F Rosa Rubicondior: Evolution! Now a Theory With Teeth!

Wednesday 9 August 2017

Evolution! Now a Theory With Teeth!

Chupkaornis keraorum.
Credit: Masato Hattori
Amateur collectors in Japan discover country's first and oldest fossil diving bird -- ScienceDaily

As a keen fossil hunter in my early days, I dreamed of finding something more than the usual marine shells - the muscles, oysters, bivalves and marine snails that are common in North Oxfordshire limestone deposits - so I can imagine how two Japanese fossil-hunting brothers, Masatoshi and Yasuji Kera, on the island of Hokkaido must have felt when they found nine skeletal elements of what turned out to be the oldest early Asian diving bird.

Fossils of these birds are relatively common in the Northern Hemisphere, but this is the first to be found in Asia and the first from the easterm margin of the Eurasian continent.

Asian hesperornithiforms are extremely rare in contrast to the much more abundant record from North America. In Asia, these fossil birds are only known from fragmentary materials from Mongolia. Here we describe the skeletal remains of a new hesperornithiform Chupkaornis keraorum gen. et sp. nov. from the Upper Cretaceous Kashima Formation (Coniacian to Santonian) of the Yezo Group in Mikasa City, Hokkaido, Japan. This is the best-preserved hesperornithiform material from Asia and it is the first report of hesperornithiforms from the eastern margin of the Eurasian continent. Chupkaornis has a unique combination of characters: finger-like projected tibiofibular crest of femur, deep, emarginated lateral excavation with a sharply defined edge of the ventral margin of the thoracic vertebrae, and the heterocoelous articular surface of the thoracic vertebrae. Our new phylogenetic analysis revises the phylogenetic relationships of Hesperornithiformes. In contrast to previous studies, Enaliornis is assigned as the most basal taxon and Baptornis is positioned as more derived than Brodavis. Chupkaornis is a sister taxon to the clade of Brodavis and higher taxa. Parahesperornis and Hesperornis are positioned within Hesperornithidae, the derived Hesperornithiformes. Many of the skeletal character changes are concentrated at the base of Hesperornithidae (Parahesperornis and more derived taxa), and involve the modification of the pelvic girdle and hind limb morphology (e.g. dorsal directed antitrochanter of pelvis, short and sprawled femur, including probable lobe-toed feet suggested by the specialized distal articular surface of first digit of fourth toe, and predominantly robust digit IV phalanges). These skeletal modifications are likely adaptations for foot-propelled diving behaviour.

What the Kera brothers had discovered were four cervical vertebrae, two thoracic vertebrae, the distal end of the left and right femora, and the middle part of the right fibula, all of the same individual.

Hespeornithiforms is the oldest group of birds that succeeded to adapt for diving in ocean. This study provides better understanding in the early evolution of this group and the origin of diving in birds.

Tanaka Tomonori, Ph.D. student.
Department of Natural History Sciences, Hokkaido University
Hesperornithiformes were foot-propelled Cretaceous diving birds from the northern hemisphere where they were widely distributed. They were the first known birds to have adapted to diving. Their reduced forelimbs suggests they were flightless. By contrast their powerful hind limbs suggests they were powerful swimmers and almost certainly marine predators. They also had teeth.

The discovery of Chupkaornis, the oldest Asian hesperornithiform, suggests that basal hesperornithiform had dispersed to the eastern margin of Asia no later than 90 million to 84 million years old, making them contemporary with dinosaurs. This shows that birds, even as they were evolving into their modern forms from their theropod dinosaur ancestors, they were already diversifying and becoming specialised for the new niches they were moving in to. This group still had teeth, for example.

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