opposite bird – Nix Illustration

Gorgonavis

Gorgonavis alcyone was an enantiornithean bird that lived in what is now Spain during the early Cretaceous, about 129-126 million years ago.

Enantiornitheans were a diverse and abundant group of Mesozoic birds that retained claws on their wings and often had toothy snouts instead of beaks, and many also had ribbon-like display feathers on their tails instead of lift-generating fans. While they externally looked a lot like modern birds they weren’t ancestral to any living forms — instead they represented a separate “cousin” lineage to euornitheans that convergently evolved similar features and lifestyles.

Although known only from an isolated skull, Gorgonavis would probably have been around 14cm long (~5.5″) with an estimated wingspan of 30cm (~12″). It seems to have been a close relative of long-snouted enantiornitheans like Longipteryx, having similar elongated jaws with teeth only at the tips.

If it was a longipterygid it would be the oldest known member of that group and the only one currently known outside of China, suggesting that particular family was much more widespread than previously thought.

Longipterygids were traditionally interpreted as kingfisher-like birds specialized to prey on insects or fish, but two specimens from China with preserved gut contents recently demonstrated that they may actually have been frugivores feeding on the fleshy fruit-like seeds of gymnosperm plants.

Elsornis

The enantiornitheans (“opposite-birds”) were the most diverse and widespread group of Mesozoic birds, existing all around the world throughout the Cretaceous period. They retained claws on their wings and had toothy snouts instead of beaks, and while most of them lacked the lift-generating tail fans of modern birds they appear to have still been very adept fliers.

But Elsornis keni here was doing something different.

Known from the Late Cretaceous of Mongolia, about 80 million years ago, this opposite-bird lived alongside famous dinosaurs like Velociraptor and Protoceratops in what is now the Gobi Desert. Only a single partial specimen has ever been found, so its full life appearance is unknown and this reconstruction is somewhat speculative, but it would have been around the size of a pigeon at 25cm long (10″) – not including any decorative tail feathers it may have had, similar to other enantiornitheans.

It wing and shoulder bones were very odd for an opposite-bird, with proportions that don’t match anything capable of competent flight. Instead Elsornis appears to have been a flightless enantiornithean, a representative of a previously unknown terrestrial lineage.

Enantiophoenix

Enantiophoenix electrophyla, an enantiornithean bird from the Late Cretaceous of Lebanon (~95 mya).

It was similar in size to a modern starling, around 20cm (8″) long, and although only known from a fragmentary fossil it had fairly chunky leg bones with large claws. It was probably a strong percher like most other avisaurid enantiornitheans.

Several tiny pieces of amber were also found within the fossil, which have been suggested to be stomach contents. This could perhaps be evidence of Enantiophoenix feeding on tree sap like modern sapsuckers, but without a known skull it’s hard to tell for certain whether it was specialized for that sort of diet or not.

Longipteryx

Longipteryx chaoyangensis, an enantiornithine from the Early Cretaceous of China, about 120 million years ago. With a body length of only around 15cm (6″), it had a long snout tipped with a few hooked teeth and feet capable of perching – features that indicate it may have lived very similarly to modern kingfishers, feeding on fish and small invertebrates in its swampy forest habitat.

The enantiornithines were a sort of “cousin” lineage to modern birds. Most had toothy jaws and clawed wings, and the wide variety in their skull shapes suggests that they were specialized for many different dietary niches. The entire group went extinct during the K-Pg mass extinction and left no living descendants, but during the Cretaceous they were the most widespread and diverse group of birds*, with fossils currently known from every continent except Antarctica.

* Depending on how you define “bird”.