Trogons (Trogoniformes) are the only group of birds with a heterodactyl foot, in which the second toe is permanently reversed.
Reconstruction of the living bar-tailed trogon (Apaloderma vittatum) in the Catalogue of the Birds in the British Museum.
Trogons and their close relatives, quetzals, are members a pantropically distributed order of birds consisting of a single family, the Trogonidae, which contains at least 43 species and 109 subspecies.
They are among the most colorful birds in the world: males are patterned with hues of iridescent green, blue, violet and purple above, and a bright red, yellow or orange abdomen; females have gray or brown plumage.
“Trogons are the only group of birds with a heterodactyl foot, in which the second toe is permanently reversed,” said Dr. Gerald Mayr from the Senckenberg Research Institute and Natural History Museum Frankfurt and his colleagues.
“These colorful birds are mainly insectivorous or frugivorous; they breed in tree holes and are able to dig their nesting cavities into rotten wood with their powerful beaks.”
“The living species occur in tropical or subtropical latitudes of Africa, Asia and the Americas, and trogons are particularly diversified in the New World, where most species and genera occur.”
“Trogons have a scant Paleogene fossil record,” the paleontologists added.
“The hitherto oldest member of the group is Septentrogon madseni from the Early Eocene (54.5 million years ago) Fur Formation in Denmark.”
“The oldest well-represented trogoniform specimens are from the latest early or earliest Middle Eocene ( 48 million years ago) of Messel in Germany.”
“This site yielded two articulated skeletons of Masillatrogon pumilio, a species that exhibits heterodactyl feet and was smaller than all living trogons.”
The newly-described fossil species already had the heterodactyl foot characterizing living trogons.
The bird had a much narrower and more gracile beak than living trogons, which denotes different ecological attributes of the ancient species.
Named Eotrogon stenorhynchus, it lived in what is now the United Kingdom during the Early Eocene epoch, some 55 million years ago.
“Eotrogon stenorhynchus already had the heterodactyl foot, even though the trochlea for the second toe is smaller than in crown group Trogoniformes,” the researchers said.
Multiple partial skeletons from the new species were recovered from the Early Eocene London Clay Formation in Walton-on-the-Naze, Essex, the United Kingdom.
“The specimens from Walton-on-the-Naze represent the earliest fossil Trogoniformes, even though — depending on their unknown exact stratigraphic provenance — they may only be slightly older than the holotype of Septentrogon madseni,” the scientists noted.
Eotrogon stenorhynchus shows some differences to living trogons in the morphology of the pectoral girdle and wing bones.
These differences suggest that the fossil species was less adapted to short-term hovering, which may indicate different foraging techniques.
“Living trogons forage by sallying flights from perches and employ a foraging technique termed sally-gleaning,” the authors said.
“Trogons are also capable of short-term hovering in order to pluck fruits or flowers.”
“The differences in the wing and pectoral girdle skeletons of early Paleogene and living trogons suggest that the fossil species were less adapted to short-term hovering and that these capabilities evolved in the trogoniform stem lineage.”
“How exactly this relates to possible differences in feeding behavior remains elusive, but we hypothesize that Paleogene stem group Trogoniformes may have been less sedentary than their living relatives, which most of the time sit motionless on their perches, waiting for feeding opportunities.”
The findings were published May 5, 2023 in the Journal of Ornithology.
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G. Mayr et al. Narrow-beaked trogons from the early Eocene London Clay of Walton-on-the-Naze (Essex, UK). J Ornithol, published online May 5, 2023; doi: 10.1007/s10336-023-02071-x
