Bird stem group

Modern birds have traditionally been considered simply as a class (Aves) within the phylum Chordata. However, fossil discoveries and genetic studies over the last few decades have demonstrated that they actually belong to the archosaurs and are thus related to the reptiles. They are now ranked as subclass Aves within class Reptilia in the classification scheme of Ruggiero et al (2015). Their total group, known also as the Avemetatarsalia, is a sister group to the Crocodylia within the clade Archosauria. It represents all the crown-group archosaurs that are more closely related to birds than to the Crocodylia (Benton, 1999).

The bird stem group contains a huge number of fossil species and represents a complex phylogeny. Many different phylogenetic trees have been published; these differ in detail, but are generally in agreement when summarized to show only the direct bird line. Such a summary is shown in the following composite tree assembled from a number of analyses:

Figure 1. Summarized composite phylogenetic tree of the stem-Aves

As indicated above, the bird crown group belongs to the clade Dinosauria. From a phylogenetic point of view, birds (clade Avialae) are dinosaurs, but are generally now referred to as “avian dinosaurs” to distinguish them from the “non-avian dinosaurs”. We will use the term “dinosaur” to refer to the non-avian dinosaurs.

Note the presence in the above tree (Figure 1) of well-known dinosaur groups such as the sauropods and tyrannosaurs, all of which form part of the bird stem group. However, given that those groups represent evolutionary dead-ends that diverged from the bird stem line, we will not consider them exhaustively here. Our focus will be on stem fossils lying close to the red line of descent shown in Figure 1 above.

In order to understand the phylogenetic relationships of these stem fossils, we need to see a more detailed tree that depicts individual stem species. However, such a tree would be too big to show all at once, so we will divide it into the following four parts, each representing a phase of evolution of the bird stem group:

From the basal node (root) of the Avemetatarsalia to the root of the Dinosauriformes;
From the Dinosauriformes root to the root of the Coelurosauria;
From the Coelurosauria root to the root of the Aviales;
From the Aviales root to the appearance of the Crown-Aves.

These four parts of the stem-bird phylogenetic time tree are shown below:

Figure 2a. Time tree of the stem-Aves (Part 1)

Figure 2b. Time tree of the stem-Aves (Part 2)

Figure 2c. Time tree of the stem-Aves (Part 3)

Figure 2d. Time tree of the stem-Aves (Part 4)

From the Avemetatarsalia basal node to the root of the Dinosauriformes
The earliest known fossil in the stem line of birds is Dongusuchus efremovi, found in the Middle Triassic (Anisian) “Donguz Suite” at several locations in southeastern European Russia (Tverdokhlebov et al, 2003). No public-domain images are available from this species, but two other stem-group species shown above, Yarasuchus deccanensis and Teleocrater rhadinus, are roughly the same age and are illustrated below, together with photographs and restorations of other stem fossils for which public-domain images are available (click on an image to see a larger version; image credits given at bottom of page):

Figure 3a. Images of stem-group birds (Avemetatarsalia basal node to the root of the Dinosauriformes)

The images above are ordered from most basal to most crownward. The most basal species (in the Aphanosauria clade) are quite lizard-like, not unlike some of the stem-Archosauria, but significant changes occurred further up the tree. The Lagerpetidae were more slender than the Aphanosauria with longer legs relative to body length, and some developed a bipedal mode of movement. Furthermore, the order Pterosauria developed wings and were able to fly. All these evolutionary changes took place during the Middle and Late Triassic, over a time period of no more than 39 million years.

From the Dinosauriformes basal node to the root of the Coelurosauria
This part of the stem-line tree (Figure 2b) illustrates the subdivision of the dinosaurs into two clades: the orders Ornithischia and Saurischia. They differ principally in the structure of their pelvic bones; the pelvis of the Ornithischia is superficially similar to that of birds (Encyclopedia Britannica).

Fossils representing the basal Saurischia are shown below (click on an image to see a larger version):

Figure 3b. Images of stem-group birds (Dinosauriformes basal node to the root of the Coelurosauria)

As in the previous set, the images above are numbered in order from most basal to most crownward. The species basal to Buriolestes schultzi, for example those belonging to the family Herrerasauridae, were bipedal and are generally similar in form to the family Lagerpetidae (see Figure 3), but they are much larger. The appearance of Buriolestes marks a major split in the dinosaur clade at which the generally herbivorous sauropods (clade Sauropodomorpha, containing such well-known dinosaurs such as Brachiosaurus, Diplodocus and Brontosaurus) separated from the theropods (clade Theropoda), which were carnivorous.

The most basal member of the Sauropodomorpha, Buriolestes schultzi, appears similar to the more basal dinosaurs, but has been shown to have some features (of the upper jaw and humerus bone) that indicate that it actually belongs to the Sauropodomorpha (Cabreira et al, 2016). However, it had a herbivorous diet.

All the species crownward of Buriolestes belong to the carnivorous theropods. They continue to exhibit a morphology similar to that of the basal saurischians, but they vary in size and some, like many in the clade Ceratosauria, have crests or horns on their heads. The species crownward of the ceratosaurs tend to look, at least superficially, like tyrannosaurs. The most crownward clades are the superfamily Allosauroidea (not illustrated above), represented mainly by the well-known Allosaurus, and the family Spinosauridae, which are characterized by long spines on their dorsal vertebrae, a huge claw on the hand, and an elongate, crocodile-like skull (Benson, 2015).

None of these dinosaur fossils appears to have obvious bird-like characteristics; there is no evidence for feathers in any of the fossils illustrated above, despite the depiction of what looks like a feathery coat on some of the life restorations (e.g. Tawa hallae, Chindesaurus bryansmalli and Cryolophosaurus ellioti).

From the Coelurosauria basal node to the root of the Aviales
This part of the tree depicts the phylogenetic relationships of the most crownward non-avian dinosaurs. It contains a large number of species, many of which are illustrated below (click on an image to see a larger version):

Figure 3c. Images of stem-group birds (Coelurosauria basal node to the root of the Aviales)

In this case, the series of images, ordered from more basal to more crownward, does indeed show an important transition, in two respects:

There is a trend towards a more slender body form, with a smaller head relative to body size, compared to the species basal to the superfamily Tyrannosauroidea (e.g. Zuolong salleei, Tanycolagreus topwilsoni and Proceratosaurus bradleyi) and the Tyrannosauroidea themselves. The latter clade, which represents a long side-branch of the tree, contains large dinosaurs such as Tyrannosaurus rex (not shown here) that were a long way from being direct ancestors of the birds.
Fossil evidence for feathers like those in modern birds (the Stage II feathers of Prum (1999), corresponding to the “basally branching” feathers described by Ksepka (2020)) appears for the first time. The most basal dinosaur known to have had feathers is the tyrannosauroid Dilong paradoxus. Other feathered dinosaurs illustrated above are Sinosauropteryx prima, Beipiaosaurus inexpectus, Caudipteryx zoui, Gigantoraptor erlianensis, Rahonavis ostromi and Jinfengopteryx elegans. Feathers shown in the life restorations of other species shown above are not based on fossil evidence.

From the Aviales basal node to the appearance of the Crown-Aves
This part of the tree illustrates the phylogeny of the most crownward phase of the bird stem group. The species for which public-domain images are available are shown below:

Figure 3c. Images of stem-group birds (Aviales basal node to the appearance of the Crown-Aves)

The above images, again ordered from most basal to most crownward, suggest a progression towards species that look very like modern birds. However, an important point to note is that nearly all of the above specimens have teeth, whereas crown birds do not. It is interesting that the presence of teeth is not confined to the most basal species, such as Archaeopteryx, but is also found in those closest to the crown (Louchart and Viriot, 2011).

The crown group of the birds comprises two clades, the Palaeognathae (ostriches and relatives) and the Neognathae (all other birds). Although the crown groups of these clades contain many fossil species, no fossils have yet been assigned either to the stem-Palaeognathae or to the stem-Neognathae (Brocklehurst and Field, 2024).

The oldest known member of the crown-Aves is the crown neognathan Asteriornis maastrichtensis, described from the Late Cretaceous (Late Maastricthian) Valkenburg Member of the Maastricht Formation at the CBR-Romontbos Quarry, Eben-Emael, Liège, Belgium (Field et al, 2020; Brocklehurst and Field, 2024). The phylogenetic position of this species within the crown-Neognathae is currently unclear (Brocklehurst and Field, 2024). The skull of the species is illustrated below, together with a life restoration (click on image for a larger view):

Figure 4. Asteriornis maastrichtensis, the oldest known crown bird

The above time tree (Figure 2) indicates that the bird stem group developed from Early Triassic (based on the appearance of the stem-Crocodylia) to Late Cretaceous time, representing a long stem-to-crown transition of at least 178 million years.

References

Benton, M. J. (1999). Scleromochlus taylori and the origin of dinosaurs and pterosaurs. Philosophical Transactions of the Royal Society of London B: Biological Sciences, 354(1388), 1423-1446.

Benton, M. J. (2015). Vertebrate Palaeontology - Fourth edition. John Wiley & Sons, 468 pages.

Brocklehurst, N., & Field, D. J. (2024). Tip dating and Bayes factors provide insight into the divergences of crown bird clades across the end-Cretaceous mass extinction. Proceedings of the Royal Society B, 291(2016), 20232618.

Cabreira, S. F., Kellner, A. W. A., Dias-da-Silva, S., da Silva, L. R., Bronzati, M., de Almeida Marsola, J. C., ... & Langer, M. C. (2016). A unique Late Triassic dinosauromorph assemblage reveals dinosaur ancestral anatomy and diet. Current Biology, 26(22), 3090-3095.

Field, D. J., Benito, J., Chen, A., Jagt, J. W., & Ksepka, D. T. (2020). Late Cretaceous neornithine from Europe illuminates the origins of crown birds. Nature, 579(7799), 397-401.

Kammerer, C. F., Nesbitt, S. J., Flynn, J. J., Ranivoharimanana, L., & Wyss, A. R. (2020). A tiny ornithodiran archosaur from the Triassic of Madagascar and the role of miniaturization in dinosaur and pterosaur ancestry. Proceedings of the National Academy of Sciences, 117(30), 17932-17936.

Ksepka, D. T. (2020). Feathered dinosaurs. Current Biology, 30(22), R1347-R1353.

Louchart, A., & Viriot, L. (2011). From snout to beak: the loss of teeth in birds. Trends in Ecology & Evolution, 26(12), 663-673.

Norman, D. B., Baron, M. G., Garcia, M. S., & Müller, R. T. (2022). Taxonomic, palaeobiological and evolutionary implications of a phylogenetic hypothesis for Ornithischia (Archosauria: Dinosauria). Zoological Journal of the Linnean Society.

Pei, R., Pittman, M., Goloboff, P. A., Dececchi, T. A., Habib, M. B., Kaye, T. G., ... & Xu, X. (2020). Potential for powered flight neared by most close avialan relatives, but few crossed its thresholds. Current Biology, 30(20), 4033-4046.

Prum, R. O. (1999). Development and evolutionary origin of feathers. Journal of Experimental Zoology, 285(4), 291-306.

Rauhut, O. W., & Pol, D. (2019). Probable basal allosauroid from the early Middle Jurassic Cañadón Asfalto Formation of Argentina highlights phylogenetic uncertainty in tetanuran theropod dinosaurs. Nature scientific reports, 9(1), 1-9.

Ruggiero, M. A., Gordon, D. P., Orrell, T. M., Bailly, N., Bourgoin, T., Brusca, R. C., ... & Kirk, P. M. (2015). A higher level classification of all living organisms. PloS one, 10(4), e0119248.

Serrano-Brañas, C. I., Espinosa-Chávez, B., Maccracken, S. A., Gutiérrez-Blando, C., de León-Dávila, C., & Ventura, J. F. (2020). Paraxenisaurus normalensis, a large deinocheirid ornithomimosaur from the Cerro del Pueblo Formation (Upper Cretaceous), Coahuila, Mexico. Journal of South American Earth Sciences, 101, 102610.

Tverdokhlebov, V. P., Tverdokhlebova, G. I., Surkov, M. V., & Benton, M. J. (2003). Tetrapod localities from the Triassic of the SE of European Russia. Earth-Science Reviews, 60(1-2), 1-66.

Image credits – stem-Aves

Figure 3a (Spondylosoma absconditum): Jaime A. Headden (User:Qilong), CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3a (Yarasuchus deccanensis): Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3a (Teleocrater rhadinus): Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
Figure 3a (Scleromochlus taylori, skeletal reconstruction): Jaime A. Headden (User:Qilong), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3a (Scleromochlus taylori, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3a (Eudimorphodon ranzii, fossil): Tommy from Arad, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3a (Eudimorphodon ranzii): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3a (Dimorphodon macronyx, fossil): Tim Evanson, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3a (Dimorphodon macronyx): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3a (Kongonaphon kely): Fanboyphilosopher (Neil Pezzoni), CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
Figure 3a (Ixalerpeton polesinensis, skeletal reconstruction): Maurissauro, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3a (Ixalerpeton polesinensis, life restoration): ULBRA Canoas, Public domain, via Wikimedia Commons
Figure 3a (Lagerpeton chanarensis, skeletal reconstruction): Maurissauro, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3a (Lagerpeton chanarensis, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3a (Dromomeron gregorii): Sterling Nesbitt, under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 license.
Figure 3a (Dromomeron romeri): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Lagosuchus talampayensis, fossil): Ogmios, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3b (Lagosuchus talampayensis, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Staurikosaurus pricei, fossil): Kabacchi, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3b (Staurikosaurus pricei, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3b (Sanjuansaurus gordilloi, skeletal reconstruction): Oscar Alcober, Ricardo Martinez, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3b (Sanjuansaurus gordilloi, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Herrerasaurus ischigualastensis, fossil): Fernando de Gorocica, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3b (Herrerasaurus ischigualastensis, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3b (Daemonosaurus chauliodus, skull): Jaime A. Headden (User:Qilong), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3b (Daemonosaurus chauliodus, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Tawa hallae): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3b (Chindesaurus briansmalli): Jeff Martz, Public domain, via Wikimedia Commons
Figure 3b (Eodromaeus murphy, fossil): Kabacchi, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3b (Eodromaeus murphy, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3b (Buriolestes schultzi, skeletal reconstruction): Maurissauro, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3b (Buriolestes schultzi, life restoration): Audrey.m.horn, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3b (Coelophysis bauri, fossil): James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3b (Coelophysis bauri, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Coelophysis rhodesiensis): Wikipteryx, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3b (Dilophosaurus wetherilli, fossil): Eden, Janine and Jim from New York City, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3b (Dilophosaurus wetherilli, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Cryolophosaurus ellioti, fossil): Hello!mrsax, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3b (Cryolophosaurus ellioti, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Sinosaurus triassicus, fossil): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3b (Sinosaurus triassicus, life restoration): Slate Weasel, CC0, via Wikimedia Commons
Figure 3b (Ceratosaurus nasicornis, fossil): Daderot, CC0, via Wikimedia Commons
Figure 3b (Ceratosaurus nasicornis, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Eoabelisaurus mefi, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Masiakasaurus knopfleri, fossil): Eduard Solà, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3b (Masiakasaurus knopfleri, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Majungasaurus crenatissimus, fossil): D. Gordon E. Robertson, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3b (Majungasaurus crenatissimus, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3b (Chuandongocoelurus primitivus, skeletal reconstruction): Paleocolour, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3b (Chuandongocoelurus primitivus, life restoration): Paleocolour, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3b (Monolophosaurus jiangi, fossil): Kabacchi, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3b (Monolophosaurus jiangi, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3b (Streptospondylus altdorfensis): Foolp, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3b (Irritator challenger, fossil): Kabacchi, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3b (Irritator challenger, life restoration): Fred Wierum, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3b (Ichthyovenator laosensis, fossil): ★Kumiko★, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3b (Ichthyovenator laosensis, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Zuolong salleei, skeletal reconstruction): IJReid, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
Figure 3c (Zuolong salleei, life restoration): FunkMonk (Michael B. H.), CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3c (Tanycolagreus topwilsoni, fossil): ★Kumiko★, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Tanycolagreus topwilsoni, life restoration): Smokeybjb, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3c (Proceratosaurus bradleyi, fossil): The Trustees of the Natural History Museum, London, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
Figure 3c (Proceratosaurus bradleyi, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Guanlong wucaii, fossil): ★Kumiko★ from Tokyo, Japan, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Guanlong wucaii, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Dilong paradoxus, fossil): Tiouraren (Y.-C. Tsai), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3c (Dilong paradoxus, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Stokesosaurus langhami, skeletal reconstruction): Conty, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3c (Stokesosaurus langhami, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Gorgosaurus libratus, fossil): Sebastian Bergmann from Siegburg, Germany, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Gorgosaurus libratus, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Daspletosaurus torosus, fossil): Kevin Guertin, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Daspletosaurus torosus, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3c (Ornitholestes hermanni, fossil): Ryan Somma, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Ornitholestes hermanni, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Sinosauropteryx prima, fossil): Sam / Olai Ose / Skjaervoy from Zhangjiagang, China, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Sinosauropteryx prima, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Compsognathus longipes, fossil): MatthiasKabel, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Figure 3c (Compsognathus longipes, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Huaxiagnathus orientalis, fossil): Laikayiu, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Figure 3c (Huaxiagnathus orientalis, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Shenzhousaurus orientalis, fossil): Jonathan Chen, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3c (Shenzhousaurus orientalis, life restoration): Danny Cicchetti, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3c (Beishanlong grandis, fossil): SSR2000, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3c (Beishanlong grandis, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Harpymimus okladnikovi, fossil): ★Kumiko★ from Tokyo, Japan, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Harpymimus okladnikovi, life restoration): Steveoc 86, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Figure 3c (Archaeornithomimus asiaticus, fossil): Gary Todd, CC0, via Wikimedia Commons
Figure 3c (Archaeornithomimus asiaticus, life restoration): Audrey.m.horn, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3c (Haplocheirus sollers, fossil): ssr ist4u from Ibaraki, Japan, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Haplocheirus sollers, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Patagonykus puertai, fossil): Kabacchi, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3c (Patagonykus puertai, life restoration): GabrielNU, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3c (Alvarezsaurus calvoi, fossil): FunkMonk, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3c (Alvarezsaurus calvoi, life restoration): Karkemish, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3c (Mononykus olecranus, fossil): Thomas Cowart from Durhamderivative work: Ivan Akira, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3c (Mononykus olecranus, life restoration): PaleoNeolitic, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
Figure 3c (Incisivosaurus gauthieri, skull): Jaime A. Headden (User:Qilong), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3c (Incisivosaurus gauthieri, life restoration): Nobu Tamura, licensed under Creative Commons Attribution- ShareAlike (CC BY-SA) license
Figure 3c (Caudipteryx zoui, fossil): Wolfgang Sauber, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3c (Caudipteryx zoui, life restoration): Conty, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3c (Gigantoraptor erlianensis, fossil): Kabacchi, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3c (Gigantoraptor erlianensis, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Falcarius utahensis, fossil): Paul Fisk from Salt Lake City, UT, USA, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Falcarius utahensis, life restoration): Dibrangosaurus, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3c (Beipiaosaurus inexpectus, fossil): Jonathan Chen, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3c (Beipiaosaurus inexpectus, life restoration): PaleoNeolitic, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
Figure 3c (Alxasaurus elesitaiensis, fossil): Sebastian Bergmann from Siegburg, Germany, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Alxasaurus elesitaiensis, life restoration): Conty, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3c (Mahakala omnogovae, skeletal reconstruction): Jaime A. Headden (User:Qilong), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3c (Mahakala omnogovae, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Shanag ashile, skull): Skye McDavid, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3c (Shanag ashile, life restoration): Danny Cicchetti, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3c (Rahonavis ostromi, skeletal reconstruction): Jaime A. Headden (User:Qilong), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3c (Rahonavis ostromi, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3c (Sinornithoides youngi, fossil): ★Kumiko★, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3c (Sinornithoides youngi, life restoration): John Conway [1], CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Figure 3c (Byronosaurus jaffei, skeletal reconstruction): Jaime Headden (User:Qilong), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3c (Byronosaurus jaffei, life restoration): FunkMonk (Michael B. H.), CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3c (Jinfengopteryx elegans, fossil): Laikayiu, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Figure 3c (Jinfengopteryx elegans, life restoration): Matt Martyniuk (Dinoguy2), CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Figure 3c (Sinovenator changii): Jaime Headden (en:User:Qilong), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3d (Xiaotingia zhengi, fossil): Bruce McAdam, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3d (Xiaotingia zhengi, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3d (Anchiornis huxleyi, fossil): Bjoertvedt, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3d (Anchiornis huxleyi, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3d (Aurornis xui, fossil): Don Hitchcock, donsmaps.com
Figure 3d (Aurornis xui, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3d (Eosinopteryx brevipenna): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3d (Archaeopteryx lithographica, fossil): James L. Amos, CC0, via Wikimedia Commons
Figure 3d (Archaeopteryx lithographica, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3d (Sapeornis chaoyangensis): Tiouraren (Y.-C. Tsai), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3d (Jeholornis prima, fossil): Tiouraren (Y.-C. Tsai), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3d (Jeholornis prima, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3d (Jixiangornis orientalis): Laikayiu, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Figure 3d (Confuciusornis sanctus, fossil): Tommy from Arad, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Figure 3d (Confuciusornis sanctus, life restoration): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3d (Patagopteryx deferrariisi, fossil): FunkMonk, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3d (Patagopteryx deferrariisi, life restoration): FunkMonk (Michael B. H.), CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Figure 3d (Hongshanornis longicresta, fossil): Chiappe et al., CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
Figure 3d (Hongshanornis longicresta, life restoration): Pavel.Riha.CB, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Figure 3d (Yanornis martini, fossil): Xiaoting Zheng, Jingmai K. O'Connor, Fritz Huchzermeyer, Xiaoli Wang, Yan Wang, Xiaomei Zhang, Zhonghe Zhou, CC BY 2.5 <https://creativecommons.org/licenses/by/2.5>, via Wikimedia Commons
Figure 3d (Yanornis martini, life restoration): Entelognathus, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3d (Yixianornis grabaui): ★Kumiko★, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Figure 3d (Hesperornis regalis, fossil): Quadell, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
Figure 3d (Hesperornis regalis, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 3d (Ichthyornis dispar, fossil): Ghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Figure 3d (Ichthyornis dispar, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
Figure 4. (Asteriornis maastrichtensis, skull and life restoration): "NetMassimo" blog of Massimo Luciani, published under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 license