiale is situated on the slab above the overall limb proportions. Fischer (1985) proposed a close re−

ulna and the phalanx proximalis digiti majoris; the articular lationship between Diomedeoides and Diomedea based on a surface with the carpometacarpus faces up. It does not differ single femur. Better preserved specimens have enabled re−

from that of extant Procellariiformes . fined hypotheses regarding the affinities of the Diomedeoi−

In the new specimen, the carpometacarpus is for the first didae, and most recently it has been hypothesized that the time well preserved (Fig. 6). The shape of the processus poorly developed processus supracondylaris dorsalis of the extensorius is peculiar in that it (gradually) slopes in a dorso− humerus suggests a position of the Diomedeoididae outside a Ą

Fig. 6. Wing and pectoral girdle elements of Diomedeoides brodkorbi from the early Oligocene of Rheinweiler, Germany, in comparison to extant

Procellariiformes . A. Diomedeoides brodkorbi (MHNF 30877); right ulna in ventral view (A 1), right carpometacarpus in dorsal view (A 2), right phalanx proximalis digiti majoris in ventral view (A 3), left coracoid in dorsal view (A 4). B. Pelagodroma marina Latham, 1790 (SMF 8312); right ulna in ventral view (B 1), right carpometacarpus in dorsal view (B 2), right phalanx proximalis digiti majoris in ventral view (B 3), left coracoid in dorsal view (B 4).

C. Fulmarus glacialis Linnaeus, 1761 (SMF 7181); right ulna in ventral view (C 1), right phalanx proximalis digiti majoris in ventral view (C 2), left coracoid in dorsal view (C 3). D. Pterodroma incerta Schlegel, 1863 (SMF 4138); dorsal view of right carpometacarpus. E. Oceanodroma castro Harcourt, 1851

(SMF 5641); dorsal view of left coracoid.

doi:10.4202/app.2009.0069

clade including the Diomedeidae , Procellariidae , Pelecanoididae , and Hydrobatinae ( Mayr 2009b) . Their exact phylogenetic position with respect to the Oceanitinae , however, remained unresolved.

An assessment of the affinities of these birds is complicated by the fact that molecular studies have returned ambiguous results bearing on the early divergences within crown group Procellariiformes . Whereas some analyses of molecular data either support the Oceanitinae ( Hackett et al. 2008) or the Diomedeidae ( Ericson et al. 2006) as the sister taxon of all remaining procellariiforms, some other analyses indicate that the Hydrobatinae split first, followed by the Oceanitinae ( Nunn and Stanley 1998; note that in Fig. 2 of this paper the names Oceanitinae and Hydrobatinae are interchanged). Likewise, the analysis of morphological features has yielded unclear results: a basal monophyletic Hydrobatidae was recovered by Bertelli and Giannini 2005, a basal Diomedeidae by Ksepka et al. 2006 and a basal Pelecanoididae by Livezey and Zusi 2007. However, it is worth mentioning that none of these studies have focused on Procellariiformes exclusively. Forbes’s (1882) anatomical study on the procellariiforms endorses the position of the Oceanitinae as sister taxon to all other members of this order.

The new osteological data obtained from specimen MHNF 30877 indicates that Diomedeoides is outside crown group Procellariiformes ( Fig. 7 ). The critical feature that strongly argues for a stem−group position of the Diomedeoididae is the deeply excavated, cup−like cotyla scapularis of the coracoid by which diomedeoidids are clearly distinguished from all extant procellariiform taxa, in which the facies articularis scapularis of the coracoid is shallow. Such a deeply excavated, cup−like articulation facet for the scapula is also present in Mesozoic non−neornithine birds such as Ichthyornis Marsh, 1872 and Hesperornis Marsh, 1872 , and is without a doubt a primitive character for Neornithes ( Mourer−Chauviré 1992a; Mayr and Weidig 2004). A cup−like cotyla scapularis of the coracoid occurs in stem group representatives of several other avian lineages, whose extant relatives have a flat facies articularis scapularis, such as the Galliformes ( Mourer−Chauviré 1992a) and Psittaciformes ( Mourer−Chauviré 1992b; Mayr 2000). Within all extant procellariiform taxa, the facies articularis scapularis of the coracoid is shallow (Fig. 6). A shallow facies articularis scapularis may serve to increase the movability of the scapula relative to the coracoid, but the exact functional significance of this feature remains unknown.

As noted in the introduction, diomedeoidids share several striking features with some members of extant Oceanitinae . Most notable among these are the long legs and greatly widened pedal phalanges ( Mayr et al. 2002; Mayr 2009b). The new specimen adds to these similarities in the long mandibular symphysis, the circular outline of the cotyla scapularis of the coracoid, and in the wider phalanx proximalis digiti majoris. Whereas the long legs and similar length of the pars symphysialis of the mandible may be plesiomorphic for Procellariiformes , the extraordinary similarities in the morphology of the pedal phalanges certainly evolved convergently ( Mayr et al. 2002; Mayr 2009b). Whether the same is true for the shape of the phalanx proximalis digiti majoris is less clear.

Procellariiform birds are today among the most diversified and numerically abundant groups of pelagic birds. Unambiguous remains of representatives of crown group Procellariiformes are, however, unknown from pre−Oligocene fossil sites, and the most abundant medium−sized seabirds in the late Paleocene and Eocene were the Prophaethontidae and the Pelagornithidae (bony−toothed birds) ( Mayr 2009a). Prophaethontidae are unknown from post−Eocene sediments, whereas late Paleogene and Neogene pelagornithids are giant forms with a wingspan above four meters ( Mayr 2009a). Because even early Oligocene procellariiforms appear to have been stem group representatives, we consider it well possible that the radiation of crown group Procellariiformes was in some way connected with the demise of the Prophaethontidae and small Pelagornithidae . Whether, however, tubenoses occupied ecological niches that became vacant after extinction of prophaethontids and bony−toothed birds, or whether the latter became extinct owing to competition with tubenoses can only be said once more data on the temporal occurrences of these birds become available.

The abundance of diomedeoidids in Central Europe during the Mid−Oligocene marine transgression may be related to the seasonal productivity of these waters. Detailed analysis from the clay pit of the Bott−Eder GmbH (“Grube Unterfeld”, Frauenweiler, Germany), which yielded several specimens of the Diomedeoididae ( Mayr et al. 2002; Mayr 2009b), indicates that primary producers appear regularly in abundance ( Grimm et al. 2002). Planktonic blooms were caused by the seasonal upwelling of bottom nutrients, triggered by differences in salinity between surface and bottom waters as a result of enhanced evaporation during the summer months ( Grimm et al. 2002; see also Micklich and Hildebrandt 2005). Phytoplankton blooms were the reason for the very diverse ichthyofauna of the area (its deposition is known as the “fish shales”), and provided the basis for a complex food web, thus supporting the presence of numerous predators such as sharks and procellariiform birds. The presence of large amounts of dinoflagellates in the layers where the bird was found confirms this hypothesis.

Figures

Fig. 2. Map of the immediate surroundings of the Rheinweiler locality.

Fig. 7. Relationships between crown group Procellariiformes and the Diomedeoididae. The two nodes are characterized by the characters: 1, cotyla scapularis of coracoid shallow; 2, mandibular symphysis short; processus supracondylaris dorsalis large; legs short (note that the Hydrobatinae are intermediate in the last two characters mentioned).