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Description

A selection of basal avialans known as hesperorniths, to scale.

Birds are unmistakable. Not one modern species could be confused for anything other than being an avian. They all have very characteristic features: feathers, modified forelimbs as wings, hard-shelled eggs, lightweight skeletons, reduced pygostyle tail, a high metabolic rates and a toothless beak. Beaks are obviously not an exclusively avian trait, as other groups have them too (such as turtles, platypus, pufferfish and cephalopods). In birds the bony mandibles are covered in a hard outer layer of keratin, known as the rhamphotheca. This structure has the cutting edges of the beak, which replaces the lack of teeth. Tooth-like projections can be seen in some birds like mergansers, falcons, hummingbirds and the extinct pelagornithids (see False Teeth - Pelagornithids ), but these are not true teeth. However this was not the case in ancestral birds groups.

One group of toothed birds were the Cretaceous hesperorniths. Apart from the earliest species, they were large slender bodied with powerful hindlimbs positioned to the side of the body towards the tail. This is perfect when swimming as it gives the bird alot of forward power, but on land it's a different story. Modern loons have a very similar hindlimb arrangement and, as a result, find walking upright difficult. They are able to push themselves along on their belly, and it can only be assumed that hesperorniths did the same when on land. Hesperornith wings were not of much assistance being probably non-functional and consisting of a rod-like humerus only, in the later species. It was quite possible that the wing bones were not even outside the body wall, although the evidence is not clear-cut. Fossils of Parahesperornis show imprints of thick hairy feathers which were perfect insulation for spending long periods in water. With this combination of features most hesperorniths almost certainly rarely left the water, only coming to shore during breeding season, to raise young and moult. Exception to this rule was the smaller Enaliornis from the Early Cretaceous, which were probably capable of flight.

Inside the mouth were simple teeth which uniquely (for any theropod) sat in a groove rather than individual sockets. The horny rhamphotheca was reduced to the tips of the jaws where the teeth were not present. No extinct bird had teeth within a 'beak', rather that they had a combination of teeth and beak.

The vast majority of species have been found in tropical and subtropical marine deposits across Cretaceous Northern Eurasia and North America with some putative remains from Argentina. Hesperornis is the best known and most speciose form, with one species also known from a freshwater ecosystem. While hesperorniths were not ancestral to modern birds, they were along with several other species such as the flighted Ichthyornis, the sister group to all modern birds. No lineage of toothed bird made it past the K-Pg extinction event.

Description

A selection of basal avialans known as hesperorniths, to scale.

Birds are unmistakable. Not one modern species could be confused for anything other than being an avian. They all have very characteristic features: feathers, modified forelimbs as wings, hard-shelled eggs, lightweight skeletons, reduced pygostyle tail, a high metabolic rates and a toothless beak. Beaks are obviously not an exclusively avian trait, as other groups have them too (such as turtles, platypus, pufferfish and cephalopods). In birds the bony mandibles are covered in a hard outer layer of keratin, known as the rhamphotheca. This structure has the cutting edges of the beak, which replaces the lack of teeth. Tooth-like projections can be seen in some birds like mergansers, falcons, hummingbirds and the extinct pelagornithids (see False Teeth - Pelagornithids ), but these are not true teeth. However this was not the case in ancestral birds groups.

One group of toothed birds were the Cretaceous hesperorniths. Apart from the earliest species, they were large slender bodied with powerful hindlimbs positioned to the side of the body towards the tail. This is perfect when swimming as it gives the bird alot of forward power, but on land it's a different story. Modern loons have a very similar hindlimb arrangement and, as a result, find walking upright difficult. They are able to push themselves along on their belly, and it can only be assumed that hesperorniths did the same when on land. Hesperornith wings were not of much assistance being probably non-functional and consisting of a rod-like humerus only, in the later species. It was quite possible that the wing bones were not even outside the body wall, although the evidence is not clear-cut. Fossils of Parahesperornis show imprints of thick hairy feathers which were perfect insulation for spending long periods in water. With this combination of features most hesperorniths almost certainly rarely left the water, only coming to shore during breeding season, to raise young and moult. Exception to this rule was the smaller Enaliornis from the Early Cretaceous, which were probably capable of flight.

Inside the mouth were simple teeth which uniquely (for any theropod) sat in a groove rather than individual sockets. The horny rhamphotheca was reduced to the tips of the jaws where the teeth were not present. No extinct bird had teeth within a 'beak', rather that they had a combination of teeth and beak.

The vast majority of species have been found in tropical and subtropical marine deposits across Cretaceous Northern Eurasia and North America with some putative remains from Argentina. Hesperornis is the best known and most speciose form, with one species also known from a freshwater ecosystem. While hesperorniths were not ancestral to modern birds, they were along with several other species such as the flighted Ichthyornis, the sister group to all modern birds. No lineage of toothed bird made it past the K-Pg extinction event.