| Ichthyornis | |
|---|---|
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| Cast skeleton,Rocky Mountain Dinosaur Resource Center | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Avialae |
| Clade: | Ornithurae |
| Clade: | †Ichthyornithes |
| Genus: | †Ichthyornis Marsh, 1873 |
| Species: | †I. dispar |
| Binomial name | |
| †Ichthyornis dispar (Marsh, 1872) | |
| Synonyms | |
AngelinornisKashin 1972 Species synonymy
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Ichthyornis (meaning "fish bird", after its fish-like vertebrae) is an extinctgenus of toothyseabird-likeornithuran from the lateCretaceous period ofNorth America. Itsfossil remains are known from thechalks ofAlberta,Alabama,Kansas (Greenhorn Limestone),New Mexico,Saskatchewan, andTexas, in strata that were laid down in theWestern Interior Seaway during theTuronian throughCampanian ages, about 95–83.5million years ago.Ichthyornis is a common component of theNiobrara Formation fauna, and numerous specimens have been found.
Ichthyornis has been historically important in shedding light onbird evolution. It was the first known prehistoric bird relative preserved with teeth, andCharles Darwin noted its significance during the early years of the theory ofevolution.Ichthyornis remains important today as it is one of the fewMesozoiceraornithurans known from more than a few specimens.
It is thought thatIchthyornis was the Cretaceous ecological equivalent of modern seabirds such asgulls,petrels, andskimmers. An average specimen was the size of apigeon, 24 centimetres (9.4 in) long, with a skeletal wingspan (not taking feathers into account) of around 43 centimetres (17 in),[2] though there is considerable size variation among known specimens, with some smaller and some much larger than the type specimen ofI. dispar.[1]
Ichthyornis is notable primarily for its combination of vertebrae which are concave both in front and back (similar to some fish, which is where it gets its name) and several more subtle features of its skeleton which set it apart from its close relatives.Ichthyornis is perhaps most well known for its teeth. The teeth were present only in the middle portion of the upper and lower jaws. The jaw tips had no teeth and were covered in a beak. The beak ofIchthyornis, like thehesperornithids, was compound and made up of several distinct plates, similar to the beak of analbatross, rather than a single sheet ofkeratin as in most modern birds.[3] The teeth were more flattened than the rounded teeth found incrocodilians, though they became wider towards the base of the crown. The tips of the teeth were curved backward and lacked any serrations.[1]They were arranged in a groove, much like those of marine reptiles.[4]
The wings and breastbone were very modern in appearance, suggesting strong flight ability and placing it with modern birds in the advanced groupCarinatae. Unlike earlieravialans such as theenantiornithines, the species appears to have matured to adulthood in a rather short, continuous process.[5]
A study on anIchthyornis endocast reveals that it had a relatively basal brain compared to modern birds, similar to that ofArchaeopteryx and other non-avian theropods. Conversely, it had a palate remarkably convergent with that of modernneognaths.[6]
Ichthyornis fossils have been found in almost all levels of the Niobrara Chalk, from beds dating to the lateConiacian age (about 89 million years ago) to theCampanian age (about 83.5 million years ago).[1][7] Even earlier remains attributed toIchthyornis have been found in theGreenhorn Formation of Kansas, dating to the earlyTuronian age (about 93 million years ago).[2] Specimens ofIchthyornis from earlier eras were, on average, smaller than later ones. The holotype specimen ofIchthyornis dispar, YPM 1450, had a humerus about 58 millimetres (2.3 in) long. In many geologically younger specimens like YPM 1742, the same wing bone was 71.5 millimetres (2.81 in) long. Both the older, smaller specimens, and the more recent, larger specimens show signs that they had reached skeletal maturity and were adults, and came from the same geographic area. It is likely thatIchthyornis dispar as a species increased in size over the several million years it inhabited the Western Interior Seaway ecosystem.[1]
Ichthyornis was one of the first Mesozoicavialans ever found and the first one known to have had teeth, making it an important discovery in the early history of paleontology. It remains important today, as it represents one of the closest non-avian relatives of modern birds, and one of a handful of Mesozoic bird relatives represented by numerous specimens.[1]Ichthyornis was discovered in 1870 byBenjamin Franklin Mudge, a professor fromKansas State Agricultural College who recovered the initial fossils from the North Fork of theSolomon River inKansas, United States. Mudge was a prolific fossil collector who shipped his discoveries to prominent scientists for study.[8] Mudge had previously had a close partnership with paleontologistEdward Drinker Cope of theAcademy of Natural Sciences inPhiladelphia. However, as described by S.W. Williston in 1898, Mudge was soon contacted byOthniel Charles Marsh, Cope's rival in the so-calledBone Wars, a rush to collect and identify fossils in the American West. Marsh wrote to Mudge in 1872 and offered to identify any important fossils free of charge, and to give Mudge sole credit for their discovery. Marsh had been a friend of Mudge when they were younger, so when Mudge learned of Marsh's request, he changed the address on the shipping crate containing theIchthyornis specimen (which had already been addressed to Cope and was ready to be sent), and shipped it to Marsh instead. Marsh had narrowly won the prestige of studying and naming the important fossil at the expense of his rival.[8]
However, Marsh did not initially recognize the true importance of the fossil. Soon after receiving it, he reported back to Mudge his opinion that the chalk slab contained the bones of two distinct animals: a small bird animal, and the toothed jaws of some unknown reptile. Marsh considered the unusual vertebrae of the bird to resemble those of a fish, so he named itIchthyornis, or "fish bird."[9] Later in 1872, Marsh described the toothed jaws as a new species of marine reptile, namedColonosaurus mudgei after their discoverer.[10] The similarity of the lower jaw and teeth to those ofmosasaurs is so great that as late as 1952, J.T. Gregory argued that it really belonged to a diminutive species or young individual related to the genusClidastes.[11]
By early in 1873, Marsh had recognized his error. Through further preparation and exposure of skull bones from the rock, he found that the toothed jaws must have come from the bird itself and not a marine reptile. Due to the previously unknown features ofIchthyornis (vertebrae concave on either side and teeth), Marsh chose to classify it in an entirely new sub-class of birds he called the Odontornithes (or "toothed birds"), and in the new orderIchthyornithes (later Ichthyornithiformes). The only other bird Marsh included in these groups was the newly namedApatornis, which he had previously named as a species ofIchthyornis,I. celer.[12] Mudge later noted the rare and unique quality of these toothed birds (includingHesperornis, which was found to also have teeth by 1877), and the irony of their association with the remains oftoothlesspterosaurs, flying reptiles which were only known to have had teeth in other regions of the world at that time.[13]
Soon after these discoveries,Ichthyornis was recognized for its significance to the theory of evolution recently published by Charles Darwin. Darwin himself told Marsh in an 1880 letter thatIchthyornis andHesperornis offered "the best support for the theory of evolution" since he had first publishedOn the Origin of Species in 1859.[1] (WhileArchaeopteryx was the first known Mesozoic avialan and is now known to have also had teeth, the first specimen with a skull was not described until 1884).[14] Others at the time also recognized the implications of a nearly modern bird with reptilian teeth, and feared the controversy it caused. One Yale student described various men and women urging Marsh to concealIchthyornis from the public because it lent too much support to evolutionary theory.[1] Many accused Marsh of having tampered with the fossils or intentionally created a hoax by associating reptilian jaws with the body of a bird, accusations that continued to surface even as late as 1967. However, an overwhelming majority of researchers have demonstrated that Marsh's interpretation of the fossils was correct, and he was fully vindicated by later finds.[1]
At the turn of the 20th century, thePeabody Museum of Natural History atYale University, where mostIchthyornis specimens were housed, began placing many of its most interesting or important specimens on display in the museum's Great Hall. Two panel mounts (that is, pieces where the skeleton is arranged and set into a plaster slab) were created forIchthyornis; one forI. dispar, and one for "I. victor". Both were created by Hugh Gibb, who prepared many of Marsh's fossils for study and display. TheI. dispar mount contained only theholotype fossils, while the "I. victor" mount was a composite incorporating a variety of different specimens to make the piece appear more complete (it did not, however, contain any part of the actual "I. victor" holotype specimen).[1]
At some point before 1937, the catalogue number of the actual "I. victor" type specimen was mistakenly reassigned to the panel mount. Later reports of the specimen, even by the Peabody Museum's staff, therefore mistakenly stated that the original "I. victor" specimen comprised most of the skeleton, when it was in fact only three bones.[1] By 1997, the situation had become so confused thatJacques Gauthier, the current curator of the museum's vertebrate paleontology collection, authorized the dismantling of both panel mounts. This allowed the bones to be properly sorted out and studied in three dimensions, which had been impossible previously when they were embedded in plaster.[1] A full re-description of these specimens was published by paleontologistJulia Clarke in 2004.[1]
Ichthyornis is close to the ancestry of modern birds, theAves, but represents an independent lineage. It was long believed that it was closely related to some other Cretaceous taxa known from very fragmentary remains –Ambiortus,Apatornis,Iaceornis andGuildavis – but these seem to be closer to the ancestors of modern birds than toIchthyornis dispar. In Clarke's 2004 review, the formerorder Ichthyornithiformes and thefamily Ichthyornithidae are now superseded by the cladeIchthyornithes, which in the paper was also defined according tophylogenetic taxonomy as all descendants of the most recent common ancestor ofIchthyornis dispar and modern birds.[1]
Of the several described species, only one,Ichthyornis dispar, is currently recognized, following the seminal review by Julia Clarke.[1] Marsh had previously named a specimen now attributed toI. dispar asGraculavus anceps. Clarke argued that because the rules for naming animals laid out by theICZN state that a type species for a genus must have originally been included in that genus,Ichthyornis anceps is ineligible to replaceI. dispar as the type species and so must be considered a junior synonym even though it was named first. However, Michael Mortimer pointed out that this is incorrect; whileI. anceps cannot become the type species ofIchthyornis, the ICZN does not preclude it from becoming the senior synonym of the type speciesI. dispar. Therefore,I. anceps should have been considered the correct name for the only recognizedIchthyornis species.[15] All other supposed species ofIchthyornis have not been supported as valid. The presumed"Ichthyornis" lentos, for example, actually belongs into the earlygalliform genusAustinornis.[1]"Ichthyornis" minusculus from theBissekty Formation (Late Cretaceous) ofKyzyl Kum,Uzbekistan, is probably an enantiornithine. All otherIchthyornis species are synonymous withI. dispar.[1]
Thecladogram below is the result of a 2014 analysis by Michael Lee and colleagues that expanded on data from an earlier study by O’Connor & Zhou in 2012. The clade names are positioned based on their definitions.[16]
{{cite journal}}: CS1 maint: multiple names: authors list (link)
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