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| Reconstruction of an avisaurid (e.g., Avisaurus darwini). Morphology of the tarsometatarsus suggests that these large birds engaged in raptorial behavior and could carry proportionally large prey. Clark, Atterholt, Scannella, Carroll & O’Connor, 2024 Illustration by Ville Sinkkonen. |
Abstract
Enantiornithines were the most diverse group of birds during the Cretaceous, comprising over half of all known species from this period. The fossil record and subsequently our knowledge of this clade is heavily skewed by the wealth of material from Lower Cretaceous deposits in China. In contrast, specimens from Upper Cretaceous deposits are rare and typically fragmentary, yet critical for understanding the extinction of this clade across the K-Pg boundary. The most complete North American Late Cretaceous enantiornithine is Mirarce eatoni, a member of the diverse clade Avisauridae. Except for Mirarce, avisaurids are known only from isolated hindlimb elements from North and South America. Here we describe three new enantiornithines from the Maastrichtian Hell Creek Formation, two of which represent new avisaurid taxa. These materials represent a substantial increase in the known diversity of Enantiornithes in the latest Cretaceous. Re-examination of material referred to Avisauridae through phylogenetic analysis provides strong support for a more exclusive Avisauridae consisting of six taxa. Exploration of the functional morphology of the avisaurid tarsometatarsus indicates potential strong constriction and raptorial attributes. The lower aspect ratio of the tarsometatarsus facilitates a more biomechanically efficient lever system which in extant birds of prey equates to lifting proportionally heavier prey items. In addition, the proportional size and distal position of the m. tibialis cranialis tubercle of the tarsometatarsus is similar to the morphology seen in extant birds of prey. Together with the deeply-grooved metatarsal trochlea facilitating robust and likely powerful pedal digits, morphologies of the hindlimb suggest avisaurids as Late Cretaceous birds of prey.
Systematic paleontology
Aves Linnaeus, 1758
Pygostylia Chiappe, 2002
Ornithothoraces Chiappe, 1995
Enantiornithes Walker, 1981
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| The three fossil specimens (from left to right), Avisaurus darwini, Avisaurus sp., and Magnusavis ekalakaensis, all of which are represented by a tarsometatarsus. They are all shown to scale with one another. |
Avisauridae Brett-Surman and Paul, 1985
Avisaurus Brett-Surman and Paul, 1985
Avisaurus darwini sp. nov.
Etymology: The specific name “darwini” is in honor of Charles Darwin, whose momentous research and publications helped define the field of evolutionary biology. Avisaurus darwini, Darwin’s bird lizard.
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| Reconstruction of an avisaurid (e.g., Avisaurus darwini). Morphology of the tarsometatarsus suggests that these large birds engaged in raptorial behavior and could carry proportionally large prey. Illustration done by Ville Sinkkonen. |
Magnusavis ekalakaenis gen. et sp. nov.
Etymology: In Latin, “Magnus” meaning big, and “avis” meaning bird, and “ekalakaensis” in honor of the town of Ekalaka, Montana, close to where this specimen was discovered. Ekalaka is Lakota for “one who wanders”. Magnusavis ekalakaenis, Ekalaka’s big bird.
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| Phylogenetic placement of new Hell Creek enantiornithines based on cladistic analysis. A) A subset of the strict consensus tree focusing on taxa around the Avisauridae, and B) the full 50% majority tree. In the majority tree, the newly diagnosed Avisauridae family is comprised of six taxa, two of which are described in this publication. C) A 1.8 m tall human to scale with A. darwini (maroon), the largest known Early Cretaceous enantiornithine Pengornis (black), a medium-sized early enantiornithine, Imparavis (white), and finally, a minuscule enantiornithine, Elektorornis (smaller inset black). The extant Buteo jamaicensis (Red-tailed Hawk) is shown in grey. |
Alexander D. Clark, Jessie Atterholt, John B. Scannella, Nathan Carroll and Jingmai K. O’Connor. 2024. New Enantiornithine Diversity in the Hell Creek Formation and the Functional Morphology of the avisaurid tarsometatarsus. PLoS ONE. 19(10): e0310686. DOI: doi.org/10.1371/journal.pone.0310686
