Variations in the hypotarsus morphology of birds and their evolutionary significance

Abstract

Mayr, G. 2016. Variations in the hypotarsus morphology of birds and their evolutionary significance -Acta Zoologica (Stockholm) 97: 196-210. Crown group (neornithine birds) exhibit a great variation in the morphology of the hypotarsus, a structure on the proximal end of the tarsometatarsus, which guides the tendons of the flexor muscles of the toes. Hypotarsus morphology is of significance for the identification of fossil taxa, and several extant groups show characteristic patterns that are of phylogenetic interest. So far, however, the diversity of hypotarsus morphologies has been little studied, and there are no comprehensive studies across all neornithine birds. In particular, the identities of the involved canals and sulci remain elusive, and some confusion exists about their correct homologies. In this study, hypotarsus morphologies are for the first time surveyed among all extant birds, and basic patterns are characterized. Instances are identified, in which particular hypotarsus morphologies are correlated with certain locomotion types, inferences are made about possible ancestral morphologies, and some patterns of phylogenetic interest are discussed.Osteological comparisons are based on skeletons in the collection of the Senckenberg Research Institute Frankfurt and the American Museum of Natural History (AMNH; few passerines). In many cases, identification of the number of tendons running through particular canals or sulci was facilitated by the examination of incompletely macerated specimens with the dried tendons still in place. For Phalacrocorax carbo, tendinal identity was verified by dissection of a fresh bird. Particular attention has been paid to the selection of adult birds for the examination of hypotarsus morphology. Examined skeletonsConcerning extant taxa, the bony hypotarsus of the following taxa was studied (the sequence of non-passeriform taxa follows Cracraft 2013):Comparative hypotarsus morphology Mayr Acta Zoologica (Stockholm) 97: 196-210 (April 2016)

Three terrestrial Pleistocene coucals (Centropus: Cuculidae) from southern Australia: biogeographical and ecological significance

“…The tarsometatarsus of Centropus is elongate, with a length more than six times greater than the proximal width ( Eudynamys and Scythrops have tarsometatarsi with a length ≤5 times greater than proximal width). As in all cuculids, the hypotarsus is bicanaliculate (Mayr, ), having two large canals of subequal size (medial slightly larger) that are completely enclosed and are placed side by side; the hypotarsus is offset laterally, sitting almost entirely plantar of the cotyla lateralis and the eminentia intercondylaris; the plantar surface of the hypotarsus is a flattened disc; the eminentia intercotylaris is narrow and very prominent proximally, and projects proximal of the area intercondylaris; the margins of the cotylae lateralis et medialis have a square appearance in proximal aspect, with the cotyla lateralis narrower and more dorsoplantarly elongate than the cotyla medialis; the fossae parahypotarsalis medialis et lateralis are deep, the medial one particularly so, with the excavation continuing distally down most of the plantar surface of the shaft, and the fossae are separated from one another by a ridge that originates from the crista intermedia hypotarsi and extends distally about half way down the plantar surface; the cristae plantares medialis et lateralis are sharp; two foramina vascularia proximalia are present, with their relative proximodistal placement varying amongst species; the sulcus extensorius is deeply excavated; the tuberositas m. tibialis cranialis is placed medially, either on the medial wall of the sulcus extensorius or at the junction of the sulcus and its medial wall, immediately distal of the foramen vasculare proximalis medialis in some species, but with a distinct gap seen in others; the foramen vasculare distale is large; the canalis interosseus distalis is absent, but a deep dorsal groove runs between trochleae metatarsi III and IV merging with the foramen vasculare distale proximally; the fossa metatarsi I is deep and elongate, but does not excavate the medial profile of the shaft in dorsal/plantar view; trochlea metatarsi II is ball‐like and ungrooved, and is less distally extended than trochlea metatarsi III; trochlea metatarsi III is wide with a broad, deep medial groove, and is the most distally projecting trochlea; trochlea metatarsi IV is very short, and has a large trochlea accessoria that is strongly recurved medially and overhangs the fossa supratrochlearis plantaris.…”

Cellular preservation of musculoskeletal specializations in the Cretaceous bird Confuciusornis

“…We infer that there was a transition between the ancestral state in reptiles, in which digital flexor tendons contribute to the complex plantar aponeurosis438, and an intermediate state in early ornithuromorph birds, involving an incipient osseous hypotarsus and tibial cartilage as interpreted here (Fig. 5)431. The largely cartilaginous nature of these two structures in Confuciusornis hints that their derived state in crown-group birds may have evolved from the gradual fusion of mineralized cartilage, similar to a traction epiphysis—a pre-existing intratendinous (‘metaplastic') mineralization that has become secondarily fused to the main bone39.…”

“…6). The ridge occurs at the position of the hypotarsus in ornithuromorph birds31, but it is less distinct and more cartilaginous compared with the latter structure, even with those seen in early ornithuromorph birds, such as the Late Cretaceous Patagopteryx, Pengornis , Yixianornis , Apsaravis and Ichthyornis , which have a flat bony projection or unprojected discrete surface without canals and sulci323334. In contrast, the ridge resembles an intermediate state of the hypotarsus in the ontogeny of extant birds, in which the hypotarsus remains cartilaginous until the latest stages or after hatching, when it ossifies from a separate centre located on its distal medial corner3536.…”

Another look at tarsometatarsi of early penguins

“…The hypotarsus, a feature of the plantar side of the proximal tarsometatarsus, is of significance for penguin taxonomy, especially in extinct taxa (e.g., Myrcha et al 2002;Ksepka et al 2012; see also Acosta Hospitaleche and Jadwiszczak 2011). This structure, modeled by the interaction with tendons, in most present−day and some early (Delphinornis) penguins is, using Mayr's (2015) terminology, essentially monosulcate, utterly dominated by the hypotarsal sulcus bordered by the medial crest and lateral tubercle made of lateral and intermedial crests, barely recognizable or undistinguishable (Fig. 6).…”