Fig 1.
The left tarsometatarsus of Avisaurus darwini (DDM 1577.730) in A) dorsal, B) plantar, C) right lateral, D) left lateral, E) proximal, and F) distal aspects. Abbreviations: dlp, distal lateral projection; dmp, distal medial projection; dvf, distal vascular foramen; fo, fossa (collateral ligament fossa); is, intermetatarsalian space; lc, lateral cotyle; lf, laterally-projecting flange; mc, medial cotyle; mtII, metatarsal II; mtIII, metatarsal III; mtIV, metatarsal IV; pf, proximoplantar fossae; pl, proximal labrum; pp, plantar projection; tct, m. tibialis cranialis tubercle. Scale bar 10mm. Line illustrations done by Samantha Clark.
Table 1.
Measurements (in mm) of each newly described specimen.
For MOR 3070, asterisks represent approximations. Total length of metatarsal II was estimated by scaling MOR 3070 to A. darwini, which is nearly the same size.
Fig 2.
The left partial tarsometatarsus of Avisaurus sp.
MOR3070 in A) dorsal, B) plantar, and C) proximal aspects. The distal portion of metatarsal II is placed in the approximate place if it was articulated. Abbreviations: dr, dorsal ridge: lc, lateral cotyle; lf, laterally-projecting flange; mc, medial cotyle; mtII, metatarsal II; mtIII, metatarsal III; mtIV, metatarsal IV; pf, proximoplantar fossae; pl, proximal labrum; tct, m. tibialis cranialis tubercle. Scale bar 10mm. Line illustrations done by Samantha Clark.
Fig 3.
The partial left tarsometatarsus of Magnusavis ekalakaensis (CCM V2019.5.1) in A) dorsal, B) plantar, and C) distal aspects, D) and the single preserved pedal phalanx in dorsal and right lateral aspects. Abbreviations; mtII, metatarsal II; mtIII, metatarsal III; pp, proximal projection; psf, plantar supratrochanter fossa; tct, m. tibialis cranialis tubercle. Scale bar 10mm. Line illustrations done by Samantha Clark.
Fig 4.
Tarsometatarsus size variation among the Avisauridae and close relatives.
A) Avisaurus darwini, B) MOR3070, C) Avisaurus archibaldi, D) Gettyia, E) Soroavisaurus, F) Mirarce, G) Intiornis, H) Neuquenornis. All to scale. Scale bar 10 mm.
Fig 5.
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.
Fig 6.
Regression analyses of A) tarsometatarsus length against time, and B) proportional tarsometatarsus mediolateral width against time. Both result in significant (p < 0.01) positive associations with an increase in body size over time. Avisaurids are colored red and all other enantiornithine are colored black.
Fig 7.
Assessing aspect ratio of A) extinct avian and non-avian theropods, and then with the addition of B) extant volant birds. The grey dots represent extinct volant theropod genera. The red dots represent non-avian theropod genera. Blue dots represent extant avian genera. Refer to S4 Table for data.
Fig 8.
A) The left tarsometatarsi in dorsal and distal aspects with the m. tibialis cranialis tubercle highlighted in a) Struthio camelus (terrestrial ratite), b) Turdus olivator (arboreal passerine), c) Cygnus buccinator (semi-aquatic anseriform), d) Buteo jamaicensis (raptorial accipitrid), e) Surnia ulula (raptorial strigid), f) Avisaurus darwini. The distal view of each taxon is represented underneath the dorsal view to show variations of the intertrochlear grooves. Tarsometatarsi are not to scale. Reference photos of tarsometatarsi sourced from morphosource. B) Tarsometatarsus length (in meters) against the proportional distal location of the m. tibialis cranialis tubercle (TCT) in extant birds and enantiornithines, including avisaurids (marked as asterisks). Extant birds are divided into their primary means of locomotion, following data from Zeffer and Norberg [52].
Fig 9.
A) Reconstruction of an articulated left tibiotarsus, tarsometatarsus, and pes of Avisaurus darwini with muscular reconstruction of the primary pedal flexors (caudal surface) and m. tibialis cranialis (cranial surface) in medial aspect. A) ankle joint shown in neutral position. B) ankle joint shown in dorsiflexion. Pedal elements are based on Mirarce which preserves nearly all pedal phalanges (Atterholt et al., 2018). The tibiotarsus is based on the distal portion preserved in Mirarce and the reconstruction given in Atterholt et al. [16]. Muscles such as the gastrocnemius and extensors are not included to more clearly show the two major flexor muscles and the m. tibialis cranialis. The tibialis cranialis muscle attaches to the hypertrophied tubercle of metatarsal II. The action of the tibialis cranialis is flexed to dorsiflex the ankle and invert the tarsometatarsus. This feature is hypertrophied in birds of prey compared to other extant birds, and is associated with an increased capacity to grasp prey with the pes. C) A member of the genus Glaucidium exhibiting the carrying strength that comes from having an enlarged and distally-migrated m. tibialis cranialis tubercle. Within this taxon, prey items may exceed three times the body mass of the owl [62, 63]. Photo used with permission and credited to Roy Priest.
Fig 10.
Action of the tibialis cranialis muscle on the ankle joint, shown in a A) resting position and a B) contracted position. As the muscle contracts, not only does the ankle dorsiflex, bringing the tibiotarsus and tarsometatarsus closer together, but the tarsometatarsus and phalanges invert (i.e., elevation of the medial border of the foot).
Fig 11.
Reconstruction of an avisaurid (e.g., A. 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.