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Description
Clockwise from left (not to scale):
Similicaudipteryx yixianensis, an oviraptorosaur. Oviraptorosaurs had short, box-shaped heads. Derived forms (caenagnathoids) had toothless beaks and often crests. Their diet is still mostly unknown. Basal oviraptorosaurs appear to have been predominantly herbivorous, but caenagnathoids are known to have eaten small animals. Several caenagnathoid specimens have been found brooding on their nests. Though they had large pennaceous feathers on their hands and tail, all known oviraptorosaurs were probably flightless. Their tail muscles appear to have been adapted specifically for flaunting the tail feathers in visual displays. So far they're only known from the Cretaceous, but presumably have a ghost lineage stretching into the Jurassic.

Mei long, a troodont. One of the two definite deinonychosaur ("raptor") lineages, troodonts were generally small, fast omnivores with long legs and short forelimbs. Most had a retractable second toe bearing an enlarged claw that was probably used as a weapon. Some have been preserved in a curled up, possibly sleeping position and others in a brooding position. Unlike most other paravians (all known aviremigians aside from oviraptorosaurs), many troodonts had a forward-pointing pubis, but basal taxa show that this is a reversal from the ancestral backward-pointing pubis. Most known troodonts are from the Cretaceous, but some Jurassic taxa have also been found.

Rahonavis ostromi, a dromaeosaurid. The other main deinonychosaur lineage, dromaeosaurids were more adapted to hypercarnivory than are most other maniraptoriforms. The retractable toe claw was very well developed in the larger and more derived taxa, though these dromaeosaurids were also less adapted for high-speed running than were their more basal relatives. Several smaller species may have been capable of limited flight, and some had long feathers on the legs that formed a second pair of wings. Definite dromaeosaurids are only known from the Cretaceous, but presumably have a ghost lineage stretching into the Jurassic.

Xiaotingia zhengi, an archaeopterygid. Archaeopterygids were basal paravians from the Late Jurassic. They may be deinonychosaurs, but could also belong to the Avialae (closer to modern birds than to deinonychosaurs) or be equally close to both deinonychosaurs and avialians. Their flight abilities were probably limited. They retained a retractable toe claw. At least some had long leg feathers.

Epidexipteryx hui, a scansoriopterygid. Scansoriopterygids were a group of very small, enigmatic maniraptors from the Late Jurassic. They may be very basal avialians, but I wouldn't be surprised if they turned out to be something completely different. They had short blunt skulls and very long fingers, and may have been specialized for climbing trees. Their first toe was placed very low on the foot, but wasn't reversed. Unlike most other paravians, they had a forward-pointing pubis and are known to have had scales underneath the tail. By the way, though I once wrote that their wing feathers may have been attached to the third finger (instead of the second finger like other aviremigians), I'm following 's suggestion at Hell Creek that this is an illusion caused by the feathers slipping off the second finger before fossilization.

Jeholornis prima. A long-tailed avialian from the Early Cretaceous that retained a retractable second toe. It is known to have eaten seeds. Its first toe may have been slightly reversed, but it did not point fully backward as in modern birds.

Sapeornis chaoyangensis, an omnivoropterygid. Omnivoropterygids were among the most basal birds to have a very short tail tipped with a pygostyle. Unlike other basal avialians, omnivoropterygids had fully reversed toes and could perch more effectively, but probably couldn't fly very well. They had short skulls and probably fed on fruit and seeds. All omnivoropterygids are known from the Early Cretaceous.

Changchengornis hengdaoziensis, a confuciusornithid. Confuciusornithids had toothless beaks evolved convergently from those of modern birds. Their first toe probably pointed inward instead of being fully reversed. Like other basal paravians they couldn't lift their wings above their backs, so they were probably gliders or limited flappers, but they were probably quite skilled at this as far as basal avialians go. At least some are known to have eaten fish. Some individuals had a pair of ribbon-shaped tail feathers, which may represent sexual dimorphism. All confuciusornithids are known from the Early Cretaceous.

Gobipteryx minuta (top), a gobipterygid. One of the most diverse and successful bird groups during the Mesozoic were the enantiornithines, all of which are known from the Cretaceous. Most were probably good fliers and had several adaptations to flight that more basal aviremigians lacked, such as an alula and the ability to lift the wings higher above the back. The first toe in enantiornithines was also fully reversed. Some retained wing claws, others shrunk them. Enantiornithine phylogeny is a mess, but one possible enantiornithine clade was the Gobipterygidae, consisting of some small enantiornithines. Most enantiornithines had teeth, but Gobipteryx minuta itself is known to have had a toothless beak. The ribbon-shaped tail feathers are there because I felt like it, but these are indeed known in some enantiornithine taxa.

Avisaurus archibaldi (bottom), an avisaurid. Another possible clade of enantiornithines was the Avisauridae, some members of which were very large (with wingspans more than a meter wide) and others very small (the size of a passerid). Some avisaurids may have been hypercarnivorous and some may have been seabirds.

Shanweiniao cooperorum, a longipteryigid. Longipterygid enantiornithines typically had very long snouts. While most enantiornithines didn't have a fan of tail feathers, instead having no long tail feathers at all or some widely-spaced ribbon-shaped tail feathers, Shanweiniao cooperorum had four ribbon-shaped tail feathers that could have formed a tail fan, which would have helped generate lift in flight.

Patagopteryx deferrariisi. A secondarily flightless bird from the Late Cretaceous. It was closer to modern birds than enantiornithines were.

Hesperornis regalis (center, top left), a hesperornithine. The hesperornithines were specialized waterbirds from the Cretaceous. Some were seabirds while others lived in freshwater. Derived taxa were flightless and many had lost their forearms. They also had flattened, grebe-like toes for swimming, although some more basal taxa had typical webbed feet. In at least the most derived species the entire upper leg was encased in the body wall. Hesperornithines had beaks formed by a compound rhamphothecae at the front of the jaws, but they also had teeth further behind. They presumably had tail fans at least ancestrally, as more basal avialians that had tail fans are known. They were fairly close to modern birds.

Ichthyornis dispar (center, top right). A medium-sized seabird from the Late Cretaceous. It was very close to modern birds. Like the hesperornithines, it had a compound rhamphotheca at the front of the jaws and teeth in the back.

Vegavis iaai (center, bottom), a neornithine. All modern birds belong to the clade Neornithes. The earliest known neornithines come from the Late Cretaceous, and neornithines were the only bird (indeed, dinosaur) clade to survive the K-Pg extinction. Ancestral neornithines appear to have been waterbirds. Some of the most basal modern bird clades already had representatives during the Late Cretaceous, for example Vegavis iaai was a Cretaceous anseriform (duck). Neornithines have a toothless beak and most are excellent fliers. They also have an extremely fast growth rate compared to other dinosaurs.

Description
Clockwise from left (not to scale):
Similicaudipteryx yixianensis, an oviraptorosaur. Oviraptorosaurs had short, box-shaped heads. Derived forms (caenagnathoids) had toothless beaks and often crests. Their diet is still mostly unknown. Basal oviraptorosaurs appear to have been predominantly herbivorous, but caenagnathoids are known to have eaten small animals. Several caenagnathoid specimens have been found brooding on their nests. Though they had large pennaceous feathers on their hands and tail, all known oviraptorosaurs were probably flightless. Their tail muscles appear to have been adapted specifically for flaunting the tail feathers in visual displays. So far they're only known from the Cretaceous, but presumably have a ghost lineage stretching into the Jurassic.

Mei long, a troodont. One of the two definite deinonychosaur ("raptor") lineages, troodonts were generally small, fast omnivores with long legs and short forelimbs. Most had a retractable second toe bearing an enlarged claw that was probably used as a weapon. Some have been preserved in a curled up, possibly sleeping position and others in a brooding position. Unlike most other paravians (all known aviremigians aside from oviraptorosaurs), many troodonts had a forward-pointing pubis, but basal taxa show that this is a reversal from the ancestral backward-pointing pubis. Most known troodonts are from the Cretaceous, but some Jurassic taxa have also been found.

Rahonavis ostromi, a dromaeosaurid. The other main deinonychosaur lineage, dromaeosaurids were more adapted to hypercarnivory than are most other maniraptoriforms. The retractable toe claw was very well developed in the larger and more derived taxa, though these dromaeosaurids were also less adapted for high-speed running than were their more basal relatives. Several smaller species may have been capable of limited flight, and some had long feathers on the legs that formed a second pair of wings. Definite dromaeosaurids are only known from the Cretaceous, but presumably have a ghost lineage stretching into the Jurassic.

Xiaotingia zhengi, an archaeopterygid. Archaeopterygids were basal paravians from the Late Jurassic. They may be deinonychosaurs, but could also belong to the Avialae (closer to modern birds than to deinonychosaurs) or be equally close to both deinonychosaurs and avialians. Their flight abilities were probably limited. They retained a retractable toe claw. At least some had long leg feathers.

Epidexipteryx hui, a scansoriopterygid. Scansoriopterygids were a group of very small, enigmatic maniraptors from the Late Jurassic. They may be very basal avialians, but I wouldn't be surprised if they turned out to be something completely different. They had short blunt skulls and very long fingers, and may have been specialized for climbing trees. Their first toe was placed very low on the foot, but wasn't reversed. Unlike most other paravians, they had a forward-pointing pubis and are known to have had scales underneath the tail. By the way, though I once wrote that their wing feathers may have been attached to the third finger (instead of the second finger like other aviremigians), I'm following 's suggestion at Hell Creek that this is an illusion caused by the feathers slipping off the second finger before fossilization.

Jeholornis prima. A long-tailed avialian from the Early Cretaceous that retained a retractable second toe. It is known to have eaten seeds. Its first toe may have been slightly reversed, but it did not point fully backward as in modern birds.

Sapeornis chaoyangensis, an omnivoropterygid. Omnivoropterygids were among the most basal birds to have a very short tail tipped with a pygostyle. Unlike other basal avialians, omnivoropterygids had fully reversed toes and could perch more effectively, but probably couldn't fly very well. They had short skulls and probably fed on fruit and seeds. All omnivoropterygids are known from the Early Cretaceous.

Changchengornis hengdaoziensis, a confuciusornithid. Confuciusornithids had toothless beaks evolved convergently from those of modern birds. Their first toe probably pointed inward instead of being fully reversed. Like other basal paravians they couldn't lift their wings above their backs, so they were probably gliders or limited flappers, but they were probably quite skilled at this as far as basal avialians go. At least some are known to have eaten fish. Some individuals had a pair of ribbon-shaped tail feathers, which may represent sexual dimorphism. All confuciusornithids are known from the Early Cretaceous.

Gobipteryx minuta (top), a gobipterygid. One of the most diverse and successful bird groups during the Mesozoic were the enantiornithines, all of which are known from the Cretaceous. Most were probably good fliers and had several adaptations to flight that more basal aviremigians lacked, such as an alula and the ability to lift the wings higher above the back. The first toe in enantiornithines was also fully reversed. Some retained wing claws, others shrunk them. Enantiornithine phylogeny is a mess, but one possible enantiornithine clade was the Gobipterygidae, consisting of some small enantiornithines. Most enantiornithines had teeth, but Gobipteryx minuta itself is known to have had a toothless beak. The ribbon-shaped tail feathers are there because I felt like it, but these are indeed known in some enantiornithine taxa.

Avisaurus archibaldi (bottom), an avisaurid. Another possible clade of enantiornithines was the Avisauridae, some members of which were very large (with wingspans more than a meter wide) and others very small (the size of a passerid). Some avisaurids may have been hypercarnivorous and some may have been seabirds.

Shanweiniao cooperorum, a longipteryigid. Longipterygid enantiornithines typically had very long snouts. While most enantiornithines didn't have a fan of tail feathers, instead having no long tail feathers at all or some widely-spaced ribbon-shaped tail feathers, Shanweiniao cooperorum had four ribbon-shaped tail feathers that could have formed a tail fan, which would have helped generate lift in flight.

Patagopteryx deferrariisi. A secondarily flightless bird from the Late Cretaceous. It was closer to modern birds than enantiornithines were.

Hesperornis regalis (center, top left), a hesperornithine. The hesperornithines were specialized waterbirds from the Cretaceous. Some were seabirds while others lived in freshwater. Derived taxa were flightless and many had lost their forearms. They also had flattened, grebe-like toes for swimming, although some more basal taxa had typical webbed feet. In at least the most derived species the entire upper leg was encased in the body wall. Hesperornithines had beaks formed by a compound rhamphothecae at the front of the jaws, but they also had teeth further behind. They presumably had tail fans at least ancestrally, as more basal avialians that had tail fans are known. They were fairly close to modern birds.

Ichthyornis dispar (center, top right). A medium-sized seabird from the Late Cretaceous. It was very close to modern birds. Like the hesperornithines, it had a compound rhamphotheca at the front of the jaws and teeth in the back.

Vegavis iaai (center, bottom), a neornithine. All modern birds belong to the clade Neornithes. The earliest known neornithines come from the Late Cretaceous, and neornithines were the only bird (indeed, dinosaur) clade to survive the K-Pg extinction. Ancestral neornithines appear to have been waterbirds. Some of the most basal modern bird clades already had representatives during the Late Cretaceous, for example Vegavis iaai was a Cretaceous anseriform (duck). Neornithines have a toothless beak and most are excellent fliers. They also have an extremely fast growth rate compared to other dinosaurs.