Odontopteryx - Description and Systematics

Description and Systematics

O. toliapica is among the smallest pseudotooth birds known to date – but this still means that to would have rivalled, if not exceeded, most living albatrosses in wingspan and the Brown Pelican (Pelecanus occidentalis) in bulk. In life, its head (including the beak) would have been 20–25 cm (8–10 in) long. Unlike in most other pseudotooth birds, its "teeth" are slanted forwards.

Like those of its relatives, the thin-walled bones of Odontopteryx broke easily and thus very few fossils – though still far more than of the average pseudotooth bird genus – are decently preserved. In combination with its small (for pseudotooth birds) size, some traits allow to identify the present genus. It resembles Dasornis in having a jugal arch that is mid-sized, tapering and stout behind the orbital process of the prefrontal bone, unlike in the large Neogene Osteodontornis. Also, its paroccipital process is much elongated back- and downwards, again like in Dasornis but unlike in Pseudodontornis longirostris. Meanwhile, the distal humerus specimen from Mexico (MHN-UABCS Te5/6–517) which may or may not belongs in the present genus differs from the corresponding bone of Osteodontornis in a more narrow and less excavated surface between the external condyle and the ectepicondylar prominence, with the pit between these closer to the bone's end. Its quadrate bone, meanwhile, differed from that of Osteodontornis in a very broadly grooved dorsal head, a wide main shaft with a strongly curved lateral ridge and a small and somewhat forward-pointing orbital process. The forward center of the quadrate's ventral articulation ridge extends downwards and to the middle, and the pterygoid process is only slightly expanded to the upper center in Odontopteryx. The socket for the quadratojugal is displaced downwards. The quadrate of P. longirostris is not very well preserved; it agees with Odontopteryx in a broad main shaft but is closer to Osteodontornis in the straight main shaft ridge and its upward-directed ventral articulation ridge's forward center. Its quadratojugal socket differs from both.

Odontopteryx differed from Pelagornis (a contemporary of Osteodontornis) and agreed with Dasornis in having a deep and long handward-pointing pneumatic foramen in the fossa pneumotricipitalis of the humerus, a latissimus dorsi muscle attachment site on the humerus that consists of two distinct segments instead of a single long, and a large knob that extends along the ulna where the ligamentum collateralis ventralis attached. Further differences between Odontopteryx and Pelagornis are found in the tarsometatarsus: in the present genus, it lacks a deep fossa of the hallux' first metatarsal bone and its middle toe trochlea is conspicuously expanded forward. The salt glands inside the eye sockets were far less developed in Odontopteryx than in Pelagornis. As the traits shared between Odontopteryx and Dasornis are probably plesiomorphic however, they cannot be used to argue for a closer relationship between the two Paleogene genera than either had with Osteodontornis and/or Pelagornis.

But even though – due to the lack of better-preserved fossils – a close relationship between Odontopteryx and Dasornis cannot be excluded for sure either, it seems that the Neogene pseudotooth birds all derive from a large Paleogene form – such as Dasornis or (if it is not actually identical with Pelagornis) the mysterious P. longirostris – and that the smallish lineage became entirely extinct before the Neogene (perhaps in the Grande Coupure). In 1891 O. toliapica was proposed as type genus of a family Odontopterygidae; recent authors generally place all pseudotooth birds in a single family. But if the evolutionary scenario outlined above is correct, the family name Pelagornithidae could be restricted to the giant lineage, and the Odontopterygidae reestablished as name for the smaller lineage. Macrodontopteryx was initially also included in the Odontopterygidae, but if not a distinct genus it is more likely a young individual of Dasornis. The only smallish Neogene pseudotooth bird known as of 2009 is "Pseudodontornis" stirtoni from New Zealand, which was about the size of O. toliapica. Its relationships are completely obscure.