Opposed thumbs are adaptations to arboreal life and rare for non-mammal vertebrates; Kunpengopterus antipollicatus, a newly-discovered species of arboreal pterosaur that inhabited a unique forest ecosystem in what is now China during the Jurassic period, shows the oldest record of such a feature.
Life reconstruction of Kunpengopterus antipollicatus in the Tiaojishan paleoforest; opposed pollex depicted as being utilized in handling food items (a palaeontinid) and in clinging to trees (a ginkgo). Image credit: Chuang Zhao.
Kunpengopterus antipollicatus is thought to have lived between 161 and 158 million years ago (Jurassic period).
The flying reptile had a small body, a primitive, elongate tail, and an estimated wingspan of 85 cm (33.5 inches).
It was a type of a darwinopteran, a primitive subgroup of predatory pterosaurs with transitional anatomy.
“Darwinopterans are a group of pterosaurs from the Jurassic of China and Europe, named after Charles Darwin due to their unique transitional anatomy that has revealed how evolution affected the anatomy of pterosaurs throughout time,” explained Dr. Rodrigo V. Pêgas, a paleontologist at the Federal University of ABC.
Nicknamed ‘Monkeydactyl,’ Kunpengopterus antipollicatus is the first known pterosaur with an opposed pollex (thumb).
This species also represents the earliest record of a true opposed thumb in Earth’s history.
“The fingers of ‘Monkeydactyl’ are tiny and partly embedded in the slab,” said Dr. Fion Waisum Ma, a Ph.D. researcher at the University of Birmingham.
“Thanks to micro-CT scanning, we could see through the rocks, create digital models and tell how the opposed thumb articulates with the other finger bones.”
“This is an interesting discovery. It provides the earliest evidence of a true opposed thumb, and it is from a pterosaur — which wasn’t known for having an opposed thumb.”
Holotype specimen of Kunpengopterus antipollicatus (A) and a schematic skeletal drawing (B). Scale bar – 50 mm. Abbreviations: ca – caudal series, cri – cervical rib, cv – cervical vertebra, d – digit, de – dentary, fe – femur, hu – humerus, hy – hyoid apparatus, mc – metacarpal, ph – phalanx, pop.il – postacetabular process of the illium, pp – prepubis, pt – pteroid, rd – radius, sk – skull, ti – tibia, ul – ulna. Image credit: Zhou et al., doi: 10.1016/j.cub.2021.03.030.
By studying its forelimb morphology and musculature, the paleontologists suggest that Kunpengopterus antipollicatus could have used its hand for grasping, which is likely an adaptation for arboreal life.
“A true opposed pollex is mostly present in mammals (e.g. primates) and some tree frogs, but extremely rare among extant reptiles except for chameleons,” the researchers said.
“This discovery adds to the list that darwinopteran pterosaurs such as Kunpengopterus antipollicatus also evolved an opposed thumb.”
The two fossilized specimens of Kunpengopterus antipollicatus — one of which with two associated eggs — were recovered form the Tiaojishan Formation of Liaoning in China.
“Tiaojishan paleoforest is home to many organisms, including three genera of darwinopteran pterosaurs,” said Dr. Xuanyu Zhou, a paleontologist at China University of Geosciences.
“On top of that, a particular darwinopteran fossil has been preserved with two associated eggs, revealing clues to pterosaur reproduction,” Dr. Pêgas said.
“They’ve always been considered precious fossils for these reasons and it is impressive that new darwinopteran species continue to surprise us!”
The study was published in the journal Current Biology.
Xuanyu Zhou et al. A new darwinopteran pterosaur reveals arborealism and an opposed thumb. Current Biology, published online April 12, 2021; doi: 10.1016/j.cub.2021.03.030