''Mirarce'' (meaning "wonderful winged messenger") is a genus of enantornithe

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

. It contains a single species, ''M. eatoni''. It was similar in size to modern

turkeys The turkey is a large bird in the genus ''Meleagris'', native to North America. There are two extant turkey species: the wild turkey (''Meleagris gallopavo'') of eastern and central North America and the ocellated turkey (''Meleagris ocell ...

Discovery

In 1992, in Utah, USA, paleontologist Howard Hutchison discovered fossilized remains of an

enantiornithine The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and c ...

bird. For a long time they have not been described; they were sometimes figured under the unofficial name of "

Kaiparowits The Paiute Indian Tribe of Utah is a federally recognized tribe of Southern Paiute and Ute Indians in southwestern Utah. Reservation The Paiute Indian Tribe of Utah (PITU) has a reservation composed of ten separate parcels of land, located in ...

enantiornithine". The holotype, UCMP 139500, is well preserved in three dimensions. It consists of a partial postcranial skeleton without a skull, including 3 cervical and 2 thoracic vertebrae, a

pygostyle Pygostyle is a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these. The pygostyle is the main component o ...

, a

furcula The (Latin for "little fork"; : furculae) or wishbone is a forked bone found in most birds and some species of non-avian dinosaurs, and is either an interclavicle or formed by the fusion of the two clavicles. In birds, its primary function is ...

, the

xiphoid process The xiphoid process (), also referred to as the ensiform process, xiphisternum, or metasternum, constitutes a small cartilaginous process (extension) located in the inferior segment of the sternum, typically ossified in adult humans. Both the Gr ...

of the

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

, a fragment of the left scapula and a

coracoid A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...

, the humerus, ulna, and radius with fragments of the manus, several fused fragments of the pelvic girdle, and some elements of the hind limbs. It is the most complete enantiornithine found in North America. In 2018, the fossil was named and described by a group of paleontologists led by Jesse Atterholt. The generic name is made up of the Latin ''mirus''; beautiful, "for an impressive level of preservation and morphological details," with the addition of the name of

Arke In Greek mythology, Arke or Arce () is one of the daughters of Thaumas, and sister to the rainbow goddess Iris. During the Titanomachy, Arke fled from the Olympians' camp and joined the Titans, unlike Iris who remained loyal to Zeus and his alli ...

( Ἄρκη \ Arkē ), the winged messenger of the Titans of Greek mythology - "for evidence pointing to an improved vehicle of this kind." The species name ''eatoni'' given in honor of Jeffrey Eaton in recognition of the decades of scientific work done on the Kaiparowits formation and the study of its fossil specimens.

Description

''Mirarce eatoni'' was a large turkey-sized bird. Skeletal morphology indicates that at the time of death, the individual was adult. All the surviving elements of the bones show complete fusion. The bones of the forelimbs deserve special attention. The humerus of the bird is short and strong. The left ulna was preserved as a mineral cast of the internal cavity with several small fragments of fossilized bone surface. Two folded spots preserved on the posterior edge of the bone body are interpreted as

quill knobs The pennaceous feather is a type of feather present in most modern birds and in some other species of maniraptoriform dinosaurs. Description A pennaceous feather has a stalk or quill. Its basal part, called a ''calamus'', is embedded in the ski ...

. These elongated folds are located along the length of the bone. Despite the fact that bone fragments do not allow measuring the size of each tubercle, determining the distance between them or estimating the number of secondary feathers, their very presence on the fossil is a very significant fact, as this is the first time the structure has been found in enantiornithines.

Phylogeny

''Mirarce'' was found to be a member of

Avisauridae Avisauridae is a family of extinct enantiornithine dinosaurs from the Cretaceous period, distinguished by several features of their ankle bones. Depending on the definition used, Avisauridae is either a broad and widespread group of advanced ena ...

, close to ''

Avisaurus ''Avisaurus'' (meaning "bird lizard") is a genus of enantiornithine avialan from the Late Cretaceous of North America. Discovery ''Avisaurus archibaldi'' was discovered in the Late Cretaceous Hell Creek Formation of North America (Maastrichtian, ...

'' itself.

Paleoecology

''Mirarce'' fossils have been found in the

Kaiparowits Formation The Kaiparowits Formation is a sedimentary geological formation, rock formation found in the Kaiparowits Plateau in Grand Staircase–Escalante National Monument, in the southern part of Utah in the western United States. It is over 2800 feet (8 ...

of western North America, which dates to the late

Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...

age of the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

. At its large size, powerful talons and leg tendons, it would have been a raptorial predator akin to modern

birds of prey Birds of prey or predatory birds, also known as (although not the same as) raptors, are hypercarnivorous bird species that actively predation, hunt and feed on other vertebrates (mainly mammals, reptiles and smaller birds). In addition to speed ...

.Clark AD, Atterholt J, Scannella JB, Carroll N, O’Connor JK (2024) New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus. PLoS ONE 19(10): e0310686. https://doi.org/10.1371/journal.pone.0310686

References

Late Cretaceous birds of North America Avisauridae Fossil taxa described in 2018 {{paleo-bird-stub