A partial skeleton of a new fossil loon (Aves, Gaviiformes) from the early Oligocene of Germany with preserved stomach content
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- Mayr, G. J Ornithol (2004) 145: 281. doi:10.1007/s10336-004-0050-9
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A partial skeleton of a new fossil loon (Aves, Gaviiformes), ?Colymboides metzleri n.sp., is described from the early Oligocene (Rupelian) of Frauenweiler in Germany. The new species resembles the early Miocene species Colymboides minutus in size and overall morphology, but differs in several osteological details. The specimen represents the first associated remains of an early Tertiary loon. Preserved stomach content further provides the first direct evidence that early Tertiary loons were already specialized towards a piscivorous diet, hunting fishes in a marine environment.
KeywordsAvesGaviiformesColymboides metzleri n. sp.OligoceneDiet
The four extant species of loons (Gaviiformes, Gaviidae) are foot-propelled diving birds that only occur in the Northern Hemisphere, breeding at northern freshwater sites, but wintering along sea costs in temperate areas (Carboneras 1992). Loons were considered to be closely related to grebes (Podicipediformes) by most earlier authors, but the similarities are now generally attributed to convergence (see Sibley and Ahlquist 1990 for a review of the history of avian classification). There is both morphological and molecular evidence that grebes are the sister taxon of flamingos (Phoenicopteriformes) (van Tuinen et al. 2001; Mayr 2004a), but the relationships of loons are uncertain.
The earliest fossil gaviiform is Neogaeornis wetzeli from the Cretaceous of Chile (Olson 1992), known from a single tarsometatarsus that resembles the highly derived bone of modern loons. Chatterjee (2002) reported another supposed loon, Polarornis gregorii, from the Cretaceous of Antarctica, of which were found the proximal part of the bill and the adjacent distal part of the cranium, parts of the otic region, four vertebrae, a small sternum fragment, a femur, and a proximal tibiotarsus. Despite this very fragmentary preservation, Chatterjee (2002) presented a detailed “reconstruction” of the entire skeleton and a complete skull in which most parts were “constructed by pressing (...) paper pulp into a mold that was sculpted first in clay” (Chatterjee 2002, pp. 128); it is difficult to understand why Chatterjee (1997, pp. 117) described the skull of Polarornis as being “fairly intact, beautifully preserved” and figured a drawing of a complete skull with no indication that substantial parts were reconstructed and are not preserved in the specimen.
Among the most completely known Tertiary fossil loons is the early Miocene species Colymboides minutus that was first described from Saint-Gérand-le-Puy in France by Milne-Edwards (1867–1871), and studied in greater detail by Storer (1956) and Cheneval (1984). Storer (1956, p. 423) considered C. minutus to be “a small loon” with hind limbs “not as highly adapted for swimming rapidly and powerfully under water as are those of living loons”, and noted (p. 425) that the “morphological differences between Colymboides and Gavia are many and rather great, but they are largely one of size and degree of specialization”. C. minutus was also reported from the early Miocene of the Czech Republic (Švec 1980, 1982). However, although all major limb elements of this species are known, no associated remains have yet been discovered.
Two other species were assigned to Colymboides: Colymboides anglicus from the Upper Eocene of England which is known from a coracoid, a referred humerus and a referred frontal part of the skull (Lydekker 1891; Harrison and Walker 1976), and Colymboides belgicus from the lowermost Oligocene of Belgium which is based on a proximal carpometacarpus and a distal ulna (Mayr and Smith 2002). Olson and Rasmussen (2001) further tentatively assigned to Colymboides an incomplete humerus and carpometacarpus from the Miocene of North America.
A number of fossil loons from the Miocene and Pliocene of Europe and North America were further assigned to the modern genus Gavia (Švec 1982; Olson and Rasmussen 2001; Mlíkovský 2002). Some of these are clearly stem group representatives of the Gaviidae, others may be within the crown group and on the stem lineage of modern species (Olson and Rasmussen 2001, p. 246).
Here I describe the first associated remains of an early Tertiary loon, from early Oligocene (Rupelian) marine deposits of Frauenweiler in Southern Germany (Trunko and Munk 1998). Apart from a rich fish fauna (Micklich and Parin 1996), this site has yielded a diverse avifauna, and the hitherto reported taxa include the procellariiform Diomedeoides (Mayr et al. 2002), the mousebird Oligocolius (Coliidae, Mayr 2000), the earliest European record of a songbird (Passeriformes, Mayr and Manegold 2004), and the stem group hummingbird Eurotrochilus (Trochilidae, Mayr 2004b).
Osteological terminology follows Baumel and Witmer (1993). Measurements are in millimeters and represent the maximum length of the bone along its longitudinal axis. The fossil specimen is deposited in Staatliches Museum für Naturkunde Stuttgart, Germany (SMNS).
Type locality and horizon
Frauenweiler south of Wiesloch (Baden-Württemberg, Germany), clay pit of the Bott-Eder GmbH (“Grube Unterfeld”); Rupelian, early Oligocene.
?Colymboides metzleri is decidedly smaller than the Upper Cretaceous loons, Colymboides anglicus, C. belgicus, and species of the modern genus Gavia, measuring about half of the size of the smallest extant species, Gavia stellata. The new species is of similar size to Colymboides minutus from which it is distinguished in a more protruding and narrower process formed by the cristae cnemiales (length 16 mm vs 11 mm in the slightly larger C. minutus), and in a proportionally shorter femur and longer tarsometatarsus (femur 25.9 mm vs 30.6–33.0 mm in C. minutus, tarsometatarsus 34.5 mm vs. 30.7–33.0 mm in C. minutus). Meaningful comparisons with Gaviella pusilla are not possible.
Dimensions of Colymboides minutus are in parentheses (after Cheneval 1984)
Left humerus, 60.5 (59.4–67.1); left ulna, ~46.6 (48.7–55.9); left radius, 45.5 (48.1–52.8); left carpometacarpus, 33.4 (29.9–34.0); left femur, 25.9 (30.6–33.0); left tarsometatarsus, 34.5 (30.7–33.0); pedal phalanges: II1, 16.5; III1, 18.7.
The species has been named after the finder of the holotype, Mr. Rudolf Metzler.
Description and comparison
Only two fragments of the tip of the beak, probably from both the lower and upper jaw, are preserved, as well as a small fragment of the cranium that allows recognition of the condylus occipitalis.
The vertebrae of early Tertiary loons have been unknown so far. In the Frauenweiler specimen, 13 or 14 praesacral vertebrae and a caudal vertebra are scattered over the slab that exhibit a similar morphology to the vertebrae of modern Gaviidae. As in modern loons, the thoracic vertebrae bear very long processus ventrales.
Likewise, the scapula of an early Tertiary loon is preserved in the Frauenweiler specimen for the first time. The left scapula lies next to the femur, and the extremitas omalis of the right scapula is situated next to the mid section of the humerus. The bone is similar to the scapula of modern loons in that its cranial section is markedly bowed medio-laterally and has an ovoid cross section (flattened in most other birds). However, it differs from modern loons in the more pointed acromion.
Although Lydekker (1891) referred a cranial portion of a sternum to Colymboides anglicus, this was shown by Harrison and Walker (1976) to be from a duck (Anatidae). In specimen SMNS 80739/2a, the sternum is seen in right lateral view. Only five processus costales can be counted in the fossil, whereas there are eight in modern loons. As in modern loons, the caudal margin exhibits a single pair of incisions and the trabecula mediana is strongly caudally elongated. The apex carinae protrudes cranially.
As far as comparable owing to preservation, humerus, ulna, and carpometacarpus closely resemble the corresponding bones of Colymboides minutus, as figured and described by Storer (1956) and Cheneval (1984). As in modern loons, the ulna is much shorter than the humerus.
The radius lies between the ulna and the carpometacarpus. As preserved, its distal end exhibits an unusual morphology in being much more flattened than in modern loons. Whether this reflects the true condition or is of diagenetic origin is, however, uncertain.
The os carpi ulnare is small as in modern Gavia. The os carpi radiale differs from that of modern loons in that it does not bear a marked tendinal furrow.
Apart from a synsacrum of Colymboides minutus figured by Storer (1956), details of the pelvis of early Tertiary loons were hitherto unknown. The pelvis of specimen SMNS 80739/2 is dissociated and apparently the ilium was not fused to the narrow synsacrum, a condition found in few modern birds including loons, penguins (Sphenisciformes) and tubenoses (Procellariiformes). Apart from being less elongated, the caudal end of the postacetabular ilium exhibits the highly derived morphology of modern loons in that the elongated spina dorsolateralis ilii bears a marked ridge along its lateral surface and there is notch in the caudal margin, between ischium and ilium (Fig. 4).
The femur is not as abbreviated and stout as that of modern Gaviidae, but appears to be proportionally shorter and somewhat stouter than in Colymboides minutus (see Measurements).
The tibiotarsus bears strongly elongated, proximally protruding cristae cnemiales (Figs. 1, 3). The process formed by the cnemial crests is proportionally longer and narrower than in Polarornis and Colymboides minutus, though less elongated than in modern loons: in ?Colymboides metzleri it measures about 16 mm, whereas it has a length of only 11 mm in the similar-sized Colymboides minutus (tibiotarsus length 69 mm), and 48 mm in Gavia stellata (tibiotarsus length 163 mm). Unfortunately, the distal end of the tibiotarsus is not preserved in the specimen, so the complete length of this bone in ?C. metzleri is unknown.
The tarsometatarsus of SMNS 80739/2 resembles the corresponding bone of Colymboides minutus. The second trochlea is strongly turned plantad. As in C. minutus but contrary to modern loons, the hypotarsus apparently lacks a bony bridge and exhibits plantarly open sulci.
The toes of early Tertiary loons have been so far unknown. The Frauenweiler specimen shows that those of Colymboides were similar to the toes of modern loons. The proximal phalanx of the fourth toe exhibits the derived morphology of loons in that the proximal end is widened and dorsally projecting. The hallux, which is very small in modern loons, appears not to be preserved in the specimen. The ungual phalanges are of similar shape to those of modern loons and differ from the broad, flattened ungual phalanges of grebes (Podicipediformes).
In the specimen a dense package of remains of small fishes (diameter of vertebrae from 0.4–1.0 mm) is preserved that unquestionably represents the former stomach content of the bird (Fig. 3).
The new Frauenweiler fossil is assigned to the Gaviiformes because of, e.g., the caudally elongated trabecula mediana of the sternum, the highly derived morphology of the postacetabular part of the pelvis (Fig. 4), the strongly elongated, proximally protruding cristae cnemiales on the tibiotarsus, and the medio-laterally compressed tarsometatarsus.
Whereas the genus Gavia is well-characterized by derived characters that are absent in Colymboides (e.g., a more elongated processus extensorius of the carpometacarpus and a more derived structure of the hypotarsus), no derived characters are known that support monophyly of the taxa assigned to Colymboides, which may turn out to be a paraphyletic genus. Although very similar in size and overall morphology to Colymboides minutus, the new species shares with modern Gaviidae a more strongly protruding and narrower process formed by the cristae cnemiales of the tibiotarsus and a proportionally shorter and slightly stouter femur (which is, however, not as short and stout as in Gavia). However, whether ?Colymboides metzleri indeed is more closely related to Gavia spp. than to Colymboides minutus needs to be further evaluated by future better preserved specimens of the Frauenweiler loon, showing additional osteological details. At present, I prefer a tentative assignment to Colymboides.
The phylogenetic position of Colymboides minutus relative to Neogaeornis and Polarornis is uncertain, owing to the incomplete preservation of the Cretaceous taxa. Polarornis significantly differs from modern loons in several characters (Chatterjee 2002, p. 133) and, if correctly assigned to the Gaviiformes, may be a synonym of Neogaeornis - a possibility already raised by Olson (1992) but not discussed by Chatterjee (2002).
The Rupelian deposits of Frauenweiler have an age of about 30–33 million years (Legendre and Lévêque 1997), and the fossil loon described in this study is about 4–8 million years younger than Colymboides anglicus from England, the type horizon of which dates back 37–38 million years (Mlíkovský 1996; Legendre and Lévêque 1997). There is thus still a gap of more than 25 million years in the fossil record of the Gaviiformes, between C. anglicus and the Upper Cretaceous taxa Neogaeornis and Polarornis.
Apart from representing the first associated remains of an early Tertiary loon, specimen SMNS 80739/2 provides the first direct evidence that early Tertiary loons were already specialized towards a piscivorous diet, hunting fishes in a marine environment.
I thank R. Böttcher (SMNS) for the loan of the fossil specimen, O. Vogel for its further preparation, and S. Tränkner for taking the photographs.