Temporal range: Late Cretaceous, 94–66 Ma
Possible Early Jurassic record
|Reconstructed skeleton of Nothronychus mckinleyi|
Therizinosauridae ("reaper lizards") is a family of theropod dinosaurs whose fossil remains have been dated to the Mid-to-Late Cretaceous period (100 to 70 mya). Even though representative fossils have only been found throughout China, Mongolia, and the United States, the range of Therizinosauridae was believed to have spanned much of the supercontinent of Laurasia at its height.
Therizinosauridae was named after the large, claw-bearing ungual found on the manus of members in the group. This feature has led to little insight about the ecology of the family, and the purpose of the claw remains unknown. Other notable aspects of the physiology of these animals include a modified pelvis, robust hind-limbs, and a highly derived, nearly avian inner-ear. Moreover, the larger superfamily of Therizinosauroidea is believed to be the earliest group in which simple feathers have been documented.
Research into therizinosaurids has also focused on uncovering more about the unique ecology and paleobiology of the family. A fair portion of modern research has concentrated on the feeding-patterns of these reptiles, as they are considered to be the best regarded candidate for the emergence of herbivory within Theropoda. While many closely related taxa are carnivorous, it is thought that the members of Therizinosauroidea, including Therizinosauridae, diverged and adopted either an herbivorous or omnivorous lifestyle.
The current scientific consensus is that therizinosaurids evolved from small, bird-like maniraptorans, and thus they fall within the coelurosaurian clade called Maniraptora. Most studies have concluded that within Maniraptora, Therizinosaurians were the first of five major groups to diverge.
The skull of therizinosaurids was specialized as well, as it was likely capped off with a beak-like rostrum in the front. It has been argued that this rostrum was likely covered with a keratinous beak, an adaption which might have helped to enhance cranial stability by mitigating the stress and strain experience by the skull during feeding.
Considered one of the top-contenders for the emergence of herbivory within Theropoda, members of the family possess a host of adaptions that hint towards an herbivorous lifestyle. In just the head alone a handful of possible adaptions for a life as an herbivore have been observed, including tightly-packed, serrated teeth, a low tooth-replacement rate, and finally inset teeth suggesting the presence of cheeks. All of these features suggest that members of this family were feeding on vegetation, as well as pre-processing it within their mouths to begin the breakdown of cellulose and lignin.
Brain and ear
The group is also notable for adaptations to the structures of their ear. The structure of their inner-ear is almost avian, with bird-like semicircular canals and an extended cochlea. For birds, an extended cochlea allows them to hear across an increased range of frequencies, suggesting a similar function in therizinosaurids. Furthermore, a lengthening of the cochlea, an adaptation which has independently evolved in a number of other theropod groups, is thought to further improve auditory acumen.
These adaptation to the inner-ear of therizinosaurids likely served a number of functions, acting to assist in anything from detection of small prey, to an increased alertness to the vocalizations of juveniles, to perhaps communicating with conspecifics. One source of evidence supporting these theories of sociality and communication is the discovery of large, monospecific bone-beds, which hint toward an avian-style social structure with a high degree of interaction with conspecifics.
Moreover, the presence of extensive pneumatic structures in the skulls of therizinosaurians hints toward the possibility of an increased sensitivity to sound direction. Features include not only extensive basicranial pneumaticity, but also the development of a basisphenoid bulla.
A suite of morphological adaptations separate therizinosaurids from other members of Therizinosauroidea. Within Therizinosauridae, broad changes to both the structure of the forelimb, pelvic, and pectoral girdles likely corresponded to changes in the lifestyle of the animals. The forelimb has undergone a drastic increase in robustness, the flexibility of the wrist has increased, and the presumed reach of the animal is believed to have lengthened. Moreover, the pectoral girdle has been modified to further augment upright reach, however the grasping ability of the animals is thought to have decreased. As with the modifications to the head, these adaptations are thought to have assisted with an herbivorous lifestyle, as they may have increased the ability to harvest and collect vegetation.
As members of Therizinosauroidea, the taxa within Therizinosauridae are set apart from other theropods by the possession of a unique opisthopubic pelvis, a feature known otherwise only in birds and ornithiscians. On a different note, members of Therizinosauridae are thought to have had a wide, spread-out foot containing four metatarsals, versus the typical three of most theropods.
While the exact origin of feathers is unclear, the earliest known example of primitive, Stage-I feathers is found within Therizinosauridae. Moreover, in addition to spine-like Stage-I feathers, certain members of Therizinosauroidea are thought to have possessed more developed, avian-like feathers. Although typically associated with flight, the feathers which could have covered these animals were not used for that purpose, and instead assisted in either thermoregulation or display. Moreover, a closer inspection of therizinosaurid specimens taken from the Yixian Formation has led to further insight into feather morphology, including the presence of parallel filaments and hollow cores.
The vast majority of all therizinosaurians have come out of North America and Asia, especially from the western US and China. However, a few fragmentary fossils have been found in Europe and Africa as well.
In North America, three species and two genera have been discovered, all from the western US and dating back to the Lower Cretaceous (120-140 mya). Although nothing definitive has yet been found, a few specimens that have come out of Canada are thought to possibly be therizinosaurians.
In Asia, the majority of fossils come from China, though some have come out of Mongolia as well. While fragmentary fossils have also been found in Japan, Russia, Uzbekistan, and Kazakhstan, nothing has yet been named from those areas. However, eight species have been found and named in China, with one additional species, Eshanosaurus deguchiianus, being named, though its claim to being a part of Therizinosauroidea somewhat in doubt.
The family Therizinosauridae was coined by Evgeny Maleev in 1954 to contain the enigmatic Therizinosaurus cheloniformis, which Maleev originally thought to be a species of giant "turtle-like lizard." It was long considered a strange, probably carnivorous, species. Relatives of Therizinosaurus were later found but not recognized as such for some time. These more complete specimens were herbivores, and had a bizarre mixture of what looked to early researchers like a mix between ornithischian and "prosauropod" dinosaurs. Collectively, these were included in the family Segnosauridae, or "slow lizards", due to their heavy bodies, short legs, and presumably slow, ground sloth-like lifestyle. Subsequent studies found that segnosaurids were neither prosauropods nor ornithischians, but actually bizarre theropods, and that the previously enigmatic Therizinosaurus was also a member of this group. Since the family Therizinosauridae was named earlier than Segnosauridae, the later name became a junior synonym of the former, despite the fact that Therizinosaurus was a new addition to the segnosaurid group.
Therizinosauridae was first given a phylogenetic definition by Paul Sereno in 1998, who defined it as all dinosaurs closer to Erlikosaurus than to Ornithomimus. As the relationship of therizinosaurids to other theropods clarified and more members of the larger therizinosaur group were found, a more restrictive definition has come to be preferred for the family-level name Therizinosauridae. Sereno himself, for example, re-defined the name in 2005 as the clade containing Therizinosaurus cheloniformis, Nothronychus mckinleyi, and Neimongosaurus yangi.
The family Therizinosauridae contains Nothronychus, Erlicosaurus, Neimongosaurus, Therizinosaurus, and Segnosaurus. However, some studies also place Erliansaurus and Erlikosaurus within the grouping. Therizinosauridae, along with Alxasauridae, and a handful of partially-classified genera, constitute the superfamily of Therizinosauroidea. Finally, Therizinosauroidea falls within the higher clade of Therizinosauria, which also includes Jianchangosaurus and Falcarius according to the latest phylogeny.
By the Barremian (125-129.4 mya), there was already a pan-Laurasian distribution of Therizinosaurians. Since Laurasia was beginning to break apart during the Cretaceous, two theories have arisen to explain how therizinosaurians could have spanned across Laurasia. One theory is that basal therizinosaurians were present on both Asia and North America before it began to drift apart, suggesting an emergence for Therizinosaurians of at least the Late Triassic. However, one obstacle to this theory is that it contradicts the current emergence date for therizinosaurians, which dates them to the Mid-Jurassic. A second theory is that the group dispersed between Asia and North America via a European land bridge during the Berriasian-Valanginian.
An ecological switch from a predatory to herbivorous lifestyle was likely to have been a driving and influential force in directing the morphological adaptations of therizinosaurians. This is perhaps even more so true for therizinosaurids, which seem to have further adapted their pelvic and pectoral girdles to increase their upright reach, an adaption believed to have aided in a life of herbivory. Moreover, understanding the feeding patterns of these dinosaurs is important for understanding their morphology and how herbivory emerged within Theropoda. While a majority of theropod dinosaurs were carnivorous, a few select groups either transitioned or returned to an herbivorous lifestyle. Of the groups thought to have returned to herbivory, therizinosaurians are thought to have made the switch first, and are subsequently the most widely regarded candidate for the emergence of herbivory within Theropoda.
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