Fig. 1. Mounted skeleton cast of Deinonychus, Field Museum.
The biogeography of Paravian dinosaurs is the study of the global distribution of
Paraves through geological history. Paraves is a
clade that includes all of the
Theropoda that are more closely related to
birds than to
oviraptorosaurs.[1] These include
Dromaeosauridae and
Troodontidae (historically grouped under
Deinonychosauria) and
Avialae (including
crown group birds, i.e. modern birds).[2] The distribution of paraves is closely related to the evolution of the clade. Understanding the changes in their distributions may shed light on problems like how and why paraves evolve, eventually gaining the ability to fly.
Paraves first appeared in the fossil record in early
Late Jurassic (163–145 million years ago),[3][4] then rapidly diversified and dispersed during
Cretaceous (145–66 million years ago).[5] They emerged during the
breakup of Pangea (since Early-Middle Jurassic),[6] which influenced the biogeographic processes such as
speciation,
geodispersal and
extinction. By the Late Cretaceous, Paraves reached global distribution with fossils found in modern Asia, Europe, Australia, Antarctica etc.[7] Almost all Paravian dinosaurs died out before or during the
end-Cretaceous mass extinction (~66 million years ago), also called the Cretaceous-Paleogene (K-Pg) mass extinction.[8][9][10]
As a result of this extinction event, only a small group of avialans –
neornithines – were able to survive.[8][11] This group of Avialae continued to flourish in Cenozoic and later evolved into all modern birds.[9]
There are limitations to be considered when studying the
paleobiogeography of Paraves. Firstly, the fossil record may not represent the actual distribution of the three clades mainly due to
taphonomic bias.[12] Also, the fossil record may be incomplete, which may lead to misinterpretations.[12]
Vicariance and geodispersal
Fig. 3. The figure shows two maps viewed from the North Pole at early and mid-Late Cretaceous. It illustrates the concept of geodispersal through the formation of the Bering Land Bridge between Asia and North America. Modified from Wen et al. (2016).[13]
Vicariance is a biogeographic process that occurs when a population is forced to separate into two or more groups due to geographic constraints.[14][15] It is a key process in the biogeographic history of Paraves and one of the main hypotheses on the global distribution of dromaeosauridae in Late
Mesozoic.[16] Pangea broke up into
Laurasia and
Gondwana, creating an oceanic barrier in between the two landmasses where terrestrial faunal exchange was near impossible. In a regional scale, the collapse of land bridges can also cause vicariance.[7]
Geodispersal is the process where populations migrate from their origins to other areas due to the removal of geophysical barriers like mountains and seas, connecting areas that are previously isolated. Unlike vicariance, geodispersal opens up
gene flow by allowing populations that had never been in contact before to interact.[17]
In reality, vicariance and geodispersal often occur together in repeated intervals. For example, the
Bering Strait (Fig. 2) between North America and Asia acts as an oceanic barrier preventing terrestrial animals from crossing from one continent to another.[18] Nonetheless, the channel was not present throughout the history of Earth. During Late Jurassic and Early Cretaceous, there were multiple occasions where the
Bering Land Bridge formed between the two continents.[19] The land bridge was made by
tectonic movements in Late Mesozoic,[18] and by lowering of global sea level due to
climatic changes in
Pleistocene.[20] It acted as an overpass for land-dwellers to move from Asia to North America, and vice versa. Evidence for this was in the speciation of
troodontids in Asia and North America respectively after the Bering Land Bridge broke off in the early Late Cretaceous.[7] The land bridge was then repeatedly inundated and exposed since the breakup of Laurasia, contributing to the episodic exchange of fauna involving
dinosaurs and
mammals alike.[18]
Paleobiogeography
During the
Early Mesozoic, the supercontinent
Pangea just finished its assembly and almost immediately started breaking apart.[6] The
rifting began to take place during Early to Middle
Jurassic (201–163 million years ago), and gradually formed two extensive landmasses –
Laurasia and
Gondwana.[21] The continents then continued to be ripped apart into smaller land that resembled modern continents throughout late
Mesozoic and
Cenozoic.[6] The breakup of continents did not happen uniformly, and modern continents were formed at different speeds while experiencing repeated collision and rifting.[22][21] This phenomenon is closely related to the dispersal and evolution of Paraves.[7]
There was a consensus among
paleontologists that Paraves first appeared in Middle to Late Jurassic (174–145 million years ago). However, it was fairly recent when the
cladeAvialae was grouped under
Paraves.[5] With the discovery of Anchiornis (earliest
feathered dinosaur with four wings) in
Tiaojishan Formation in China,[4] scientists were able to identify the appearance of Paraves within the Middle Jurassic. Anchiornis is a feathered dinosaur with
anatomical features similar to that of Archaeopteryx, historically the oldest known avialan. Initially considered a basal troodontid, many paleontologists have also considered it an early avialan. Feathered dinosaurs have helped to confirm that the
dinosaur-to-bird hypothesis is almost certainly true.[2][3]
Anchiornis is seen as one of the first paravian dinosaurs to have ever existed,[4] along with several other
genera from the same
family (
Anchiornithidae) such as Aurornis[5] and Caihong.[23] Their fossils were found in the same rock formation in China, suggesting that Paraves likely originated from Asia.[4][23]
Following the appearance of Anchiornis and other basal paravians, a series of
vicariance events were inferred to have taken place. The widespread distribution and
speciation of Dromaeosauridae and Troodontidae in Late Mesozoic coincided with the timing of the Pangean breakup, thus giving rise to the hypothesis of
allopatric speciation initiated by continental fragmentation.[7]
Fig. 5. A map showing the distribution of dromaeosauridae, troodontidae and avialae respectively at the end of Late Jurassic. The red line indicates the separation of Laurasia and Gondwana (no faunal exchange on land). Abbreviations: N, North America; S, South America; A, Asia; E, Europe; F, Africa; T, Antarctica; U, Australia; I, India; M, Madagascar. Modified from Ding et al. (2019).[7]
The oldest Archaeopteryx (Avialae) was found in the
Solnhofen Limestone in Germany,[26] while the oldest evidence of Dromaeosauridae was unearthed in the
Morrison Formation in North America.[27] Troodontidae continued to diversify and can be found in Asia, Europe and North America.[29][4][7][25][28] They were most probably dispersed from Asia to other continents on land.[7]
Around the same time, Avialae began to obtain the ability to fly.[30] This may contribute to the rapid diversification and dispersal of avialans in later stages.[7][30]
Cretaceous
Fig. 6. A map showing the distribution of Paraves in Early Cretaceous. The red line indicates the separation of South America and Africa. The dotted lines indicate the position of the continents at the end of Jurassic. Abbreviations: N, North America; S, South America; A, Asia; E, Europe; F, Africa; T, Antarctica; U, Australia; I, India; M, Madagascar. Modified from Ding et al. (2019).[7]
In
Early Cretaceous (145–100 million years ago), the opening of the
South Atlantic Ocean commenced and South America became fully detached from Africa at approximately 100 Ma.[21] Despite the fragmentation of land, terrestrial animals can still cross from continent to continent, facilitated by the land bridges and shallow seas.[15]
Table 2. Distribution of each clade in Early Cretaceous
Apart from
Troodontidae,
Dromaeosauridae and
Avialae began spreading to other continents. Dromaeosauridae dispersed to Asia and Africa,[7] likely made possible by the Apulian Route connecting Eurasia and Africa,[35] and the Bering Land Bridge linking North America and Asia.[18] The Apulian Route was established in early Late Jurassic and broke off towards the end of Jurassic.[35] At this stage, paravian distribution concentrated mostly on the northern hemisphere, with exceptions of avialae found in South America and Australia.[7]
Fig. 7. A map showing the distribution of Paraves in Late Cretaceous. The dotted lines indicate the position of the continents at the end of Early Cretaceous. Abbreviations: N, North America; S, South America; A, Asia; E, Europe; F, Africa; T, Antarctica; U, Australia; I, India; M, Madagascar. Modified from Ding et al. (2019).[7]
Table 3. Distribution of each clade in Late Cretaceous
In
Late Cretaceous (100–66 million years ago), both
Dromaeosauridae and
Avialae had reached global distribution, while
Troodontidae remained found only in Asia (including India) and North America.[7]There were many more avialan records in Antarctica and Australia in Early Cretaceous compared to that of Late Cretaceous.[7]
Paleontologists also deduced a slight change in diet in avialans during Cretaceous.[30] While most Paraves in
Late Jurassic were
carnivorous (meat-eater), some avialans were found to be
seed-eating in this period. This change in diet and the sudden avian
diversification coincided with the spread of
angiosperms (flowering plants), implying that there may have been
coevolution taking place where the avialans and the flowering plants impact each other's evolution.[30]
End-Cretaceous mass extinction
The
Cretaceous-Paleogene mass extinction (~66 million years ago) is one of the most-studied extinction events. While not being the largest known mass extinction event, it is famous for its impact on dinosaurs.[45] Like all other dinosaurian species, almost all Paraves died out sometime between the latest Cretaceous (
Maastrichtian) and the start of
Paleogene – with one exception. Given the title "the most successful dinosaurs" by paleontologists,
neornithines managed to survive the
mass extinction and continued to flourish in present days.[8][30]
The reason as to why only
neornithines lived was still a very much debated topic among
vertebrate paleontologists. Some believed that it was related to their global distribution and the cause of
mass extinction.[8][11] The fossil record of late Cretaceous neornithines concentrated in the
southern hemisphere where
Gondwana once was.[11] It is then hypothesised that life in the southern hemisphere suffered less[8] because the impact of the meteor at
Chicxulub was northward-facing.[46]
Another hypothesis suggested that the extinction of non-neornithines was unlikely a result of
mass extinction but was instead caused by a change in vegetation pattern.[30] It is found that a portion of non-neornithines was already extinct before the
impact event.[30] This can be accounted for by the regional scale vegetation loss that occurred in North America,[47] which largely affected the atmospheric composition[48] and disrupted the food chain.[30]
Both hypotheses, along with any other possibilities, have yet to be proven with definitive evidence. One certain thing is that
neornithines did not diversify much in Cretaceous compared to that in the early
Cenozoic, unlike other groups of birds.[10][49] The fossil record for neornithines in
Late Cretaceous was sparse, but there was an explosive increase in fossils in Early Cenozoic.[10] As the radiation of neornithines happened together with the rise of mammals, it seems logical that the two events were related.
Paleogene-present
Fig. 8. A diagram showing the diversification of avialans before and after the K-Pg mass extinction event. The size roughly represents the number of species in that group. Basal birds diversified into different groups in the Cretaceous, while neornithines only began their rapid diversification in Paleocene. Modified from Feduccia (2014).[10]
As the Earth entered a new era, the continents started to move into their modern positions.[50] Australia and South America finally separated from Antarctica[51] while the Indian subplate began its collision into Asia, creating the world's largest mountain range the
Himalayas.[52] At the same time, the global average temperature cooled down[53][54] since
Late Cretaceous with a
Thermal Maximum at
Paleocene-
Eocene boundary and
Middle Miocene Climatic Optimum.[55] Between these warm episodes were periods of cooling (
Oligocene global cooling),[30] where seawater level fell and land bridges between Africa and Eurasia (i.e.
Gomphotherium landbridge),[56] and Eurasia and North America (i.e.
Bering Land Bridge)[20] were formed.
Vegetation patterns underwent drastic changes in Cenozoic. With long periods of global warming, grasslands spread to regions of higher latitudes.[57] This phenomenon facilitated the dispersal of avialans that lived in non-arboreal environments,[30] examples found in
Green River Formation (USA) and
Messel Oil Shale (Germany).
Neornithines underwent a rapid increase in number in
Paleogene in a relatively short time, though crown group birds were still sparse.[10]
Since the continents were almost entirely separated from each other,[50]neornithines began to
speciate independently.[30] Most of the
mutations occurred in birds without long-distance flight abilities, and the most prominent changes were found in birds living on isolated islands like New Zealand and Australia.[30][58] It is hypothesised that due to the lack of large
carnivorous (meat-eating)
predators on these islands due to the extinction of dinosaurs, birds were able to evolve and
adapt to the new environments.[30] Such
mutations include an increase in body sizes but reduced wings and development of flightlessness, as found in birds like
moas (completely extinct by the year 1440[59]),
kiwis and
ostriches.[30] Though they were not closely related to one another, this evolution pattern indicates that birds evolve similarly in isolated environments without major threats of predators.[58]
Summary of paleobiogeography
Table 4. Summary
Continent
Middle Jurassic
(170-166Ma)
Late Jurassic
(166-145Ma)
Early Cretaceous
(145-100Ma)
Late Cretaceous
(100-66Ma)
Post K-Pg Mass Extinction
(66Ma-present)
D
T
A
D
T
A
D
T
A
D
T
A
D
T
A
Asia
⧠
⧠
★
⧠
⚫︎
★
⧠
⚫︎
⚫︎
Europe
⧠
⚫︎
⚫︎
★
⚫︎
⚫︎
Africa
★
★
⚫︎
⚫︎
North America
★
⧠
★
⧠
★
⧠
⚫︎
⚫︎
South America
⚫︎
★
⚫︎
⚫︎
Antarctica
★
⚫︎
⚫︎
Australia
⚫︎
⚫︎
D – Dromaeosauridae (★)
T – Troodontidae (⧠)
A – Avialae (⚫︎)
Table 5. Distribution of Dromaeosauridae through time
Continent
Middle Jurassic
Late Jurassic
Early Cretaceous
Late Cretaceous
Post K-Pg Mass Extinction
Asia
✓
✓
Europe
✓
Africa
✓
✓
North America
✓
✓
✓
South America
✓
Antarctica
✓
Australia
Table 6. Distribution of Troodontidae through time
Continent
Middle Jurassic
Late Jurassic
Early Cretaceous
Late Cretaceous
Post K-Pg Mass Extinction
Asia
✓
✓
✓
✓
Europe
✓
Africa
North America
✓
✓
✓
South America
Antarctica
Australia
Table 7. Distribution of Avialae through time
Continent
Middle Jurassic
Late Jurassic
Early Cretaceous
Late Cretaceous
Post K-Pg Mass Extinction
Asia
✓
✓
✓
Europe
✓
✓
✓
✓
Africa
✓
✓
North America
✓
✓
South America
✓
✓
✓
Antarctica
✓
✓
Australia
✓
✓
Limitations
Flight capabilities
There are some limitations in studying the biogeography of Paraves, including their development of
flight. Flight capabilities in Paraves were developed since the Late Jurassic.[30] While only several groups of
paravian dinosaurs have such abilities, it is certainly a possibility that it contributed to their dispersal in late Mesozoic.[25] In these cases, the biogeographic record of Paraves cannot be interpreted solely by whether the land was connected or not. Modern
migratory birds can travel covering long distances without relying much on land. This means that the development of long-distance flight occurred at some point between the first appearance of bird-like dinosaurs and present day.[30] It is found that, at least until
Early Cretaceous, Paraves can only glide between trees instead of flying like modern birds.[60] Thus, their flight abilities do not affect the dispersal of Paraves.[7] In
Cenozoic, avian flight abilities were more or less developed in
neornithines, though there are exceptions that are flightless birds mainly found in isolated islands.[58] Since then, flight in avians had become a major factor contributing to the dispersal of birds.[30]
Taphonomic bias
Taphonomic bias is caused by the difference in how organisms decay and fossilise. It is another challenge faced by paleontologists as the fossil records are often incomplete.[61] Moreover, some localities have better-preserved record due to their geological history than the others, leading to misinterpretations in biogeography.[62] This is especially true for dinosaurian fossil records since localities in North America and China had a much higher abundance of fossils preserved in
sedimentary rocks.[7][30] In fact, there is only one known rock formation that holds a relatively complete and dated record of the latest
Cretaceous (
Maastrichtian), named the
Hell Creek Formation in western North America.[45] The fossils for birds (including basal
avialans) in
Late Cretaceous are incomplete as well, given the sparse record discovered in Australia and Antarctica.[49]
Other possible types of taphonomic bias include collecting bias. Fossils of species that lived in a specific environment can also be poorly preserved. An example being fossils of arboreal
neornithines (birds that live in trees) are rarely found in the late
Mesozoic and earliest
Paleocene.[30] This may lead to the conclusion that they did not diversify in that period due to underrepresentation, which may or may not be true.[30]
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