Early Eocene Fossil Bird Reveals How Trogons Evolved Their Unique Feet

A nearly complete fossil skeleton from Denmark shows that the distinctive heterodactyl feet of modern trogons evolved gradually. The newly described species, Daniatrogon kochi, lived during the early Eocene epoch approximately 55 million years ago and had not yet developed the fully inverted second toe seen in living trogons.

A transitional fossil in trogon evolution

Trogons are colorful, medium-sized birds found in tropical and subtropical forests in Africa, Asia and the Americas. They are recognized by several unusual anatomical features, including their heterodactyl feet.

In modern trogons, the second toe points backward and, together with the hallux, opposes the two forward-facing toes. This arrangement gives the birds a distinctive grasping foot adapted to life on branches.

In Daniatrogon kochi, however, the second toe was not completely reversed. Its claw pointed in the same general direction as the claws of the other forward-facing toes, although the digit may have been slightly rotated inward.

All other fossil trogons known from articulated skeletons have completely inverted second toes. The Danish fossil therefore provides a rare glimpse of an earlier stage in the evolution of the defining trogon foot.

Fossil trogon discovered on the island of Mors

The almost complete skeleton was discovered in 1989 by fossil collector Carsten Koch on the coast of the Danish island of Mors. It comes from the Fur Formation, a sequence of early Eocene rocks in Denmark.

The species was named Daniatrogon kochi in honor of Koch. Its discovery adds to the early Paleogene fossil record of trogons, whose earliest known fossils come from the early Eocene Fur Formation in Denmark and roughly equivalent deposits of the London Clay at Walton-on-the-Naze in England.

“The black-bellied rooster is a group of birds currently found in tropical and subtropical regions of Africa, Asia and the Americas,” Dr. Gerald Mayr of the Senckenberg Research Institute and Natural History Museum in Frankfurt wrote in the study.

“These colorful, primarily insectivorous or frugivorous birds are well characterized by heterodactyl feet in which the second toe is permanently inverted and, along with the big toe, opposes the other two fore toes.”

According to Mayr, apart from possible misidentified heterodactyl feet in Early Cretaceous enantiornithines and unconfirmed records from Early Oligocene birds, this diagnostic feature is unique to trogons.

Why did trogons evolve inverted toes?

The fossil challenges some assumptions about the function of the trogon foot. Heterodactyl feet have generally been interpreted as an adaptation for perching, compensating for the relatively weak hallux.

However, Daniatrogon kochi had very long toes, similar to those of the Messel trogon Masilatrogon. This suggests that the digits became shorter only after trogons evolved their inverted second toe.

Mayr proposes that early stem-group trogons were more active in moving through branches and vegetation while searching for food. Strong grasping feet would have helped them maintain their grip during this behavior.

“Modern trogons are rather sedentary birds that forage by jumping from perches,” the researchers wrote. “Stem-group trogons may have been more active in moving through branches and vegetation while foraging for food. Enhanced foot grasping would have been an advantage for this activity.”

The reduction of the hallux in modern trogons may therefore be associated with the evolution of a more sedentary lifestyle. Once the second toe became strongly inverted, one well-developed backward-facing digit may have been sufficient for perching.

Foot evolution preceded changes to the beak

Other early trogons had narrower and less heavily ossified beaks than modern species. Their beaks may have been used to excavate soft, rotting wood.

Mayr suggests that early trogons may have nested in existing cavities or, like some related birds, used their feet to dig into soil. The fossil indicates that changes to the feet occurred before the beak became broader and more robust.

Compared with living trogons, the narrower beaks of Daniatrogon, Eotrogon and Primotrogon may indicate that these stem-group species nested in preexisting holes or used their feet to burrow into the ground.

The study suggests that early trogons were more ecologically diverse than their living relatives, with greater variation in both beak and foot shape.

Early trogons and the evolution of modern birds

The trogon lineage is estimated to have split from the lineage leading to hoopoes, hornbills and their relatives approximately 62 million years ago. Early members of these groups were remarkably similar from the neck down.

“The main differences between early Trogoniformes and early Bucerotiformes—hoopoes, hornbills and their relatives—concern cranial features,” Mayr concluded. “This may indicate that the divergence between Trogonidae and Bucerotiformes was initially driven by different feeding specializations.”

The study was published on September 21, 2026, in the Journal of Systematic Palaeontology.

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Gerald Mayr. 2026. “The evolution of a morphological novelty: the exceptional skeleton of the early Eocene trogon Daniatrogon kochi (Trogoniformes: Trogonidae) reveals the origin of a unique foot morphology.” Journal of Systematic Palaeontology 24 (2): 2727037. doi: 10.1080/14772019.2026.2727037.