Hesperornithes
Encyclopedia
Hesperornithes is an extinct and highly specialized clade
of Cretaceous
toothed bird
s. Hesperornithine birds, apparently limited to former aquatic habitats in the Northern Hemisphere, include genera
such as Hesperornis, Parahesperornis, Baptornis, Enaliornis, and probably Potamornis, all strong-swimming predatory waterbirds. Many, if not all species were completely flightless. The largest known hesperornithine, described in 1999 and named Canadaga arctica, may have reached a maximum adult length of over 1.5 meters (five feet).
Hesperornithine birds were the only Mesozoic
birds to colonize the oceans. Hesperornithine birds went extinct at the K-T boundary, along with enantiornithine protobirds, all non-avian dinosaurs, and many other mostly reptillian life forms.
s for analysis. In most cases, what holds true for one hesperornithine also applies to the others, as these birds were quite stereotypical and extreme in their autapomorphies.
Although some of the smaller species might have been able to fly, Hesperornis and Baptornis had only vestigial wings. Like living foot-propelled diving birds, the femur and metatarsus were short, whereas the tibia was long. The legs were also set far back on the body, as in loon
s, grebe
s or penguin
s. Hesperornithids must have been powerful swimmers and divers but extremely ungainly on the land, and probably spent little time ashore except to nest.
Some researchers think that on land they had to slide on their bellies and push with their legs; the hip and knee joints were shaped such that these birds could not move them dorsoventrally, and in a resting position the feet projected sideways from the body, which would have prevented them from walking upright. Whereas Hesperornis had probably lobes of skin on its toes similar to the grebe
s of today, the toes of Baptornis could not rotate (necessary in lobed feet to reduce drag) and thus were more likely webbed, as in loon
s. The dense bones of these birds decreased their buoyancy, making diving easier.
The beak was long, probably hooked at the tip, and bore a series of simple, sharp teeth which were set into a longitudinal groove. These probably helped to seize fish
, as does the serrated beak of mergansers today; unlike the reptilian teeth of other toothed birds, those of the hesperornithids were unique. Given that teeth are induced by a rather simple genetic mechanism, it is even possible that these birds re-evolved them and that some ancestors of the hesperornithids were toothless.
in the paraphyletic "Odontornithes
" by Othniel Charles Marsh
, in 1873. In 1875, they were separated as Odontolcae. The group was often considered to be allied to loons and grebes,
or to the Paleognathae
. These similarities, however, as the more recently determined fact that the osteon
s of their bones - at least in Hesperornis - were arranged in a pattern similar to that in Neognathae
, are today considered to be due to convergent evolution
.
Currently, the hesperornithine birds are recognized as a very specialized lineage that is not ancestral or otherwise closely related to the birds of today. Still, their relationship is close enough that they probably diverged from the ancestors of today's birds as late as the earliest Cretaceous
.
The discovery that Gansus yumenensis, which lived some time before the main radiation of hesperornithids, was a primitive member of the Ornithurae
- that is, birds which had developed the type of pygostyle
that is also found in modern birds - suggests that the ancestors of Hesperornithes - which also ornithuran - may have been birds generally similar to Gansus and living some 130-150 mya. Given the tendency of birds that have become flightless to increase in size markedly in just a few million years, the probability of Hesperornithes deriving from an animal at least superficially similar and not too far related to Gansus is quite high. The new fossil material of that species should assist in determining the number of synapomorphies
of hesperornithines and other ornithurans, which have until now been muddied by the numerous autapomorphies of the former.
Loss and/or fusion of caudal vertebrae in pygostyle
-like structures was a general trend in Cretaceous avian
evolution, and a full pygostyle and associated structures may have evolved more than once to similar shapes. The fact that Gansus had non-pneumatized, dense bones, like those of hesperornithines, although it was not a specialized diver, is interesting to note. Similarly, the bone structure of Hesperornis indicates that as opposed to Enantiornithes
and in line with other Ornithurae it showed rapid, uninterrupted growth to adult size.
The earliest known hesperornithine is the Early Cretaceous
Enaliornis
although these birds are somewhat tentatively assigned to this group because of the bad preservation of their remains; the majority of hesperornithine taxa
are known from the Late Cretaceous
. Small hesperornithine bones are known from the freshwater deposits of the Late Cretaceous of the Judith River Group
as well as the Hell Creek
and Lance Formation
s as well as Eurasia
n sites. These birds were about the size of a cormorant or a loon. Less well-verified is the presence of the lineage around Antarctica, which was a temperate
, ice-free region in their time.
Subclass HESPERORNITHES
This is usually treated as equivalent with Hesperornithiformes. However, given the uncertainties of the evolution of teeth and flightlessness in the Hesperornithes, as noted above, it is here considered a distinct group, so that ancestral taxa do not have to be placed together with more advanced ones.
Sometimes assigned to the Hesperornithes, but actual relationships unclear
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
toothed bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s. Hesperornithine birds, apparently limited to former aquatic habitats in the Northern Hemisphere, include genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
such as Hesperornis, Parahesperornis, Baptornis, Enaliornis, and probably Potamornis, all strong-swimming predatory waterbirds. Many, if not all species were completely flightless. The largest known hesperornithine, described in 1999 and named Canadaga arctica, may have reached a maximum adult length of over 1.5 meters (five feet).
Hesperornithine birds were the only Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
birds to colonize the oceans. Hesperornithine birds went extinct at the K-T boundary, along with enantiornithine protobirds, all non-avian dinosaurs, and many other mostly reptillian life forms.
Anatomy and ecology
Most of what is known about this group rests on analyses of single species, as few provide sufficiently diverse fossilFossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s for analysis. In most cases, what holds true for one hesperornithine also applies to the others, as these birds were quite stereotypical and extreme in their autapomorphies.
Although some of the smaller species might have been able to fly, Hesperornis and Baptornis had only vestigial wings. Like living foot-propelled diving birds, the femur and metatarsus were short, whereas the tibia was long. The legs were also set far back on the body, as in loon
Loon
The loons or divers are a group of aquatic birds found in many parts of North America and northern Eurasia...
s, grebe
Grebe
A grebe is a member of the Podicipediformes order, a widely distributed order of freshwater diving birds, some of which visit the sea when migrating and in winter...
s or penguin
Penguin
Penguins are a group of aquatic, flightless birds living almost exclusively in the southern hemisphere, especially in Antarctica. Highly adapted for life in the water, penguins have countershaded dark and white plumage, and their wings have become flippers...
s. Hesperornithids must have been powerful swimmers and divers but extremely ungainly on the land, and probably spent little time ashore except to nest.
Some researchers think that on land they had to slide on their bellies and push with their legs; the hip and knee joints were shaped such that these birds could not move them dorsoventrally, and in a resting position the feet projected sideways from the body, which would have prevented them from walking upright. Whereas Hesperornis had probably lobes of skin on its toes similar to the grebe
Grebe
A grebe is a member of the Podicipediformes order, a widely distributed order of freshwater diving birds, some of which visit the sea when migrating and in winter...
s of today, the toes of Baptornis could not rotate (necessary in lobed feet to reduce drag) and thus were more likely webbed, as in loon
Loon
The loons or divers are a group of aquatic birds found in many parts of North America and northern Eurasia...
s. The dense bones of these birds decreased their buoyancy, making diving easier.
The beak was long, probably hooked at the tip, and bore a series of simple, sharp teeth which were set into a longitudinal groove. These probably helped to seize fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, as does the serrated beak of mergansers today; unlike the reptilian teeth of other toothed birds, those of the hesperornithids were unique. Given that teeth are induced by a rather simple genetic mechanism, it is even possible that these birds re-evolved them and that some ancestors of the hesperornithids were toothless.
Systematics and evolution
These birds were originally combined with IchthyornisIchthyornis
Ichthyornis is a genus of toothed seabirds from the Late Cretaceous of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansas, New Mexico, Saskatchewan, and Texas, in strata that were laid down in the Western Interior Seaway during the Turonian-Campanian ages,...
in the paraphyletic "Odontornithes
Odontornithes
Odontornithes is an obsolete and disused taxonomic term proposed by O. C. Marsh for birds possessing teeth, notably the genera Hesperornis and Ichthyornis from the Cretaceous deposits of Kansas....
" by Othniel Charles Marsh
Othniel Charles Marsh
Othniel Charles Marsh was an American paleontologist. Marsh was one of the preeminent scientists in the field; the discovery or description of dozens of news species and theories on the origins of birds are among his legacies.Born into a modest family, Marsh was able to afford higher education...
, in 1873. In 1875, they were separated as Odontolcae. The group was often considered to be allied to loons and grebes,
or to the Paleognathae
Paleognathae
The Palaeognathae or paleognaths are one of the two living superorders of birds. The other living superorder is Neognathae. Together these two clades form the subclass Neornithes....
. These similarities, however, as the more recently determined fact that the osteon
Osteon
The osteon, or Haversian system, is the fundamental functional unit of much compact bone. Osteons, roughly cylindrical structures that are typically several millimeters long and around 0.2mm in diameter, are present in many of the bones of most mammals, birds, reptiles, and amphibians.Each osteon...
s of their bones - at least in Hesperornis - were arranged in a pattern similar to that in Neognathae
Neognathae
Neognaths are birds within the subclass Neornithes of the class Aves. The Neognathae include virtually all living birds; their sister taxon Palaeognathae contains the tinamous and the flightless ratites....
, are today considered to be due to convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
Currently, the hesperornithine birds are recognized as a very specialized lineage that is not ancestral or otherwise closely related to the birds of today. Still, their relationship is close enough that they probably diverged from the ancestors of today's birds as late as the earliest Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
.
The discovery that Gansus yumenensis, which lived some time before the main radiation of hesperornithids, was a primitive member of the Ornithurae
Ornithurae
Ornithurae is the name of a natural group which includes all modern birds as well as their extinct relatives with plough-shaped pygostyles, a bone at the end of the tail which allows the tail feathers to fan and retract....
- that is, birds which had developed the type of pygostyle
Pygostyle
Pygostyle refers to a number of the final few caudal vertebrae fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these....
that is also found in modern birds - suggests that the ancestors of Hesperornithes - which also ornithuran - may have been birds generally similar to Gansus and living some 130-150 mya. Given the tendency of birds that have become flightless to increase in size markedly in just a few million years, the probability of Hesperornithes deriving from an animal at least superficially similar and not too far related to Gansus is quite high. The new fossil material of that species should assist in determining the number of synapomorphies
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
of hesperornithines and other ornithurans, which have until now been muddied by the numerous autapomorphies of the former.
Loss and/or fusion of caudal vertebrae in pygostyle
Pygostyle
Pygostyle refers to a number of the final few caudal vertebrae fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these....
-like structures was a general trend in Cretaceous avian
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
evolution, and a full pygostyle and associated structures may have evolved more than once to similar shapes. The fact that Gansus had non-pneumatized, dense bones, like those of hesperornithines, although it was not a specialized diver, is interesting to note. Similarly, the bone structure of Hesperornis indicates that as opposed to Enantiornithes
Enantiornithes
Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...
and in line with other Ornithurae it showed rapid, uninterrupted growth to adult size.
The earliest known hesperornithine is the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
Enaliornis
Enaliornis
Enaliornis is a genus of hesperornithine bird which lived in the late Early Cretaceous, making it the oldest known hesperornithine. Fossils have been found near Cambridge, England...
although these birds are somewhat tentatively assigned to this group because of the bad preservation of their remains; the majority of hesperornithine taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
are known from the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
. Small hesperornithine bones are known from the freshwater deposits of the Late Cretaceous of the Judith River Group
Judith River Group
The Judith River Group is a group of geologic formations in western North America dating from the late Cretaceous and noted as a site for the extensive excavation of dinosaur fossils. The formation is named after the Judith River in Montana. The group is also called the Judith River Wedge...
as well as the Hell Creek
Hell Creek Formation
The Hell Creek Formation is an intensely-studied division of Upper Cretaceous to lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana...
and Lance Formation
Lance Formation
The Lance Formation is a division of Late Cretaceous rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the latest Mesozoic vertebrate faunas...
s as well as Eurasia
Eurasia
Eurasia is a continent or supercontinent comprising the traditional continents of Europe and Asia ; covering about 52,990,000 km2 or about 10.6% of the Earth's surface located primarily in the eastern and northern hemispheres...
n sites. These birds were about the size of a cormorant or a loon. Less well-verified is the presence of the lineage around Antarctica, which was a temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...
, ice-free region in their time.
Systematics
This is usually treated as equivalent with Hesperornithiformes. However, given the uncertainties of the evolution of teeth and flightlessness in the Hesperornithes, as noted above, it is here considered a distinct group, so that ancestral taxa do not have to be placed together with more advanced ones.
- Basal and unresolved taxa
- Genus AsiahesperornisAsiahesperornisAsiahesperornis is a prehistoric flightless bird genus from the Late Cretaceous. The single known species is Asiahesperornis bazhanovi. It lived in what today is Kazakhstan, at its time the shores of the shallow Turgai Sea....
(Late Cretaceous) - hesperornithid? - Genus JudinornisJudinornisJudinornis is a prehistoric flightless bird genus from the Late Cretaceous. The single known species is Judinornis nogontsavensis...
(Late Cretaceous) - Genus PasquiaornisPasquiaornisPasquiaornis is a prehistoric flightless bird genus from the Late Cretaceous. It lived during the late Cenomanian, some 95-93 million years ago....
(Late Cretaceous) - Hesperornithes gen. et sp. indet. (Zhuravlovskaya/Zhuravlevo Late Cretaceous of Kushmurun, Kazakhstan)
- Hesperornithes gen. et sp. indet. TMP 89.81.12 (Dinosaur Park Late Cretaceous of Iddesleigh, Canada)
- Genus Asiahesperornis
- Order HESPERORNITHIFORMES
- Family Enaliornithidae
- Genus EnaliornisEnaliornisEnaliornis is a genus of hesperornithine bird which lived in the late Early Cretaceous, making it the oldest known hesperornithine. Fossils have been found near Cambridge, England...
(Early Cretaceous)
- Genus Enaliornis
- Family Baptornithidae
- Genus BaptornisBaptornisBaptornis is an extinct genus of flightless aquatic bird from the Late Cretaceous, some 87-80 million years ago . The fossils of Baptornis advenus, the type species, were discovered in Kansas, which at its time was mostly covered by the Western Interior Seaway, a shallow shelf sea...
(Late Cretaceous) - includes Parascaniornis
- Genus Baptornis
- Family Hesperornithidae
- Genus HesperornisHesperornisHesperornis is a genus of flightless aquatic birds that spanned the first half of the Campanian age of the Late Cretaceous period . One of the lesser-known discoveries of the paleontologist O. C. Marsh in the late 19th century Bone Wars, it was an important early find in the history of avian...
(Late Cretaceous) - Genus ParahesperornisParahesperornisParahesperornis is a prehistoric flightless bird genus from the Late Cretaceous. Its range in space and time may have been extensive, but its remains are rather few and far between, at least compared with its contemporary relatives in Hesperornis. Remains are known from central North America,...
(Late Cretaceous) - Genus CanadagaCanadagaCanadaga is a flightless bird genus from the Late Cretaceous. The single known species is Canadaga arctica. It lived in the shallow seas around what today is Bylot Island in Nunavut, Canada...
(Late Cretaceous) - Genus Coniornis (Late Cretaceous) - sometimes included in Hesperornis
- Genus Hesperornis
- Family Enaliornithidae
Sometimes assigned to the Hesperornithes, but actual relationships unclear
- "cf. ParahesperornisParahesperornisParahesperornis is a prehistoric flightless bird genus from the Late Cretaceous. Its range in space and time may have been extensive, but its remains are rather few and far between, at least compared with its contemporary relatives in Hesperornis. Remains are known from central North America,...
" (Nemegt Late Cretaceous of Tsagaan Kushu, Mongolia) - hesperornithiform (hesperornithid)? - Genus NeogaeornisNeogaeornisNeogaeornis is a controversial prehistoric genus of diving bird. The single known species, Neogaeornis wetzeli, was described from fossils found in the Campanian to Maastrichtian Quiriquina Formation of Chile. It lived about 70-67 million years ago...
(Late Cretaceous) - baptornithid, neornithine (gaviiform, procellariiform)? - Genus PotamornisPotamornisPotamornis is a prehistoric bird genus that dated back to the late Maastrichtian. Its scrappy remains were found in the Lance Formation at Buck Creek, USA, and a single species has been named and described in 2001: Potamornis skutchi....
(Lance Late Cretaceous of Buck Creek, USA) - hesperornithid? baptornithid? enaliornithid?