Birds are a diverse group of vertebrates that evolved from theropod dinosaurs during the Jurassic period. There are around 10,000 living species of birds worldwide, grouped into different clades based on anatomical and genetic similarities. Two of the major subclades within the clade Ornithothoraces are Enantiornithes and Euornithes.
Overview of Bird Evolution
The earliest known birds evolved from small, feathered theropod dinosaurs during the Middle to Late Jurassic period, around 170 million years ago. Some key evolutionary adaptations that allowed for avian flight include feathers, a wishbone, a keeled breastbone, and hollow bones. Archaeopteryx, which lived around 150 million years ago, is one of the earliest and most primitive known birds.
By the Cretaceous period, two major lineages of birds had evolved: Enantiornithes and Euornithes. Enantiornithes (meaning “opposite birds”) was the most diverse and abundant group of early birds but went extinct at the end of the Cretaceous along with non-avian dinosaurs. Euornithes (meaning “true birds”) gave rise to the birds found today and survived the Cretaceous-Paleogene extinction event around 66 million years ago.
Enantiornithes
The Enantiornithes were a diverse group of early birds that thrived during the Cretaceous period alongside dinosaurs. They inhabited environments worldwide, with fossils found on every continent. Over 80 species have been identified so far.
Some key features of Enantiornithes include:
- Light, hollow bones like modern birds
- Toothed, jaws with small teeth set in sockets
- Claws on their wings
- Small size, ranging from sparrow to crow-sized
- Variable plumage, with some having feathers similar to modern birds
Enantiornithes means “opposite birds” in Greek, which refers to the orientation of some bones in their shoulder joints being mirrored compared to modern birds. This different shoulder arrangement allowed their wings to flex backwards as well as forwards.
Some well-known examples of Enantiornithes include:
- Sinornis: One of the first Enantiornithes discovered, dating to ~125 million years ago in China
- Gobipteryx: A sparrow-sized bird with well-preserved feathers, dating to ~75 million years ago in Mongolia
- Neuquenornis: Found in South America, known for its long, tooth-lined jaws
- Pengornis: A crow-sized species with evidence of feathers, dating to ~125 million years ago in China
Enantiornithes were a diverse and successful bird group, but they perished in the Cretaceous-Paleogene mass extinction event alongside their dinosaur relatives. Their extinction allowed the ancestors of modern birds (Euornithes) to diversify and fill newly opened ecological niches.
Key Takeaways about Enantiornithes
- Lived during the Cretaceous period, ~145-66 million years ago
- Globally distributed with 80+ known species
- Key features include toothed jaws, clawed wings, small size
- Extinct at the end of the Cretaceous period
- Mirrored shoulder joint compared to modern birds
Euornithes
Euornithes includes modern birds and their most immediate ancestors. They evolved later in the Cretaceous period, arising from early avian groups like Enantiornithes. Euornithes survived the mass extinction at the end of the Cretaceous and diversified into the birds we see today.
Some key features of Euornithes include:
- Toothless, beaked jaws
- Fan of tail feathers
- Enlarged breastbone for flight muscle attachment
- Wings without claws
- Modern arrangement of shoulder bones
The earliest known Euornithes is Archaeorhynchus, dating to about 125 million years ago in China. It was a small, tern-like seabird with reduced teeth and a fan-shaped tail. Other early members include Yanornis, Yixianornis, and Apsaravis, all from the Early Cretaceous of China.
By the Late Cretaceous, Euornithes had diversified into the two main lineages that exist today: the Paleognathae (ratites and tinamous) and the Neognathae (all other living birds). Key Late Cretaceous euornithines include:
- Patagopteryx: A basal member related to today’s ratites and tinamous
- Vegavis: Related to modern ducks and geese (part of the Galloanserae group)
- Asteriornis: A relative of anseriforms (ducks, geese, swans) and galliforms (landfowl)
- Teviornis: An early screameringe, related to modern songbirds and perching birds
The Cretaceous-Paleogene extinction wiped out all Enantiornithes, allowing Euornithes to diversify and take over ecological roles once filled by their extinct relatives. Today’s 10,000+ bird species all descend from these Cretaceous Euornithes survivors.
Key Takeaways about Euornithes
- Evolved later in the Cretaceous period, ~125-66 million years ago
- Survived Cretaceous-Paleogene extinction event
- Gave rise to all modern birds
- Key features include toothless beaks, fanned tails
- Two main lineages: Paleognathae and Neognathae
Comparing the Two Clades
Enantiornithes and Euornithes co-existed for millions of years throughout the Cretaceous period but had distinct features and evolutionary histories:
Enantiornithes | Euornithes |
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While Enantiornithes were abundant and diverse in the Cretaceous, they did not survive into the Cenozoic era. Only Euornithes, the ancestors of today’s birds, managed to persist after the extinction event that wiped out non-avian dinosaurs. Their survival allowed birds to become the diverse and successful group we know today, occupying diverse niches across the globe.
Conclusion
Enantiornithes and Euornithes represent the two major lineages of Ornithothoraces, the bird group that includes all avians more derived than Archaeopteryx. Enantiornithes evolved earlier but went extinct at the end of the Cretaceous, while Euornithes gave rise to modern birds. Key differences include the presence of teeth, wing claws, and shoulder anatomy in Enantiornithes compared to the toothless beaked jaws and fanned tails of Euornithes. Only Euornithes managed to survive the mass extinction event 66 million years ago, allowing them to diversify into the tens of thousands of bird species that inhabit the planet today.