Therizinosauridae (meaning 'scythe lizards')[1] is an extinct family of derived (advanced) therizinosauroid dinosaurs whose fossil remains have been found in mostly Late Cretaceous boundary.

Even though representative fossils have only been found throughout Asia and North America, the range of Therizinosauridae is believed to have spanned much of the supercontinent of Laurasia based on several footprints and isolated remains in Europe and Africa.

[12] The braincases are directed to the bottom, co-ossified with well-developed sideways oriented paroccipital projections, highly pneumatized and had a prominent central foramen.

At least two different tooth morphologies are observed among therizinosaurids; the first is represented by relatively homodont, oval to lanceolate-shaped teeth with moderate coarse denticles (serrations) on the crowns (upper exposed part).

The tubercles are not as strongly developed as in other therizinosaurids though, in addition, Therizinosaurus had some of the longest forelimbs known for any bipedal dinosaurs: the preserved right arm in specimen IGM 100/15 has a total length of 2.4 m (7.9 ft).

[2][27] The vertebral column was highly pneumatized (air-spaced) and is relatively well documented from several badly to well-preserved elements among genera but specimens of Nanshiungosaurus and Nothronychus preserve the most complete series of vertebrae.

When compared to early members, therizinosaurids had a reduction in the number of caudals and a minor chevron constriction, which indicates a shorter and flexible tail.

The most modified element within the therizinosaurid build was the possession of a unique opisthopubic pelvis (pubis and ischium extending backwards), a feature known otherwise only in birds and ornithischians.

[6] The family Therizinosauridae was coined by Evgeny Maleev in 1954 to contain the enigmatic Therizinosaurus, who interpreted this taxon as representing giant marine turtles.

[1] With the description of Segnosaurus in 1979, the paleontologist Altangerel Perle coined the family Segnosauridae to contain this enigmatic taxon and tentatively considered this group to represent theropods.

Additionally, Perle compared the forelimbs in these two taxa and concluded that they were characterized by elongated arms, possibly belonging to a single taxonomic group.

[30] In a review article on the book The Dinosauria in 1990, Barsbold and Teresa Maryańska considered Segnosauria to be an enigmatic group of saurischians with a position subject to change.

Lastly, Barsbold and Maryańska noted the striking similarities between the pelvises of Nanshiungosaurus and Segnosaurus, such as the opisthopubic condition and large iliac blade.

It has been argued that this rostrum was likely covered with a keratinous beak, an adaption that might have helped to enhance cranial stability by mitigating the stress and strain experienced by the skull during feeding.

These differentiations include the relatively indistinct and symmetrical teeth with moderate serrations (denticles) in Erlikosaurus, and the enlarged serrations in Segnosaurus composed of additional carinae and folded carinae with denticulated front edges, which together created a roughened, shredding surface near the base of the tooth crowns that was apparently unique to Segnosaurus and suggest they consumed unique food resources or used highly specialized feeding strategies, and had a higher degree of oral food processing than other therizinosaurids.

However, he could neither confirm nor disregard that the hand claws could have been fully used for sexual display, self-defense, intraspecific competition, mate-gripping during mating or grasping stabilization when foraging.

The adaptations to the inner-ear and forebrain of therizinosaurids likely served a number of functions, such as well-developed senses of smell, complex social behavior, increased alertness to the vocalizations of juveniles or even communicating with conspecifics, moreover, the large pneumatic chambers on the sensorial areas in the skulls of therizinosaurids (Erlikosaurus or Nothronychus mckinleyi) indicates that the tympanic systems would result in increased and optimal low frequency sound reception, possibly infrasound.

[60][61] However, in 2007 these were described by paleontologist Martin Kundrát and colleagues and tentatively identified as therizinosaurids based on anatomical features such as the tooth-less premaxilla with a downturned edge, dentary with a lateral shelf, teeth with leaf-shaped crowns, humerus with a prominent deltopectoral crest, ilium with an expanded anterior end, and the elongated, sharply-pointed manual unguals.

The habitat that the parents nested in was a semi-arid flood plain and the egg clutches were covered in organic-rich material during incubation as some extant archosaurs do today (crocodiles and megapode birds).

The last and more advanced stage is D where the embryos had completely ossified vertebral centra and a partially reduced neurocentral suture in their cervical vertebrae.

This indicates that embryonic therizinosaurids reached a more mature skeleton than other archosaur hatchlings in ovo and stayed within the egg for a longer period to enlarge their proportions despite the advanced ossification.

Subterraneously constructed nests could be an indicative of the lack of parental care during the incubation period, furthermore, the skeletal maturity of stage D embryos was considerably adapted to allow immediate locomotion after birth, potentially suggesting a superprecocial behaviour.

[47] Multiple of their anatomical and physiological traits such as leaf-shaped, coarsely serrated teeth, strong arm build with large claws, a notoriously elongated neck and the development of a keratinous beak situate therizinosaurids as browser herbivores.

More specifically, therizinosaurids inhabited high-browsing niches in their ecosystems and commonly lived in semi-arid to wetland-like habitats composed of high vegetation as seen on the fluvial-lacrustrine setting of most specimens.

[27][55][65] As reflected by at least 31 therizinosaurid footprints at the Cantwell Formation of Alaska, some species formed small herds, which is consistent with the complex brain and ear structure in these theropods.

The co-occurrence with hadrosaurids on this area may also indicate that these very different dinosaurs benefited from an ecological interaction, just as some animals today congregate for mutual beneficial reasons, such as augmented resource acquisition or lesser predation pressure.

[52][53] The idea of a land bridge is even more supported by the multiple co-occurrence of hadrosaurid and therizinosaurid footprints at the lower Cantwell Formation of the Denali National Park, which reflects an important faunal exchange between landmasses.

As indicated by the discovery of a waterlily-like impression representing a single fossil leaf from the same site, the trackway was made by the dinosaurs as they crossed a shallow body of water away from the main river channels, which is curious given that during the Late Cretaceous this part of North America was a semi-arid habitat.

The diversity of the ichnotaxa in this site supports the idea of similar dinosaur faunas between Alaska and Asia during the Late Cretaceous period, specifically with the Nemegt Formation which had relatively wet environments.

Fiorillo and colleagues suggested that Alaska represented a "gateway" for faunal exchange between the two continental landmasses and the existence of a "Cretaceous" Beringian land bridge further allowed this mixing of faunas, which was encouraged as similar habitats were present within Asia and North America.