The peculiar scapula of the late Eocene Elaphrocnemus phasianus M

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Senckenbergiana lethaea 88 (2) 195 – 198 1 text-fig. Frankfurt am Main, 30.12.2008

The peculiar scapula of the late Eocene Elaphrocnemus phasianus M

^ilne

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, 1892 (Aves, Cariamae)

With 1 Text-figure

Gerald Mayr & Cécile Mourer-Chauviré

Abstract

The Eocene bird Elaphrocnemus phasianus is a stem lineage representative of the Cariamidae (seriemas) and among the most abundant medium-sized avian species in the Quercy fissure fillings in France. We describe the previously unknown scapula of this species, which exhibits a highly unusual morphology.

Most notably, the acromion is greatly elongated and there is a concave facies articularis coracoidei rather than a tuberculum coracoideum as in most other birds. These features also distinguish Elaphrocnemus from other representatives of the Cariamae. The long acromion probably serves to strengthen the canalis triosseus, which in other Cariamae is achieved by a bony bridge connecting the processus procoracoideus and acrocoracoideus of the coracoid. The peculiarities of the pectoral girdle and wing of Elaphrocnemus may indicate that this taxon had better flight capabilities than other Cariamae.

Key words: fossil birds, Paleogene, Elaphrocnemus, Quercy

Zusammenfassung

Die eozäne Vogelart Elaphrocnemus phasianus ist ein Stammgruppenvertreter der Cariamidae (Seriemas) und eine der häufigsten mittelgroßen Vogelarten in den Spaltenfüllungen von Quercy in Frankreich.

Wir beschreiben die bisher unbekannte Scapula dieses Vogels, die eine höchst ungewöhnliche Morpholo- gie aufweist. Insbesondere ist das Acromion stark verlängert und es gibt eine konkave Facies articularis coracoidei statt eines Tuberculum coracoideum wie bei den meisten anderen Vögeln. Diese Merkmale unterscheiden Elaphrocnemus auch von anderen Vertretern der Cariamae. Das lange Acromion diente vermutlich dazu, den Canalis triosseus zu verstärken, was in anderen Cariamae durch eine knöcherne Brücke erzielt wird, welche die Processus procoracoideus und acrocoracoideus des Coracoids verbindet.

Die Eigenheiten des Brustgürtels und Flügels von Elaphrocnemus weisen darauf hin, daß dieses Taxon vermutlich eine bessere Flugfähigkeit als andere Cariamae hatte.

Schlüsselworte: fossile Vögel, Paläogen, Elaphrocnemus, Quercy

Dr Gerald Mayr (Corresponding author <gerald.mayr@senckenberg.de>), Forschungsinstitut Senckenberg, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany; Dr Cécile Mourer-Chauviré, UMR 5125 Paléoenvironnements et Paléobiosphère, Centre des Sciences de la Terre, Université Claude Bernard-Lyon I, 27-43 Boulevard du 11 Novembre, F-69622 Villeurbanne Cedex, France.

Introduction

Elaphrocnemus is one of the most abundant medium-sized avi- an taxa in the 19^th century collections from the middle Eocene to late Oligocene age Quercy fissure fillings in France. Three species were distinguished by Mourer-Chauviré (1983), i.e.

E. phasianus Milne-Edwards, 1892, E. crex Milne-Edwards, 1892, and E. brodkorbi Mourer-Chauviré, 1983. Mourer- Chauviré (1983) also recognized that the wing bones assigned to the taxon “Filholornis” actually belong to Elaphrocnemus, and Mayr & Mourer-Chauviré (2006) assigned an almost complete skull and other cranial remains to E. phasianus. The

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196 The peculiar scapula of the late Eocene Elaphrocnemus phasianus Milne-Edwards, 1892 (Aves, Cariamae)

exact age of the specimens from the old collections is unknown, but new excavations have shown that Elaphrocnemus phasianus occurs in late Eocene deposits and E. crex in Oligocene sites (Mourer-Chauviré 1983).

Elaphrocnemus was assigned to the Idiornithidae by Mourer-Chauviré (1983) and earlier authors (e.g. Cra- craft 1973), which also includes the middle Eocene to late Oligocene Idiornis Milne-Edwards, 1892 and three further, less well-known taxa from the Quercy fissure fillings. Togeth- er with the Phorusrhacidae (Paleocene to Pliocene of South America, Pleistocene of North America) and Bathornithidae (late Eocene and Oligocene of North America), these birds are stem group representatives of the Cariamidae (seriemas), whose two extant species have a South American distribution.

The taxon including extant Cariamidae and their fossil stem lineage representatives is usually termed Cariamae.

Elaphrocnemus distinctly differs from Idiornis and extant Cariamidae in several osteological features, and may be outside a clade including Idiornis, Phorusrhacidae, and Cariamidae (Mayr 2002). In the present note we describe the previously unknown scapula of Elaphrocnemus phasianus, which exhibits a very unusual morphology without counterpart among other birds.

Anatomical terminology follows Baumel & Witmer (1993) and Vanden Berge & Zweers (1993). The fossil specimens are deposited in the collections of the Naturhistorisches Museum Basel, Switzerland (NMB) and the Université des Sciences et Techniques du Languedoc, Montpellier, France (USTL).

Systematic Paleontology

Aves Linnaeus, 1758

Cariamae (sensu Mourer-Chauviré 1983) Elaphrocnemus phasianus Milne-Edwards, 1892 Referred specimens: NMB Q.D.378, NMB Q.D.293 (left scapulae lacking the caudal ends, Text-fig. 1).

Locality and horizon: Unknown locality and horizon of the Quercy fissure fillings in France; probably late Eocene (see In- troduction).

Measurements (in mm): NMB Q.D.378: maximum length as preserved, 41.7; maximum diameter of facies articularis humeralis, 4.7. NMB Q.D.293: maximum length as preserved, 32.6; maximum diameter of facies articularis humeralis, 5.3.

Description and comparison: The most unusual feature of the two scapulae is the greatly elongated acromion, which exceeds that of most extant birds, including the Cariamidae, in relative length (only in Pelecanidae [pelicans] is there a simi- larly elongated acromion). The acromion of the scapula of E.

phasianus is further characterized and distinguished from that of the Cariamidae by its square tip which forms a small medial hook, the rugose and irregular surface of its medial margin, and the presence of a ridge along the margo dorsalis (Text-figs 1A-F). As in extant Cariamidae the facies articularis coracoidei forms an angle with the comparatively small facies articularis humeralis. In contrast to the former, however, the facies articularis coracoidei of E. phasianus is concave, which corresponds

to the fact that the facies articularis scapularis of the coracoid of E. phasianus is convex and not flat or concave as in other birds (Text-fig. 1G).

The scapula of Idiornis gallicus (Milne-Edwards, 1892) differs from that of E. phasianus in that there is a tuberculum coracoideum (Mourer-Chauviré 1983: 101), which corresponds to the fact that the coracoid of Idiornis has a concave cotyla scapularis. The acromion of the scapulae of the bathornithid (see Olson 1985) Bathornis grallator (Wetmore, 1944) and the Phorusrhacidae is very short (Sinclair & Farr 1932, Wetmore 1944: fig. 8). The acromion of the only known scapula of bathornithid Paracrax wetmorei is broken, and there is a large pneumatic foramen caudal to the facies articularis humeralis (Cracraft 1968). The scapulae of other fossil Car- iamae are unknown.

Discussion

Both scapulae articulate well with the coracoid of E. phasianus (NMB Q.D.242) (Text-fig. 1H). The presence of a concave facies articularis coracoidei allows an unambiguous assignment to Elaphrocnemus, which is the only avian taxon of matching size known from the Quercy fissure fillings with a convex facies articularis scapularis of the coracoid. This identification is also supported by the fact that, although scapulae are rather rare among the 19^th century material of the Quercy fissure fillings, two specimens are present in the collection of NMB, in which bones of E. phasianus are the most abundant remains of medium-sized birds from the Quercy deposits.

Mourer-Chauviré (1983) assigned a different type of scapula to the larger species E. crex, which has an only slightly elongated acromion. This bone (USTL ITD 527; Text-fig. 1I) was associated with remains of E. crex in the Quercy locality Itardies. Given the great similarity of the coracoids, it is however unlikely that the scapulae of E. phasianus and E. crex are so different. Because USTL ITD 527 further lacks a concave facies articularis coracoidei, we conclude that this specimen has been erroneously referred to Elaphrocnemus.

The coracoid of Elaphrocnemus differs from that of other Cariamae in that the processus procoracoideus is greatly re- duced. In Idiornis and extant Cariamidae this process fuses with the processus acrocoracoideus to form a closed canalis triosseus which guides the tendon of musculus supracoracoi- deus (Text-fig. 1J). In Elaphrocnemus, by contrast, closure of the canalis triosseus is accomplished by the very long acromion of the scapula (Text-fig. 1H).

Also with regard to the morphology of the wing bones, Elaphrocnemus distinctly differs from other representatives of the Cariamae. Most notably, the humerus exhibits a strongly protruding, triangular crista deltopectoralis, which resembles that of extant Columbidae (pigeons and doves) and Psittaci- dae (parrots) in its shape (Mourer-Chauviré 1983: pl. 1).

The functional significance of the large crista deltopectoralis of the Columbidae and Psittacidae has been detailed by Steg- mann (1964), who noted that in these taxa the furcula is very weak owing to the presence of a large crop. One of the major functions of the avian furcula is to serve as an insertion site for the cranial portion of the musculus pectoralis. The cranial curvature of the scapi clavicularum of the normally developed furcula results in a cranial position of a portion of this muscle

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Text-fig. 1. Incomplete left scapulae (A-F) and left coracoid (G) of Elaphrocnemus phasianus Milne-Edwards, 1892 from an unknown horizon of the Quercy fissure fillings; (H) coracoid (NMB Q.D.242) and scapula (NMB Q.D.293) in articulation; (I) the extremitas cranialis of a scapula (USTL ITD 527) which was assigned to Elaphrocnemus crex Milne-Edwards, 1892 by Mourer-Chauviré (1983), and the scapula and coracoid of the extant Cariama cristata (K, J; right side, reversed to facilitate comparisons). (A, C, E) NMB Q.D.293 in medial, ventrolateral, and ventral view; (B, D, F) NMB Q.D.378 in medial, ventrolateral, and ventral view; (G) NMB Q.D.242 in dorsal view; (H) specimens NMB Q.D.242 and NMB Q.D.293 in articulation. Abbreviation: acr - acromion, fac - facies articularis coracoidei, fah - facies articularis humeralis.

Scale bars equal 5 mm (same scale bar for all figures except H).

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relative to the humerus, which enables the bird to draw the wings forward. In birds with a weakly developed furcula without cranial curvature, however, the musculus pectoralis cannot perform this function, which is taken over by the unusually well-developed musculus coracobrachialis cranialis (“anterior”) in Psittacidae and Columbidae. This muscle originates on the processus acrocoracoideus of the coracoid and inserts on the planum intertuberculare of the humerus (Vanden Berge &

Zweers 1993). Its large development in Columbidae and Psit- tacidae is correlated with the reduction of the proximal portion of the crista deltopectoralis, which thus has a pointed, triangular shape.

We assume that the same considerations also apply in the case of Elaphrocnemus. Like Phorusrhacidae and extant Cariamidae, this taxon probably also had a very weak furcula (which may or may not have been correlated with presence of a large crop). However, and as indicated by the well-developed crista deltopectoralis, Elaphrocnemus seems to have had bet- ter flight capabilities than other Cariamae. This corresponds to the fact that it also has a proportionally shorter tarsometa- tarsus than Idiornis and extant Cariamidae (Mourer-Chau- viré 1983) and thus appears to have been less cursorial. The long acromion of Elaphrocnemus may serve to strengthen the processus acrocoracoideus, on which the musculus coracobrachialis cranialis inserts. Just because of their uniqueness, the latter feature and the peculiar shape of the articulation facets between coracoid and scapula are likely to be apomorphic features of Elaphrocnemus.

Because the Cariamae had a wide distribution in the Paleo- gene, a better knowledge of their phylogenetic interrelation- ships would be of great interest from a biogeographic point of view. By outgroup comparison with other birds outside the Cariamae, the differences of the carpometacarpus (absence of a tubercle on the ventral margin of the os metacarpale minor) and hypotarsus (which is not block-like as in Idiornis, Pho- rusrhacidae, and Cariamidae) seem to be plesiomorphic and support a position of Elaphrocnemus outside a clade including Idiornis and extant Cariamidae, as assumed by Mayr (2002).

Certainly, however, this hypothesis still needs to be evaluated in a more comprehensive analysis, which also includes representatives of the Bathornithidae, and which is beyond the scope of the present note.

Acknowledgements

We thank B. Thüring (NMB) for the loan of the fossil specimens, S. Tränkner (SMF) for taking the photographs, and Z. M. Bochenski and E. Bourdon for reviewing the manuscript.

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Manuscript received: 14 March 2008 Reviewed manuscript accepted: 17 July 2008