Tenuicruris

MEANING: Slender leg

PERIOD: Early Cretaceous

CONTINENT: Australia

Tenuicruris is an enantiornithine bird that lived during the Early Cretaceous period in what is now Australia.

Abstract from paper: This study describes Tenuicruris piscanexum, gen. et sp. nov., the second Cretaceous bird named from Australia, based on a well-preserved tibiotarsus. Detailed morphological analysis and phylogenetic assessment place this new taxon within Enantiornithes, highlighting its unique combination of features, including a well-developed craniolateral cnemial crest and distinctive tarsometatarsal morphology. The novel methodology of pairwise analysis was employed to compare the new tibiotarsus with Nanantius, which confirmed both specimens had unique morphologies. Additionally, we report on the discovery of a tarsometatarsus, which although not formally named, was also phylogenetically placed within Enantiornithes.

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Tenuicruris is from the Early Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Early Cretaceous making up roughly the first half, lasting from about 143 to 100 million years ago. The poles were ice-free, due to the relatively warm climate, and forests extended into high latitudes. The continued breakup of the continents created new coastlines and isolated landmasses, influencing the evolution of distinct dinosaur faunas.

It was a time of transition, as many groups of animals and plants began to take on more modern forms while others declined or disappeared. Pterosaurs continued to thrive, though early birds were becoming more diverse and widespread. Mammals remained small but adapted to a variety of ecological niches. In the oceans, ichthyosaurs and plesiosaurs were common, and early mosasaurs began to appear.

Dinosaurs remained the dominant land animals, with groups like iguanodontians, spinosaurids, and carcharodontosaurids rising to prominence. While sauropods declined in some regions, they remained abundant in the Southern Hemisphere. The first true ceratopsians appeared, and ankylosaurs replaced stegosaurs in their niche. Dromaeosaurs and other small theropods diversified. During this time, the first flowering plants evolved, gradually changing global ecosystems by providing new food sources for herbivores.

Tenuicruris is an enantiornithean avialan. Enantiornithes is a diverse group of early birds, also known as the "opposite birds." They were the most successful group of Mesozoic birds, spanning the Cretaceous and found on nearly every continent. They ranged from sparrow-sized to birds with wingspans of a meter or more. The group's name refers to a distinctive detail of the shoulder: the joint between the scapula and coracoid is reversed compared to modern birds. Like other early birds, most retained clawed hands, conical teeth, and a reduced but often still visible bony tail base, though many also developed a pygostyle for supporting tail feathers.

Enantiornithes were capable fliers and occupied a wide range of ecological roles. Some were tree-dwelling insectivores, others fed on seeds, fish, or possibly sap, and their beaks and toe proportions vary widely, pointing to considerable ecological diversity. Exceptional fossils preserve elaborate plumage, including ribbon-like tail feathers in some males, suggesting sexual display was important. Many hatchlings appear to have been precocial, able to fly soon after hatching, a strategy quite different from most modern birds. Enantiornithes were wiped out at the end of the Cretaceous, leaving Neornithes, the ancestors of all living birds, as the only surviving avian lineage.