Falcarius Temporal range: Early Cretaceous, ~139–134.6 Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N ↓
Reconstructed skeleton, Natural History Museum of Utah
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Clade:	Maniraptora
Clade:	†Therizinosauria
Genus:	†Falcarius Kirkland et al. 2005
Type species
†Falcarius utahensis Kirkland et al. 2005

Falcarius (meaning "sickle cutter") is a genus of primitive therizinosaur dinosaur that lived during the Early Cretaceous period in what is now North America. Its remains were first collected in the Cedar Mountain Formation in 1999, with subsequent findings made during the 2000s. The genus is known from multiple specimens ranging from immature to fully-grown individuals.

Falcarius was a 4 m (13 ft) long bipedal herbivore with a small head and an elongated neck and tail. Unlike advanced therizinosaurs, Falcarius had a propubic pelvis and three-toed feet with a reduced hallux (first digit).

Falcarius is the basalmost known definitive therizinosaurian genus, and has been considered a transitional form connecting the typical theropod bodyplan to the unusual morphology of Therizinosauridae. Its description in 2005, following that of the basal therizinosauroid Beipiaosaurus from the Early Cretaceous of China in 1999, helped clarify the early evolution of the Therizinosauria and its relationships within the larger group of Theropoda.

History of discovery

The remains of Falcarius were first discovered in 1999 by commercial fossil collector Lawrence Walker at the Crystal Geyser Quarry site in Grand County. He informed paleontologist James I. Kirkland of the find, who with a team of the Utah Geological Survey from 2001 onwards began to uncover the bones that proved to be present in a two-acre (8,000-square-meter) area of Utah's Cedar Mountain Formation (Yellow Cat member). Falcarius thus lived in the Early Cretaceous period. Two extensive bone beds were discovered, including the remains of hundreds, perhaps thousands, of individuals of the new species. The minimum number of individual animals was in 2006 estimated at three hundred.^[2] In 2005 over two thousand specimens had been excavated, mostly consisting of disarticulated bones.^[3] These included the remains of juvenile animals.^[2] In 2008 a second site was reported, the Suarez Quarry, with mainly adult individuals, but of a perhaps slightly different type. In 2010 the number of specimens from the original quarry had increased to over 2,700,^[4] and was later that year reported to have risen to over three thousand.^[5]

The first remains were partially described and discussed in 2004 on several abstracts by paleontologists Lindsay E. Zanno, then a doctoral student at the university, James I. Kirkland, Scott D. Sampson, chief curator at the University of Utah's Utah Museum of Natural History, Donald D. DeBlieux, David K. Smith and R. Kent Sanders.^[6][7][8] It was not formally named until a subsequent paper published in the May 2005 issue of the journal Nature.^[3][9]

Zanno subsequently and informally described Falcarius as "the ultimate in bizarre: a cross between an ostrich, a gorilla, and Edward Scissorhands."^[9] The generic name, Falcarius, is literally taken from the Latin falcarius (meaning sickle cutter) in reference to the large hand claws. The specific name, utahensis, reflects the provenance from Utah. The holotype specimen, UMNH VP 15000, consists of a partial braincase and a large number of paratypes were assigned.^[3] Falcarius has since been described in detail by Zanno in a series of studies, beginning with her thesis in 2004.^[10]

Description

Falcarius is known from multiple specimens at different ontogenic stages. While the smallest specimen was approximately less than 1 m (3.3 ft), average adults were about 4–5 m (13–16 ft) in length weighing approximately 100 kg (220 lb).^[2][4][11]

Skull

The head anatomy of Falcarius is partially known. The skull was small and elongated. With its long neck, Falcarius could apparently reach about 1.5 m (4.9 ft) off the ground to munch leaves or fruit, possibly higher. The teeth numbered at least sixteen in the maxilla of the upper jaw. The dentary (part of the lower jaw) carried twenty-eight teeth. Its small leaf-shaped and very finely serrated maxillary teeth indicate that it consumed plant material. The front-most five teeth of the lower jaw are much longer, straighter and more pointed though, and might indicate a partially omnivorous diet including meat, e.g. small animals such as lizards. In the back of the head, the braincase was relatively large. Its lower elements were moderately inflated by pneumatised, hollowed-out, bone tissue. The large skull opening in the snout, the fenestra antorbitalis, is positioned in a depression which reaches the side of the nasal bone. There are at least five pairs of conical teeth in the front of the lower jaws. These teeth are hollowed-out at the inside. On the underside of the vertebrae of the neck there is a depression with at the mid-line a ridge. On the underside of the braincase there is a well-developed depression, the recessus basisphenoidalis. At the back of the head there are deep depressions below the occipital condyle and on the subotics, each with several pneumatic grooves.^[4][12]

Postcranial skeleton

The partitioning of the vertebral column followed that of most maniraptorans. The neck was very long with elongated cervical vertebrae. The tail was relatively long. The arm was moderately long with a somewhat robust humerus. The relatively large, and slightly recurved, pointed ten- to thirteen-centimeter (four- to five-inch) hand claws were likely used in self-defence.^[13][4]

Falcarius is known from many specimens, including complete forelimb specimens. Most of the bones of the pectoral girdle and forelimb are known, although sternal bones are not preserved. Both a left and right scapula are preserved, and they are both mostly complete as well. The blades of the scapulae are about 22.5 cm (225 mm) long, and also quite slender, with barely any expansion. They are curved instead of straight. The scapula is placed in a relatively high position. Two coracoids are also preserved, although they differ more than the scapulae in morphology. The right coracoid is the better preserved of the two. The coracoid is not specially enlarged. The furcula is elongated and narrow, its branches placed at an angle of 104°. It is V-shaped and possesses a small hypocleidium at the front. The arm as a whole is not especially developed in length but shows signs of an increasing robustness. The humerus has a rather short triangular deltopectoral crest. The epicondyles are exceptionally thick, indicating a strong musculature. The ulna is lightly curved.^[13][4]

The hand is rather long but not very robust, with in general an elongated metacarpus, although the first metacarpal is short. The first finger diverges from the second finger, the longest of the hand. The third finger is very thin. The hand claws are moderately curved, rather pointed, and moderately long. The vertebrae of the back have below their front extensions, the prezygapophyses, a groove which is divided in three smaller depressions. On the thumb claw the raised attachment point for the tendon of the extensor muscle is bordered by deep grooves for ligaments. The middle vertebrae of the back have a second ridge extending from the base of the upper rib joint, the diapophysis, to the back of the vertebra. In the arm, the shaft of the humerus is reinforced by powerful, somewhat obliquely placed, thickenings extending towards the condyles of the lower joint, which swellings have a hollow front edge. The front edge of the hip joint has a prominent swelling, pointing sideways to the back, on the pubis. In the pelvis the ilium was by a moderate inclination partly adapted to a more raised position of the trunk. The pelvis was propubic or mesopubic: the pubic bone pointed more or less forwards. The leg was relatively long and adapted to running, with the lower leg longer than the thighbone. In the foot there were three weight-bearing toes. The first toe did not reach the ground nor did the first metatarsal touch the ankle.^[13][4]

Classification

The describers in 2005 assigned Falcarius to the Therizinosauroidea sensu of Zhang, in a basal position.^[3] Zanno later placed it as the basalmost taxon in the Therizinosauria, just outside of Therizinosauroidea sensu of Sereno.^[5]

Because of the primitive position and relative completeness of Falcarius, it is a good taxon to use to compare to related groups and descendant taxa. Falcarius resembles dinosaurs belonging to the Therizinosauridae, part of the group Maniraptora. Falcarius itself probably does not belong to this former group, although it does belong to the more inclusive group Therizinosauria. The group is characterized by wider hips, a comparatively large braincase and long necks typical of plant eaters. The less basal Asian specimens were covered in quill-like feathers; this is assumed for Falcarius as well.^[5] In general Falcarius can be seen as a transitional form between the original theropod build and the more derived Therizinosauridae. Falcarius shared some derived traits with the therizinosaurids: it had a long neck, small head with teeth adapted for eating plant material, a more robust arm with large hand-claws and a more vertical position of the body. On the other hand, it resembled more typical theropods in having a long tail, a propubic pelvis, a long lower leg and a foot whose innermost toe did not reach the ground.^[4]

The cladogram below shows the results of a phylogenetic analysis of Therizinosauria performed by Yoshitsugu Kobayashi and team in 2022, based on their description of Paralitherizinosaurus. Falcarius is recovered as the basalmost therizinosaurian taxon:^[14]