An ancient genus of toothy seabird-like ornithurans from the late Cretaceous era of North America is called Ichthyornis, which means "fish bird" due to their fish-like vertebrae. From layers deposited in the Western Interior Seaway from the Turonian to Campanian eras, around 95–83.5 million years ago, fossilized remnants of this species have been identified in the chalks of Alberta, Alabama, Kansas (Greenhorn Limestone), New Mexico, Saskatchewan, and Texas. Many examples of Ichthyornis have been discovered, and it is a common species in the fauna of the Niobrara Formation. In the past, Ichthyornis has proven crucial in illuminating the development of birds. Charles Darwin recognized the significance of this early ancient bird cousin preserved with teeth when he initially proposed the idea of evolution. Being one of the few ornithurans from the Mesozoic age known from more than a few specimens, Ichthyornis is still significant today.
Description of Ichthyornis
Ichthyornis is considered the ecological counterpart of contemporary seabirds including gulls, petrels, and skimmers during the Cretaceous period. The average specimen measured 24 centimeters (9.4 in) in length and had a skeletal wingspan of approximately 43 centimeters (17 in) when feathers are excluded. However, there is a significant range in size among the known specimens, with some being significantly smaller and others much larger than the type specimen of I. dispar. The combination of vertebrae that are concave in both the front and back, giving it the name Ichthyornis, is what makes it remarkable. Other minor characteristics of its skeleton also distinguish it from its near relatives. Maybe the most well-known feature of Ichthyornis is its teeth. The central region of the upper and lower jaws was the sole area with teeth. The tops of the jaws were beaked and toothless. Unlike most contemporary birds, Ichthyornis's beak was composed of many discrete plates, like the beak of an albatross, as opposed to a single sheet of keratin like that of hesperornithids. The teeth were broader around the base of the crown, although they were more flattened than the rounded teeth of crocodilians. The teeth's tips were not serrated; instead, they were bent backward. The very contemporary appearance of the wings and breastbone suggested a great capacity to fly, placing it in the advanced group Carinatae alongside modern birds. In contrast to previous avialans like enantiornithines, the species seems to have reached adulthood by a quick, continuous process. According to research on an endocast of Ichthyornis, the species' brain was comparable to that of Archaeopteryx and other non-avian theropods, and it was rather basic in comparison to current birds. On the other hand, its palette was very similar to that of contemporary neognaths.
Timespan
Fossils of Ichthyornis have been discovered in nearly every stratum of the Niobrara Chalk, ranging from beds that date from the Campanian era (about 83.5 million years ago) to the late Coniacian age (about 89 million years ago). In Kansas, even older remnants from the early Turonian period—roughly 93 million years ago—have been discovered in the Greenhorn Formation. These bones are assigned to Ichthyornis. The average size of Ichthyornis specimens varied between earlier and later times. The humerus of Ichthyornis dispar's holotype specimen, YPM 1450, was around 58 millimeters (2.3 inches) in length. The same wing bone measured 71.5 millimeters (2.81 inches) in length in several geologically younger specimens, such as YPM 1742.
History of Study
In the early history of paleontology, Ichthyornis was a significant find as it was the first known Mesozoic avian and the first to be discovered with teeth. Being one of the few Mesozoic bird cousins represented by a large number of specimens, it is nevertheless significant today as one of the closest non-avian ancestors of current birds. Benjamin Franklin Mudge, a professor at Kansas State Agricultural College, found Ichthyornis in 1870 after he collected the first fossils from the North Fork of the Solomon River in Kansas, USA. Mudge collected fossils extensively and sent his finds to eminent experts for analysis.
Classification
Although Ichthyornis is a separate lineage, it is closely related to the Aves, the ancestors of modern birds. It was long thought to be closely related to several other Cretaceous species, including as Ambiortus, Apatornis, Iaceornis, and Guildavis, which are known from extremely fragmented remains; however, they appear to be more closely connected to the progenitors of contemporary birds than to Ichthyornis dispar. According to phylogenetic taxonomy, the clade Ichthyornithes, which is defined as all descendants of the most recent common ancestor of Ichthyornis dispar and modern birds, has replaced the former order Ichthyornithiformes and the family Ichthyornithidae in Clarke's 2004 review. After Julia Clarke's landmark assessment, only one of the numerous identified species—Ichthyornis dispar—is now acknowledged. Marsh had previously designated as Graculavus anceps a specimen that is currently assigned to I. dispar. Clarke argued that because the rules for naming animals laid out by the ICZN state that a type species for a genus must have originally been included in that genus, Ichthyornis anceps is ineligible to replace I. dispar as the type species and so must be considered a junior synonym even though it was named first. The ICZN does not prevent I. anceps from becoming the senior synonym of the type species I. dispar, however Michael Mortimer pointed out that this is untrue. I. anceps cannot become the type species of Ichthyornis. Thus, Ichthyornis anceps should have been the proper name for the one species of Ichthyornis that has been identified. None of the other proposed Ichthyornis species have been shown to be genuine. One such example is the supposed "Ichthyornis" lentos, which is actually a member of the early galliform genus Austinornis. It is likely that "Ichthyornis" minusculus, which is found in Kyzyl Kum, Uzbekistan's Bissekty Formation (Late Cretaceous), is an enantiornithine. Ichthyornis dispar is synonymous with all other species. The data from a 2012 research by O'Connor & Zhou were expanded upon in a 2014 analysis conducted by Michael Lee and colleagues, yielding the cladogram that is shown below. Clade names are arranged according to their meanings.
| Ornithurae |
| ||||||||||||||||||
Reconstruction of Ichthyornis.
Cast skeleton of Ichthyornis which is in the Rocky Mountain Dinosaur Resource Center.
Sources[1]