Neornithine bird coracoid from the Upper Cretaceous of Patagonia

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Ameghiniana
ISSN 0002-7014 versión impresa
Ameghiniana v.43 n.1 Buenos Aires ene./mar. 2006
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Neornithine bird coracoid from the Upper Cretaceous of Patagonia
Federico L. Agnolin 1 , Fernando E. Novas 1 and Gabriel Lio 1
1 Laboratorio de Anatomía Comparada, Museo Argentino de Ciencias Naturales, Avenida Ángel Gallardo
470, 1405 Buenos Aires, Argentina. fernovas@yahoo.com.ar
Introduction
The fossil record of Mesozoic neornithine birds is restricted to the Upper Cretaceous
(Campanian, Maastrichtian) from America, Europe, Asia, and Antarctica (Hope, 2002).
Most of the Cretaceous neornithines recorded at present, correspond to aquatic groups
(e.g., Charadriiformes, Anseriformes; Olson and Parris, 1987; Elzanowski, 1995), but a
few terrestrial forms were also documented ( e.g. , Psittaciformes, Galliformes, and
probably Paleognathae; Stidham, 1998; Hope, 2002). This diversity of taxa constitutes the
best available evidence to discuss the timing and branching sequence of clades of modern
birds.
With regard to the Mesozoic record of birds from South America, it is dominated by the
Enantiornithes (Walker, 1981; Chiappe, 1996), an extinct group of worldwide distribution,
considered to be the sister taxon of Ornithuromorpha (e.g., Patagopteryx plus Ornithurae;
Chiappe, 2001). In contrast, the remains of Mesozoic neornithine birds in South America
are restricted to a tarsometatarsus of the presumed loon (Gaviiformes) Neogaeornis
wetzeli (Olson, 1992), from the Maastrichtian of Chile.
The specimen here described consists of a partial coracoid, found in the Portezuelo
Formation (Turonian-Coniacian, Late Cretaceous; Cruz et al ., 1989; Leanza, 1999) of
Sierra del Portezuelo, NW Patagonia (figure 1). Albeit fragmentary, the bone shows distinct
neornithine features. The fossil was found in association with remains of pelecipods, turtle
plates, and a number of appendicular bones and vertebrae pertaining to small-sized
ornithopods, as well as teeth of dipnoans, crocodiles, sauropods, and non-avian theropods.
A few meters above this fossiliferous level, several non-avian theropods were recovered:
the alvarezsaurid Patagonykus puertai Novas, 1997, the bizarre tetanuran Megaraptor
namunhuaiquii Novas, 1998, the basal dromaeosaurid Unenlagia comahuensis Novas and
Puerta, 1997, and Neuquenraptor argentinus Novas and Pol, 2005.

Figure 1. Map indicating fossil locality / mapa indicando la localidad fosilífera.
The present discovery enlarges our knowledge of the Late Cretaceous terrestrial faunas of
Patagonia, but also adds relevant data about the timing and early radiation of neornithine
birds.
Institutional abbreviations. PVPH, Museo Carmen Funes, Paleontología Vertebrados, Plaza Huincul.
Systematic paleontology
A ves Linne, 1758
N eornithes Gadow, 1893
Genus and species indeterminate

Referred material. PVPH 237, proximal end of a right coracoid.
Locality and Horizon. Sierra del Portezuelo, Neuquén Province, Patagonia, Argentina.
Portezuelo Formation (Turonian-Coniacian, Late Cretaceous; Cruz et al ., 1989; Leanza,
1999).
Description
The coracoid (figure 2.B-E) is broken at mid-shaft, lacking its sternal extremity; its preserved
shoulder end is not abraded, but retains the well-finished external surface of the bone. The
coracoid is small, with a maximum preserved length of 9.2 mm and a maximum width of
5.5 mm. Its whole length is estimated in 3 cm, a size comparable with that of the
Californian quail ( Lophortyx californica Shaw), which reaches approximately 25 cm long
from beak to tail (Olrog, 1968).
Figure 2. PVPH 237, A , proximal end of right coracoid in ventral; B, medial; C, dorsal and D, proximal
views. Scale bar: 2 mm / PVPH 237, A, extremo proximal de coracoides derecho en vistas ventral; B,
medial; C, dorsal y D, proximal. Escala: 2 mm. Abbreviations: ap, acrocoracoidal process; bt, brachial
tubercle; cs, cotyla scapularis ; fac, facies articularis humeralis ; mah, scar for the M. acrocoracohumeralis
; pp, procoracoidal process; sms, sulcus M. supracoracoidei / Abreviaturas: ap, proceso acrocoracoideo;
bt, tubérculo braquial; cs, cotyla scapularis; fac, facies articularis humeralis; mah, superficie para el M.
acrocoracohumeralis ; pp, proceso procoracoidal; sms, sulcus M. supracoracoidei.
The acrocoracoid is dorsomedially projected. The acromial process and the brachial
tubercle of the acrocoracoid are short and rounded, being separated by a shallow groove,
as it occurs in the Early Tertiary galliforms Ameripodius (Quercymegapodiidae;
Alvarenga, 1995; Mourer-Chauviré, 2000) and Paraortyx (Paraortygidae; Mayr, 2000).
The ligament scar of the Muscle acrocoracohumeralis is wide and deep, as in most
galliforms (Mayr, 2000). The sulcus for the M. supracoracoidei is deep and wide, as it
occurs (albeit not uniquely) in Galliformes. The brachial notch is absent. The facies

articularis humeralis is flat, facing mostly dorsally. The external margin of this surface is
remarkably projected outwards, thus forming a free lateral flange (figure 2.C), thus
resembling that of megapodiid and quercymegapodiid galliforms.
The cotyla scapularis is wide, shallow, and oval in shape. A procoracoidal process is
nearly absent, a condition that shares with galliforms. The coracoidal neck is slender,
straight, and rounded in cross-section.
Comparison
The coracoid has derived characters that are present in most birds, exclusive of
Archaeopteryx : strutlike condition and a rounded cotyla scapularis , indicating a mobile
articulation with the scapula (Chiappe, 2001). However, the presence of a well defined and
concave cotyla scapularis , as well as, a laterally projected facies articularis humeralis ,
are synapomorphies shared with Ichthyornis , Ambiortus , and Neornithes, thus suggesting
that the Neuquenian bird belongs to a group of birds more derived than
Hesperornithiformes, Patagopteryx , and Enantiornithes (figure 3). Moreover, the coracoid
exhibits two derived traits considered diagnostic of Neornithes (Hope, 2002): facies
articularis humeralis not extended distally beyond the cotyla scapularis , and absence of a
medial tilting of the facies articularis humeralis . Within Neornithes, the coracoid from
Patagonia shares an apomorphic trait with Galliformes (figure 3): presence of a distinct scar
for the insertion of M . Acrocoracohumeralis (Mayr, 2000). The coracoid also resembles
galliforms in having a reduced procoracoidal process (Alvarenga, 1995), a condition also
present in Tinamiformes (C. Tambussi, pers. comm. 2004).
Figure 3. Comparison between PVPH 237 and several coracoids of living and extinct birds, in medial view.
Not to scale / comparación entre PVPH 237 y algunos coracoides de aves vivientes y extintas, en vista
medial. No a escala. A, PVPH 237; B, Ameripodius silvasantosi (from Alvarenga, 1995); C, Ichthyornis
sp. (from Hope, 2002); D, Enantiornis leali (from Walker, 1981). Abbreviations: cs, cotyla scapularis ; fac,
facies articularis humeralis / A, PVPH 237; B, Ameripodius silvasantosi (tomado de Alvarenga, 1995); C,
Ichthyornis sp. (tomado de Hope, 2002); D, Enantiornis leali (tomado de Walker, 1981). Abreviaturas: cs,
cotyla scapularis ; fac, facies articularis humeralis .
Within Galliformes, the coracoid here described resembles the extinct
Quercymegapodiidae, Gallinuloididae, and Paraortygidae, and the living Megapodiidae
(Mayr, 2000; Mourer-Chauviré, 1992) in having a laterally expanded facies articularis
humeralis , and an ovoid-shaped and concave cotyla scapularis . In contrast, in most
living members of Galliformes the cotyla scapularis is elliptical and flat or slightly convex
(Alvarenga, 1995; Mayr, 2000), and the facies articularis humeralis is not laterally
expanded.

Discussion
Up to now, undoubted neornithine remains come from beds not older than Santonian
(Hope, 2002). Purported neornithine records of older age ( e.g. , Early Cretaceous) have
been questioned (e.g., Padian and Chiappe, 1998; Hope, 2002). Then, the coracoid from
Neuquén constitutes one of the oldest known Neornithes yet recorded.
Classically, the origin of the extant lineages of birds was interpreted as to have occurred at
the end of the Cretaceous and to have been succeeded by their evolutionary "explosion" in
early Tertiary times (Olson, 1985; Feduccia, 1995). However, such view has been recently
challenged by both paleontological evidence (Hope, 2002) and calibration of molecular
phylogenies (Hedges et al ., 1996; Cooper and David, 1997). Currently, most authors
consider that Neornithes radiated well before the end of the Cretaceous, interpretation that
is in accordance with that predicated by molecular biologists, which calculated that major
divergence time for neornithine orders around 90-100 my (e.g., Hedges et al ., 1996; Dyke
and Van Tuinen, 2004), or even earlier (Cooper and David, 1997). Discovery of
Neornithine remains in the Turonian-Coniacian Portezuelo Formation (88-92 My) is in
agreement with this last interpretation, suggesting that the divergence of modern groups of
birds was well in progress during the Turonian.
Acknowledgements
We thank to C.P. Tambussi (Museo de La Plata), J. Navas, and P. Tubaro (Museo de Ciencias Naturales,
Buenos Aires) for access to ornithological collections under their care; F. Tricárico for SEM (Scan
Electronic Microscope) images; C.P. Tambusi, L.M. Chiappe, H. Alvarenga, and A.M. Báez for valuable
comments and discussion of the manuscript; The National Geographic Society and Agencia Nacional de
Promoción Científica y Tecnológica, for their financial support.
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Recibido: 8 de julio de 2004.
Aceptado: 30 de junio de 2005.
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