The only other group of birds to have a foot in which the second toe is entirely reversed is the trogons (Trogoniformes).
Restoration of the living bar-tailed trogon (Apaloderma vittatumin the Brochure of the Birds in the British Museum
Both trogons and their near relatives the quetzals belong to a family, the Trogonidae, that spans the whole tropics and includes at least 43 distinct species and 109 subspecies.
They boast among of the world’s most vivid plumage, with males sporting patterns of green, blue, violet, and purple up top and brilliant red, yellow, or orange on their bellies, while females are often more subdued in coloration.
“Trogons are the only group of birds with a heterodactyl foot, in which the 2nd toe is completely reversed,” noted Dr. Gerald Mayr from the Senckenberg Research Institute and Natural History Museum Frankfurt and his colleagues.
These colourful birds eat mostly insects and fruit, breed in tree hollows, and carve out sturdy nesting chambers from rotted wood with their powerful beaks.
“The living types occur in tropical or subtropical latitudes of Africa, Asia, and the Americas,” with the New World hosting the greatest diversity of trogon species.
“Trogons have a little Paleogene fossil record,” the palaeontologists say.
Septentrogon madseni, discovered in Denmark’s Early Eocene Fur Formation (54.5 Ma), is the earliest known member of the group.
Recent early or early Middle Eocene (48 Ma) of Messel, Germany, has yielded the first well-represented trogoniform fossils.
Two articulated skeletons of a species of Masillatrogon with heterodactyl feet and a body size smaller than any extant trogon were discovered at this location.
The heterodactyl foot is what distinguishes modern trogons from the newly identified fossil varieties.
The bird’s slimmer and more gracile beak than modern trogons represents several environmentally beneficial traits of extinct species.
The Eotrogon stenorhynchus lived in the UK during the Early Eocene period, which occurred around 55 million years ago.
The authors noted that “Eotrogon stenorhynchus already possessed the heterodactyl foot, albeit with a smaller trochlea for the second toe than in crown group Trogoniformes.”
Walton-on-the-Naze, Essex, United Kingdom’s Early Eocene London Clay Formation yielded many fragmentary skeletons belonging to the new taxa.
The researchers remembered that “the specimens from Walton-on-the-Naze represent the earliest fossil Trogoniformes,” yet “depending upon their unidentified specific stratigraphic provenance,” they could be somewhat older than the holotype of Septentrogon madseni.
The bones in Eotrogon stenorhynchus’s pectoral girdle and wings have a somewhat different shape from those of extant trogons.
These variations suggest that the extinct species used different strategies for foraging than those optimised for short-term hovering.
“Living trogons forage by sallying flights from perches and utilise a foraging method called sally-gleaning,” the authors write.
Similarly, “Trogons are likewise efficient in short-term hovering in order to pluck fruits or flowers.”
There are differences in the wing and pectoral girdle bones of early Paleogene trogons and modern trogons, suggesting that the fossil species were less adapted to short-term hovering.
“How exactly this relates with possible differences in feeding habits remains elusive,” the authors write. “However, we assume that Paleogene stem group Trogoniformes might have been less inactive than their living loved ones.”
The results were published in the Journal of Ornithology on May 5, 2023.
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