Temporal paradox (paleontology)
Encyclopedia
The temporal paradox, or time problem is a controversial issue in the evolutionary relationships of birds. It was described by paleornithologist
Alan Feduccia
.
and those believed to be most closely related to birds are known mostly from the Cretaceous
, by which time birds had already evolved and diversified. If bird-like dinosaurs are the ancestors of birds they should, then, be older than birds, but Archaeopteryx
is 155 million years old, while the very bird-like Deinonychus
is 35 million years younger. This idea is sometimes summarized as "you can't be your own grandmother". As Dodson
pointed out:
First, no one has proposed that maniraptoran dinosaurs of the Cretaceous are the ancestors of birds. They have merely found that dinosaurs like dromaeosaurs, troodontids and oviraptorosaurs are close relatives of birds. The true ancestors are thought to be older than Archaeopteryx, perhaps Early Jurassic
or even older. The scarcity of maniraptoran fossils from then is not surprising since fossilization is a rare event requiring special circumstances, and we may never find fossils of animals in sediments from ages that they actually inhabited.
Second, fragmentary remains of maniraptoran dinosaurs actually had been known from Jurassic deposits in China, North America, and Europe for many years. For example, the femur of a tiny maniraptoran from the Late Jurassic
of Colorado was reported by Padian and Jensen in 1989 while teeth of dromaeosaurids and troodontids are known from Jurassic England
.. Complete skeletons of Middle-Late Jurassic maniraptorans were subsequently described from China. The known diversity of pre-Tithonian
(and thus pre-Archaeopteryx) non-avian maniraptorans includes Ornitholestes
, the possible therizinosaur
Eshanosaurus
, the troodontids
Anchiornis
and the as yet un-named Morrison
WDC DML 001
, the scansoriopterygids
Epidexipteryx
and Epidendrosaurus and the basal alvarezsaur
Haplocheirus
.
Third, if the temporal paradox would indicate that birds should not have evolved from dinosaurs, then what animals are more likely ancestors considering their age? Brochu and Norell (2001) analyzed this question using six of the other archosaurs that have been proposed as bird ancestors, and found that all of them create temporal paradoxes — long stretches between the ancestor and Archaeopteryx where there are no intermediate fossils — that are actually worse. The MSM value for the theropod option was 0.438 - 0.466.
However, because of computational limitations, six taxa considered in SCI and SMIG calculations (Compsognathus, Eoraptor, Herrerasauridae, Marasuchus, Pseudolagosuchus, and Choristodera) were not included in calculation of MSM. Brochu and Norell (2001)
Thus, even if one used the logic of the temporal paradox, one should still prefer dinosaurs as the ancestors to birds.
Pol and Norell (2006) calculated MSM* values for the same proposed bird ancestors and obtained the same relative results. The MSM* value for the theropod option was 0.31 - 0.40 .
Paleornithology
Paleornithology also known as Avian Paleontology is the scientific study of bird evolution and fossil birds. It is a mix of ornithology and paleontology. Paleornithology began with the discovery of Archaeopteryx. The reptilian relationship of birds and their ancestors, the theropod dinosaurs, are...
Alan Feduccia
Alan Feduccia
Alan Feduccia is a paleornithologist, specializing in the origins and phylogeny of birds. He is now Professor Emeritus at the University of North Carolina. Feduccia's principal authored works include two books, The Age of Birds and The Origin and Evolution of Birds, and numerous papers in various...
.
Objection to consensus
The concept of a "temporal paradox" is based on the following facts. The consensus view is that birds evolved from dinosaurs, but the most bird-like dinosaurs, including almost all of the feathered dinosaursFeathered dinosaurs
The realization that dinosaurs are closely related to birds raised the obvious possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that clearly non-avialan dinosaur fossils were discovered with preserved feathers...
and those believed to be most closely related to birds are known mostly from the 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...
, by which time birds had already evolved and diversified. If bird-like dinosaurs are the ancestors of birds they should, then, be older than birds, but Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
is 155 million years old, while the very bird-like Deinonychus
Deinonychus
Deinonychus was a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter long dinosaur lived during the early Cretaceous Period, about 115–108 million years ago . Fossils have been recovered from the U.S...
is 35 million years younger. This idea is sometimes summarized as "you can't be your own grandmother". As Dodson
Peter Dodson
Peter Dodson is an American paleontologist who has published many papers and written and collaborated on books about dinosaurs. An authority on Ceratopsians, he has also authored several papers and textbooks on hadrosaurs and sauropods, and is a co-editor of The Dinosauria, widely considered the...
pointed out:
I hasten to add that none of the known small theropods, including DeinonychusDeinonychusDeinonychus was a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter long dinosaur lived during the early Cretaceous Period, about 115–108 million years ago . Fossils have been recovered from the U.S...
, DromaeosaurusDromaeosaurusDromaeosaurus was a genus of theropod dinosaur which lived during the Late Cretaceous period , sometime between 76.5 and 74.8 million years ago, in the western United States and Alberta, Canada. The name means 'running lizard'....
, VelociraptorVelociraptorVelociraptor is a genus of dromaeosaurid theropod dinosaur that existed approximately 75 to 71 million years ago during the later part of the Cretaceous Period. Two species are currently recognized, although others have been assigned in the past. The type species is V. mongoliensis; fossils...
, UnenlagiaUnenlagiaUnenlagia is a genus of, possibly dromaeosaurid, theropod dinosaur from the Late Cretaceous of Argentina....
, nor SinosauropteryxSinosauropteryxSinosauropteryx was the first genus of dinosaur outside of Avialae to be found with evidence of feathers. They were covered with "furry" coats of very simple filament-like feathers...
, ProtarchaeopteryxProtarchaeopteryxProtarchaeopteryx is a genus of turkey-sized feathered theropod dinosaur from China. Known from the Jianshangou bed of the Yixian Formation, it lived during the early Aptian age of the Early Cretaceous, approximately 124.6 million years ago...
, nor CaudipteryxCaudipteryxCaudipteryx is a genus of peacock-sized theropod dinosaurs that lived in the Aptian age of the early Cretaceous Period . They were feathered and remarkably birdlike in their overall appearance....
is itself relevant to the origin of birds; these are all CretaceousCretaceousThe 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...
fossils ... and as such can at best represent only structural stages through which an avian ancestor may be hypothezised to have passed.
Problems
Numerous researchers have discredited the idea of the temporal paradox. Witmer (2002) summarized this critical literature by pointing out that there are at least three lines of evidence that contradict it.First, no one has proposed that maniraptoran dinosaurs of the Cretaceous are the ancestors of birds. They have merely found that dinosaurs like dromaeosaurs, troodontids and oviraptorosaurs are close relatives of birds. The true ancestors are thought to be older than Archaeopteryx, perhaps Early Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
or even older. The scarcity of maniraptoran fossils from then is not surprising since fossilization is a rare event requiring special circumstances, and we may never find fossils of animals in sediments from ages that they actually inhabited.
Second, fragmentary remains of maniraptoran dinosaurs actually had been known from Jurassic deposits in China, North America, and Europe for many years. For example, the femur of a tiny maniraptoran from the Late Jurassic
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...
of Colorado was reported by Padian and Jensen in 1989 while teeth of dromaeosaurids and troodontids are known from Jurassic England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
.. Complete skeletons of Middle-Late Jurassic maniraptorans were subsequently described from China. The known diversity of pre-Tithonian
Tithonian
In the geologic timescale the Tithonian is the latest age of the Late Jurassic epoch or the uppermost stage of the Upper Jurassic series. It spans the time between 150.8 ± 4 Ma and 145.5 ± 4 Ma...
(and thus pre-Archaeopteryx) non-avian maniraptorans includes Ornitholestes
Ornitholestes
Ornitholestes was a small theropod dinosaur of the late Jurassic of Western Laurasia . To date, it is known only from a single partial skeleton, and badly crushed skull found at the Bone Cabin Quarry near Medicine Bow, Wyoming, in 1900...
, the possible therizinosaur
Therizinosaur
Therizinosaurs are theropod dinosaurs belonging to the clade Therizinosauria. Therizinosaur fossils have been found in Early through Late Cretaceous deposits in Mongolia, the People's Republic of China and Western North America...
Eshanosaurus
Eshanosaurus
Eshanosaurus is a possible genus of therizinosaurian dinosaur from the early Jurassic Period. It is known only from a partial lower jawbone. The fossil was found in China...
, the troodontids
Troodontidae
Troodontidae is a family of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and scrappy and they have therefore been allied, at various times, with many dinosaurian lineages...
Anchiornis
Anchiornis
Anchiornis is a genus of small, feathered, deinonychosaurian dinosaur. The genus Anchiornis contains the type species Anchiornis huxleyi, named in honor of Thomas Henry Huxley, an early proponent of biological evolution, and the first to propose a close evolutionary relationship between birds and...
and the as yet un-named Morrison
Morrison Formation
The Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is found in the western United States, which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone and limestone and is light grey, greenish...
WDC DML 001
WDC DML 001
WDC DML 001 is an as-yet undescribed, substantially complete, fossil of a small troodontid dinosaur from the Late Jurassic Morrison Formation of east – central Wyoming...
, the scansoriopterygids
Scansoriopterygidae
Scansoriopterygidae is a family of maniraptoran dinosaurs, known from three well-preserved fossils unearthed in the Daohugou fossil beds of Liaoning, China....
Epidexipteryx
Epidexipteryx
Epidexipteryx is a genus of small avialan dinosaur, known from one fossil specimen in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Epidexipteryx represents the earliest known example of ornamental feathers in the fossil record. The type specimen is...
and Epidendrosaurus and the basal alvarezsaur
Alvarezsauridae
Alvarezsauridae is an enigmatic family of small, long-legged running dinosaurs. Although originally thought to represent the earliest known flightless birds, a consensus of recent work suggests that they are primitive members of the Maniraptora. Other work found them to be the sister group to the...
Haplocheirus
Haplocheirus
Haplocheirus is a genus of alvarezsauroid theropod dinosaur. It is the most basal member of its clade. It is the oldest known alvarezsauroid, predating all other members by about 63 million years. This also makes it about 15 million years older than the oldest known bird Archaeopteryx...
.
Third, if the temporal paradox would indicate that birds should not have evolved from dinosaurs, then what animals are more likely ancestors considering their age? Brochu and Norell (2001) analyzed this question using six of the other archosaurs that have been proposed as bird ancestors, and found that all of them create temporal paradoxes — long stretches between the ancestor and Archaeopteryx where there are no intermediate fossils — that are actually worse. The MSM value for the theropod option was 0.438 - 0.466.
However, because of computational limitations, six taxa considered in SCI and SMIG calculations (Compsognathus, Eoraptor, Herrerasauridae, Marasuchus, Pseudolagosuchus, and Choristodera) were not included in calculation of MSM. Brochu and Norell (2001)
Thus, even if one used the logic of the temporal paradox, one should still prefer dinosaurs as the ancestors to birds.
Pol and Norell (2006) calculated MSM* values for the same proposed bird ancestors and obtained the same relative results. The MSM* value for the theropod option was 0.31 - 0.40 .