– 17 – Paleornithological Research 2013 Proceed. 8th Inter nat. Meeting Society of Avian Paleontology and Evolution Ursula B. Göhlich & Andreas Kroh (Eds) The tarsometatarsus of the Middle Eocene loon Colymbiculus udovichenkoi GERALD MAYR1, LEONID GOROBETS2 & EVGENIJ ZVONOK3 1 Senckenberg Research Institute and Natural History Museum Frankfurt, Frankfurt am Main, Germany; E-mail: [email protected] 2 Taras Shevchenko National University of Kiev, Dept. of Ecology and Environmental Protection, Kiev, Ukraine 3 Institute of Geological Sciences of NAS of Ukraine, Branch of Paleontology and Stratigraphy, Kiev, Ukraine Abstract — We describe the previously unknown tarsometatarsus of the earliest unambiguously identified loon, Colymbiculus udovichenkoi, from the Middle Eocene of the Ukraine. Except for being more elongate and apart from details of the hypotarsus morphology, the bone resembles the tarsometatarsus of the Early Miocene Colym- boides minutus. We consider the hypotarsus morphology of Colymbiculus to be plesiomorphic for Gaviiformes. Colymboides and crown group Gaviiformes are each characterized by an autapomorphic hypotarsus morphology, which precludes the former from being directly ancestral to the latter. The similarities shared by C. udovichenkoi and C. minutus, including their small size, are likely to be plesiomorphic for Gaviiformes. Although the disap- pearance of small stem group Gaviiformes may be related to the retreat of loons to cold Northern latitudes, more data are needed to firmly establish this hypothesis. We finally note that early Paleogene stem group Gaviiformes markedly differ from putative Late Cretaceous loons, whose identification needs to be verified by further fossil specimens. Key words: Colymbiculus, Colymboides, fossil birds, Gaviiformes, Eocene, Lutetian, Ukraine Introduction 1982 from the Early Miocene of the Czech Republic (ŠVEC 1982), the earliest stem line- Loons (Gaviiformes) have a fairly comprehen- age representative, being distinctly smaller than sive Neogene fossil record (OLSON 1985; MAYR its modern congeners. Disregarding the poorly 2009a), but the early evolutionary history of known Gaviella pusilla (SHUFELDT, 1915), whose these foot-propelled diving birds is still incom- exact age is uncertain but possibly Oligocene pletely known. The first fossil loon recognized as (WETMORE 1940), loons first appear in North such, and also one of the best-represented spe- America in the Early Miocene (RASMUSSEN 1998). cies, is Colymboides minutus MILNE-EDWARDS, All Neogene North American species were clas- 1867 from the Early Miocene of France and sified in the taxon Gavia (OLSON 1985; OLSON & the Czech Republic (MILNE-EDWARDS 1867–71, RASMUSSEN 2001). STORER 1956; CHENEVAL 1984; MLÍKOVSKÝ 2002). Pre-Miocene Gaviiformes are morphologi- Other species from the Neogene of Europe were cally clearly distinguished from their extant assigned to Gavia, the taxon that includes the relatives. Most of the fossils were, at least ten- four extant species, with Gavia egeriana ŠVEC, tatively, assigned to Colymboides, and the two © Verlag Naturhistorisches Museum Wien, 2013 published 10 Dec. 2013 – 18 – named Paleogene species of this taxon are Col- Systematic Paleontology ymboides anglicus LYDEKKER, 1891 from the Late Eocene of England (HARRISON 1976; HARRISON & Gaviiformes WETMORE & MILLER, 1926 WALKER 1976) and ?Colymboides metzleri MAYR, Gaviidae ALLEN, 1897 2004 from the Early Oligocene of Germany and Belgium (MAYR 2004, 2009b). Contemporane- Colymbiculus udovichenkoi MAYR & ZVONOK, ous with the latter species there was also a large 2011 gaviiform species of undetermined affinities in the Early Oligocene of Belgium (MAYR & SMITH Referred specimen: SMF Av 592 (complete left 2002). A Colymboides-like loon of uncertain spe- tarsometatarsus). cies identity was further reported from the Late Locality and horizon: Ikovo sand quarry, Oligocene of Germany (MAYR & POSCHMANN about 1 km west Novopskov, Luhansk Province, 2009), and undescribed remains of Colymboides Ukraine (49°31′42″ N, 39°3′59.5″ E); Middle were also found in the Late Oligocene of Kazakh- Eocene (Lower Lutetian). stan (KUROCHKIN 1976). Measurements: Length, 39.4; proximal Putative Upper Cretaceous records of Gavii- width, 7.0; distal width, 6.0; minimum mediolat- formes were described from Chile (Neogaeornis eral width of shaft, 2.9. wetzeli OLSON, 1992) and Antarctica (Polarornis Description and comparisons: The tar- gregorii CHATTERJEE, 2002), but these are based sometatarsus of Colymbiculus udovichenkoi on a single bone and a very fragmentary partial resembles that of Colymboides minutus, but is skeleton, respectively, and their identification proportionally longer relative to the humerus (the needs to be corroborated with more material (see tarsometatarsus of other Colymboides species is also below). The earliest well-documented stem unknown or too poorly preserved for detailed group representative of Gaviiformes is Colym- comparisons): Whereas the lengths of humerus biculus udovichenkoi MAYR & ZVONOK, 2011, and ulna of Colymbiculus correspond with the which occurs in Middle Eocene (about 48 mil- lower size range of C. minutus (~63.3 and 49.1 lion years ago) marine sediments of the eastern versus 62.0–66.0 and 49.8–56.0, respectively; Tethys Sea in Ukraine (MAYR & ZVONOK 2011, STORER 1956, MAYR & ZVONOK 2011, 2012), that 2012). Colymbiculus udovichenkoi is quite of the tarsometatarsus is distinctly larger (39.4 abundant at the type locality, but up to now the mm versus 30.8-32.8 mm; STORER 1956). Further material assigned to this small species consisted unlike C. minutus, the hypotarsus of C. udovi- only of four skeletal elements, i.e., the humerus, chenkoi does not exhibit a canal for the tendon ulna, radius, and tibiotarsus. In light of the fact of musculus flexor digitorum longus (Fig. 1E, that loons are highly specialized foot-propelled I). Instead, there are three well-developed sulci diving birds, one of the most significant bones of for, from medial to lateral, musculus flexor digi- C. udovichenkoi – the tarsometatarsus – remained torum longus, m. flexor perforatus digiti II, and thus unknown. This bone was found in the 2012 m. flexor hallucis longus. The lateral surface of excavation campaign at the type locality and is the crista medialis hypotarsi bears a shallow fur- described in the present study. row for the tendon of musculus flexor perforans et perforatus digiti II. The small foramina vascu- laria proximalia are closely adjacent. The distal Material and methods end of the bone corresponds well with Colym- boides minutus, and as in the latter and modern Osteological terminology follows BAUMEL & loons the trochlea metatarsi II is short and plan- WITMER (1993). Measurements are in millimeters. tarly deflected. The foramen vasculare distale is The fossils are deposited in Naturhistorisches situated farther distally than in extant loons. The Museum Basel, Switzerland (NMB) and canalis interosseus distalis is dorsally open. A Forschungsinstitut Senckenberg, Frankfurt am fossa metatarsi I is absent. Main (SMF). The tarsometatarsus of Colymbiculus is less mediolaterally compressed than that of extant SAPE Proceedings 2013 – 19 – FIGURE 1. Tarsometatarsus of A–F, Colymbiculus udovichenkoi from the Middle Eocene of Ukraine in com- parison with G–I, the Early Miocene Colymboides minutus and J–L, the extant Gavia stellata. A–F, left tarso- metatarsus (SMF Av 592) in dorsal (A), medial (B), lateral (C), plantar (D), proximal (E), and distal (F) view. G, H, left tarsometatarsus (NMB MA. 2668) in plantar (G) and dorsal (H) view. I, right tarsometatarsus (NMB S.G. 20829) in proximal view. J–L, left tarsometatarsus in plantar (J), dorsal (K), and proximal (L) view. M, left carpometacarpus (SMF Av 593, ventral view) of an unidentified small bird from the Middle Eocene of the Ikovo sand quarry. Abbreviations: adII, sulcus for tendon of musculus abductor digiti II; fdl, canal for tendon of musculus flexor digitorum longus; fhl, canal for tendon of musculus flexor hallucis longus; fpd, canal for tendon of musculus flexor perforatus digiti II; fpp2, canal for tendon of musculus flexor perforans et perforatus digiti II; fvd, foramen vasculare distale; pex, base of broken processus extensorius; sft, canal for superficial flexor ten- dons. In (F) the trochleae are numbered. Scale bars equal 5 mm. Gaviidae. There is further no marked sulcus for was less specialized for foot-propelled diving the tendon of musculus abductor digiti II. Proxi- than extant Gaviiformes, in which the tarsometa- mal of the foramen vasculare distale, on the tarsus is much more mediolaterally compressed dorsal surface of the bone, the sulcus extensorius and in which the tibiotarsus exhibits much bet- is less marked than in extant loons. In the hypo- ter developed cnemial crests (MAYR & ZVONOK tarsus of modern Gaviidae the cristae mediales et 2012). Colymbiculus udovichenkoi further dis- lateralis hypotarsi are much better developed and tinctly differs from Colymboides minutus and fuse plantarly, thus enclosing a large canal for extant loons in the much more pronounced crista the superficial flexor tendons (Fig. 1L; WILCOX deltopectoralis of the humerus (MAYR & ZVONOK 1952). There is a marked sulcus for these tendons 2012), and reduction of the deltopectoral crest is on the plantar surface of the tarsometatarsus, here considered a synapomorphy of Colymboides which is absent in Colymbiculus. and Gavia. With Colymbiculus thus being the sister taxon of the clade (Colymboides