Prehistoric Life/Dinosaurs/Birdlike Theropods

Let’s talk about the popular word “maniraptors”. This term means “robbing hand”, and refers to their large, grasping hands with three fingers each (although some had lost some digits). Maniraptorans make together a natural subgroup of coelurosaurian theropods containing the most bird-related (and bird-looking) non-avian dinosaurs: dromaeosaurids, troodontids, oviraptorosaurs, and other groups.

Technically, also birds. All maniraptoran coelurosaurs shared forelimbs with a bony-structure more or less similar to birds’ wings, and most had true vaned feathers instead of simple down-like protofeathers and plumaceous feathers seen in non-maniraptoran coelurosaurs. All dinosaurs listed in this folder are maniraptors, except for ornithomimosaurs -- ironically, they have been considered the dinobirds for almost a century –- and some “Liaoning coelurosaurs”, which actually should be placed in the “Other Small Theropods” section, but are here for convenience. For true birds and more primitive small theropods, read the following two pages.

The first Raptor Attack: Dromaeosaurus and "post-Jurassic Park" raptors

The same size of Velociraptor but with a shorter head and stronger jaws and teeth, Dromaeosaurus is less-frequently portrayed than the Power Trio made up of Utahraptor, Velociraptor, and Deinonychus. Despite this, Dromaeosaurus appears regularly in dino-books and has also made some apparitions in TV documentaries. If you see a dromeosaurid interacting with Tyrannosaurus Rex or Triceratops in Late Cretaceous North America, it would be Dromaeosaurus -- unless the writers Did Not Do the Research or They Just Didn't Care: some docus have shown Deinonychus in this role. Walking With Dinosaurs dealed with the problem in a bizarre way: here, the dromeosaurids are officially Dromaeosaurus… but have the shape of Deinonychus. Actually, every dromeosaurid in the original series was a Deinonychus, Utahraptor included -- which, even though their name clearly means “Utah thief”, were portrayed living in Europe for some reason. And to make the “utahraptors” and the “dromeosauruses” distinguishable, they show up with a different coloration. In this show, Utahraptors are also portrayed in the way dromeosaurids were once represented in paleo-art: naked-skinned, colored like big cats, chasing an iguanodont in pack, jumping on it using their sickle-claws as spurs, and eventually killing it with (a quite exaggerated) ease. Many dino-books have made this thing Up to Eleven with Dromaeosaurus, depicting scenes in which these turkey-sized predators chase in packs adult Edmontosaurus and Triceratops 500 times heavier, and disemboweling them with their sickle-claws. Current paleontology suggests that Dromaeosaurus and the other “raptors” hunted small prey and only ate the carcasses of the giant herbivores. But stop now. Dromaeosaurus, Velociraptor, Deinonychus and Utahraptor, together with some other genera, used to make the dromaeosaurids before the Jurassic Park times and also few years later (middle 1990s). We now know they actually do not match  the great variety within their family. Especially since the beginning of the 2000s, mny new dromaeosaur species have been discovered, most of them having received the suffix raptor. Examples are the North-American Bambiraptor (so called because its skeleton was from a juvenile); the European Pyroraptor; and the South American, short-armed Austroraptor. One exception is Achillobator, which lived in Late Cretaceous Mongolia and, with its 6 m long body, was only slightly smaller than Utahraptor from Utah. These were all ground-dwelling kinds with a running-plan like the traditionally-intended “raptors”. But other “new” dromeosaurids have turned out to be smaller, more specialized animals often with some tree-climbing adaptations. Because of their apparently non-raptor-like nature, some of them were not even initially thought to be dromaeosaurs: this explains why they haven’t got the suffix –raptor. The tiny Rahonavis from Madagascar was initially thought to be some sort of bird. The same about Unenlagia, the first dromaeosaurid discovered in South America. Another non-raptor-looking dromaeosaur found in 2000 (in spite of being a climbing kind it ends in –raptor nonetheless), is now one of the most portrayed bird-like dinosaurs: obviously, we’re talking about Microraptor. See also the “Liaoning Coelurosaurs” entry below.
 * Raptor Attack exhaustively talks about dromaeosaurids. We’ll talk more about their namesake Dromaeosaurus, and other non-stock relatives. The very first discovered dromaeosaurid (1920s), Dromaeosaurus has an unexpectedly generic meaning: just “running lizard”. This because its sickle claws were missing in its original skeleton, and scientists believed it was a generic type of small theropod. The image of a hook-footed dinosaur came to light only after the description of Deinonychus in the sixties.

Big-brained and gecko-eyed: Saurornithoides and Saurornitholestes

Like Velociraptor, Saurornithoides was considered a “generic coelurosaur” until the 1970s, when it and Troodon (then still called “Stenonychosaurus”) were put in their own family, the "saurornithoidids" (now called “troodontids”). Since the 1990s, the troodontid family has acquired several new relatives, usually Cretaceous: some are cited below in the “Liaoning Coelurosaurs” section. Traditionally, Troodon and Saurornithoides have been depicted as cunning nocturnal hunters who used their large, forward-facing eyes (usually shown with cat-like or even gecko-like pupils), as well as their great intelligence to catch small mammals, grasping them with their three-fingered hands weaponed with curved claws and opposable thumbs. More realistically, their eyes were bird-like with round pupils, their hands were not so prehensile, and their great smartness is debatable (see Raptor Attack). Furthermore, according to recent research, at least Troodon was more probably omnivorous or even herbivorous (a bit like ornithomimids), because its teeth were tiny and not-so-sharp, resembling those of plant-eating dinos. However, Saurornithoides, having slightly larger head and teeth than Troodon, was more Velociraptor-like than the latter, and maybe it really corresponded to the former portrait of a specialized hunter. The similar-named, similar-sized and similar-looking Saurornitholestes was not a troodontid but one of the few dromaeosaurids known before Jurassic Park (that's why it has not the suffix -raptor). Its name is a Portmanteau of Saurornithoides and Ornitholestes). Saurornitholestes lived in Late Cretaceous North America together with Dromaeosaurus, and even though has left much more fossil material, it has not received the same level of attention in media.
 * Many dinosaurs have “-saurus” at the end, but some examples have this reversed: the hadrosaur Saurolophus, the ankylosaur Sauropelta, and this one as well: Saurornithoides (“bird-like lizard”). One of the several Troodon relatives found in Asia, Saurornithoides shared the same Late Cretaceous habitat with two iconic similar-sized theropods, Velociraptor and Oviraptor. These three maniraptorans were discovered by the American expedition in Mongolia led by Roy Chapman Andrews in the 1920s.

Ostrich-mimic: Struthiomimus, "Dromiceiomimus", and basal ornithomimosaurs

Most ornithomimids have bird-related prefixes and the suffix –mimus in their name. Ornithomimus simply means “bird-mimic”, Gallimimus “rooster-mimic” (even though it hardly resembles one…). While Struthiomimus means "ostrich-mimic" - maybe the most apt name, corresponding with the traditional nickname of the group, “Ostrich-mimic dinosaurs”. Struthiomimus lived in Late Cretaceous North America like Ornithomimus, was the same size, and the two were so similar each other, they were once believed one and the same. However, Struthiomimus had stronger forelimbs and claws than Ornithomimus, and was definitively recognized distinct in the 1970s. A third ornithomimid was described in the same years in North America, with a name that makes a sort of tongue twister: Dromiceiomimus, “emu-mimic”. This one was also virtually identical to its neighbors, only with wider eyes and longer legs, and was also cited as the “fastest-running dinosaur”. Since the 2000s this animal has been considered a new species of the genus Ornithomimus, and has disappeared from the official dinosaur list. Most ornithomimosaurs, however, have been found in Asia: other than the gigantic Gallimimus and the even more gigantic Deinocheirus (see the bottom of the page), we can mention Archaeornithomimus (“ancient Ornithomimus”) and Garudimimus “Garuda-mimic”, both archaic yet already toothless animals. But even more basal ornithomimosaurs still retained small teeth: Pelecanimimus (“pelican mimic”, because its skeleton was discovered with the print of a pelican-like gular pouch) was from Early Cretaceous Spain. Strangely, ornithomimids are one of the few coelurosaurian groups which haven't left any species in the famous Liaoning sites (which have preserved some of the most famous “feathered dinosaur fossils”), so, we don’t know what kind of covering they had. Being more primitive than deinonychosaurs and oviraptorosaurs, a purey down-liker covering seems to be the most likely; but this has yet to appear in Fictionland.
 * Which is the most iconic ornithomimid in pop-consciousness? Well, it depends on age and location. The last-generation dino-fans would respond by saying Gallimimus; the long-standing ones have a better chance to say Ornithomimus… at least, if they’re from Eagle Land. Indeed, in some other countries, this role has always been ruled by a third animal, Struthiomimus.

Egg-thieves, or what?: Chirostenotes and Citipati

But in recent years, someone has again made these genera distinct. Since the skull showed only a small relief on the nose, Chirostenotes was shown almost-crestless in old portraits; but the skull was incomplete, and now is thought it had a large crest similar to that of Citipati: that is, the animal which people usually think of when they think “Oviraptor”. Indeed, most alleged “Oviraptor” remains have been reclassified into other genera: among these the famous “dinosaur on its nest” found in the 1990s, which also contributed to debunk the classic “egg-robbing specialist” reputation. As a whole, oviraptorosaurs were quite similar to each other, but their skull ornamentations were more varied, and only the "most evolved" species were toothless and had the well-known cassowary-like crest. The oviraptorosaurs' way of life still remains an enigma: today most scientists think they were omnivorous animals that ate berries as well as insects (flowering plants were already diverse in the Cretaceous); this however doesn't exclude that they occasionally ate some unattended eggs if they got the chance: their short strong jaws and agile hands are, after all, well-suited for this –- many modern mammalian carnivores, though mainly carnivorous, also eat eggs if they have the chance. Some other primitive oviraptorosaurs are listed below: among them, several are known from feathered fossils from Liaoning.
 * Oviraptor had many relatives. Most of them have been found since the 1990s in Cretaceous Asia, but one from in North America has also been known since the 1910s: Chirostenotes, to this day still one of the few North-American oviraptorosaurs. Despite this “privileged” position, Chirostenotes remains a rather obscure dinosaur, and has had a convoluted Science Marches On-story. Since its first skeleton lacked the skull, was initially classified a “generic toothed coelurosaur”. Its toothless skull was found in 1940, but was named Caenagnathus and believed to be a bird. In the 1980s, scientists decided that Caenagnathus and Chirostenotes were one and the same, and “Caenagnathus” fell in disuse, having been created after the other name.

The first known feathered dinosaur: Avimimus

It actually had a short head and short arms typical of oviraptorousars, so it couldn’t fly. However, it should be noted that Avimimus lacked an oviraptorian crest, and also had serrations in its beak which could have worked as teeth. Since the end of the 1990s, more and more new feathered dinosaurs fossils were found in China, making Avimimus ‘s relevance mainly historical at this point.
 * When did the Great Feather Adventure begin? The answer: in 1980, in the Mongolian Gobi Desert, the same place where Oviraptor and Velociraptor were first discovered. That year, a new kind of Late Cretaceous “coelurosaur” was described from a partial skeleton, which astonished the scientist who found it. He chose to name his find Avimimus - “bird mimic”, the same as Ornithomimus, only with a Latin prefix instead of Greek. Despite this, Avimimus was not an ornithomimid, but an only 5 ft long, late-surviving basal oviraptorosaur. Nothing special per-se… except for one thing: it was the very first dinosaur whose skeleton showed some evidence of feathers. Not prints on the rock however, only a crest on its arm-bones that resembled that of modern birds. For about 15 years since then, Avimimus has been the only non-avian dinosaur regularly portrayed with feathers – often in an incorrect way: certain depictions showed it as a short-winged Archaeopteryx with the same head-shape, jaws filled with teeth, and splayed forelimbs, as if was about to take off.

Birds are dinosaurs: Liaoning theropods

Sinosauropteryx (“Chinese feathered lizard”) was the first to be discovered (1996); a compsognathid, it was the very first non-avian dinosaur to have shown prints of feathers; being it was a non-maniraptoran coelurosaur, these were still down-like, unlike modern feathers. The very first Liaoning coelurosaurs, discovered with vaned feathers in 1997 and 1998, were much closer to birds: these were Protarchaeopteryx (“First Archaeopteryx”) and Caudipteryx (“feathered tail”). Both were basal oviraptorosaurs similar to Avimimus, but only 2-3 ft long and with teeth; they had pennaceous feathers on their forearms and their tail feathers were homologous to those of the famous Archaeopteryx (not casually, these three dinosaurs have been named with the suffix -pteryx). However, the wing-feathers of Caudipteryx and Protarchaeopteryx were short and symmetrical, unlike those of true birds, and thus totally unsuitable for flight. Soon after, the list of feathered dinosaur fossils increased dramatically each year. The herbivorous Beipiaosaurus is perhaps the most specialized among them, being a small therizinosaur (see at the bottom of the page) with a down-like covering and some thin feathers on its forearm. In 2004, even a feathered tyrannosauroid was discovered: Dilong, a slender coelurosaur with little external resemblance to a T. rex, preserving some down-like feathers. Other Liaoning coelurosaurs showed up in the third episode "Dino-Birds" of the miniseries Prehistoric Park. The chosen ones were: the buck-toothed oviraptorosaur Incisivosaurus because of its funny look, the troodontid Mei, which is now “the shortest-named dino” along with the alvarezsaurid Kol (the former record-holders were the Australian ankylosaur Minmi and the oviraptorosaur Khaan), and, naturally, the ever-present Microraptor (see below). In “Dino-Birds”, Nigel Marven makes a Time Travel to the Cretaceous China to save some Microraptors from extinction; before ending the mission, he encounters several Mei (referred to by their whole scientific name: Mei long) apparently sleeping in the same bird-like position in which the type specimen was found fossilized (they were actually dead), while the other Mei long act as the “danger of the forest”, and to fit their fearsome better role, are oversized and lack feathers. Returning to Real Life, the most extraordinary Liaoning discovery has come in 2009 from Jurassic rocks: Anchiornis (literally “near bird”). This pigeon-sized troodontid (once the smallest non-avian dinosaur known, but now usurped of this record by a fragmentary tiny indeterminate maniraptor from Britain) has, amazingly, preserved not only its whole plumage, but even the original colors. Since colors have almost never preserved in vertebrate fossil record, it’s easy to understand the extraordinariness of such a discovery. Even the aforementioned Sinosauropteryx has left some traces of color, as did probably other feathered dinosaur fossils, such as the basal bird Confuciusornis and the dromaeosaurid Sinornithosaurus. Still, Anchiornis remains the most well-preserved, and it is the only non-avian dinosaur whose precise appearance is known with a reasonable degree of sureness. However, since fossilization processes often change the original patterns of live animals, the true colors of Anchiornis could possibly have faded or even changed a fair bit in 160 million years. We may never know how close our restorations are.
 * The first unequivocal non-avian dinosaur fossils with actual feathers preserved came to light in the second half of the 1990s in Liaoning (province of China). They were extraordinarily well-preserved, better than almost any other known dinosaur fossil; they were all small-sized (the biggest was only 8 ft long); they hailed mostly from the Early Cretaceous (unlike Avimimus); and they represented almost all of the main coelurosaur subgroups, giving a sort of snapshot of the coelurosaurian fauna of the time. More than 20 genera have been described so far, and others could still join them in the future: we’ll mention only some examples.

Four-winged dinosaur: Microraptor

These wings had the same structure as the wings of true birds, with asymmetrical, vane-like feathers on the forelimbs, likewise on the hindlimbs, and placed in a "fan" at the tip of its long tail: in short, very similar to the kind of plumage of the well-known Archaeopteryx (itself recently found to have had remnants of such large feathers on its legs). Of course, paleontologists and dino-fans have begun Wild Mass Guessing about its way of life. Since its discovery, Microraptor has been suggested to have been a tree-climber, with forelimbs as developed as the hindlimbs, both fitted with robust claws apt for climbing upright tree-trunks; however, [a study] published in 2011 suggests it might have been terrestrial instead. The way it traversed the air is also controversial; with true flight like modern birds, or just simple gliding like modern “flying” squirrels, “flying” fish and “flying” lizards? Currently many scientists think Microraptor was actually a flier (although not as good as modern birds): not only that, it seemed to be even better adapted for flight than Archaeopteryx. Whatever the case was in Real Life, Microraptor immediately became the center of much interest soon after the year 2000, becoming rapidly popular in illustrated books (also because was the considered the smallest dinosaur at the time); it became even more widely-known after being portrayed as one of the main animal characters in the aforementioned Prehistoric Park (where it was portrayed with the classic, splayed-limbs gliding style, now known to be anatomically impossible). Soon afterwards, it started to gain attention by the broader pop-cultural world, and it could at this point be qualified as a true Stock Dinosaur (even if only in the Rarely-Seen section). There is another story to be told about Microraptor. Before being discovered properly, the tail of one specimen had been mixed with the front end of a true bird found in Liaoning, Yanornis; the so-created Mix and Match Critter was published in media as a new kind of bird-dinosaur, “Archaeoraptor”, but this hoax was exposed after qualified scientists studied the specimen -- in fact the world-infamous article that published the fake was so hastily put-together, they didn't even bother to check if it was a true fossil or not... leading to one of the biggest controversies of modern paleontology. But although “Archaeoraptor” itself didn't exist, its tail belonged to a real animal, one that redefined our understanding of dinosaurs even more than an actual “Archaeoraptor” would have. This is an often overlooked detail, especially by creationists and conspiracy theorists who still can't let go of the controversy.
 * Discovered in 2000, Microraptor is another Liaoning coelurosaur, named “small thief”; as the "-raptor" suffix suggests, it was a dromaeosaurid. It was a find that strongly surprised not only casual paleo-fans but also the entire paleontologist community. And not because it was a feathered dino fossil (such animals were already known from the same site); nor just because it was the smallest non-avian dinosaur known at that point (merely 1.5ft long, but this record is contended now by other non-avian maniraptors). It was its unique body-plan that astonished us all. A four-winged dinosaur! More precisely, its hindlimbs had a feather-covering incredibly similar to that of its forelimbs, giving it its unbelievable appearance.
 * If this is true, it would mean that flight evolved before the appearance of the so-called “first-bird”, because Microraptor was less close to modern birds than Archaeopteryx was. And since flight was achieved in basal dromaeosaurids, this would mean that… yes, Velociraptor and all other dromaeosaurids may have descended from flying ancestors! One scientist did go Up to Eleven declaring that all maniraptorans descended from flying ancestors: this would mean, Troodon, Oviraptor, and even the huge Therizinosaurus were ancestrally creatures of the air, which, like ostriches or rheas, returned to a more ground-level way of life and increased their size.

Dino-anteaters, or what?: Mononykus, Alvarezsaurus, and relatives

We still don't know how Mononykus could have used its “hooks”: maybe it destroyed termite-mounds with them? One close relative, Shuvuuia (which just means “bird” in Mongolian), was a close relative found in 1999, and lived alongside Mononykus. Unlike the latter, Shuvuuia has left some cranial remains, which show a mobile upper jaw totally similar to a modern bird's; but this discovery has only made their way of life even more enigmatic. These dinosaurs, along with other relatives, form the Alvarezsaurids, a mainly Cretaceous family named after Alvarezsaurus, a more primitive South American genus discovered incidentally in the same year as Mononykus. It’s significant that Alvarezsaurus was initially thought a late-surviving ceratosaur that convergently became similar to an ornithomimid. Whereas Mononykus was identified as a sort of running bird, closer to a house sparrow than Archaeopteryx was. Actually, the classification of the family has always been very problematic: they have been variably put next to ornithomimids, to troodontids, or to Archaeopteryx (in this case, they would be the closest bird relatives); this is because their specialized hands made comparisons with other theropods a difficult task. However, the discovery in 2010 of a basal relative called Haplocheirus with a complete, three-fingered hand has since confirmed alvarezsaurs as non-avian maniraptors. Also note that, since “Mononychus” was changed in “Mononykus”, most alvarezsaurid genera have since called with the suffix –onykus, in a classic Follow the Leader example. One of them has been found in Alberta, living alongside many popular Late Cretaceous dinosaurs: Albertonykus.
 * Lets leave Liaoning behind and go discover another odd-looking bird-like dinosaur, Mononykus. Discovered in 1993 and initially called “Mononychus” (but that name was already taken by an insect), this is an enigmatic animal which shared Late Cretaceous Mongolia with Avimimus, Velociraptor, Saurornithoides, Oviraptor and several other coelurosaurs. Only 3 ft long, its name means “one claw” because of its strange, one-fingered hands (the other two digits usually present in coelurosaurian hands were simple stubs on Mononykus).

Deadly Embrace: Deinocheirus

Some thought it had forelimbs longer than the hindlimbs, but this is unlikely, since this would have forced the animal to walk on four legs: an impossibility, since its hands were inapt for walking. It’s more probable that Deinocheirus had the same body shape of the classic ornithomimids. If its forelimbs had the same proportions of a Gallimimus, Deinocheirus could have been as long as a Spinosaurus, and even taller, thanks to the longer neck. Some scientists have even said that it could reach the fifth story of a building if alive today, and could have weighed as much as two elephants, that is to say, two T. rexes. But most experts don’t agree with these extreme ideas, and put Deinocheirus in the same size-range as Tyrannosaurus or Allosaurus. Moreover, being an ornithomimosaur, it would be rather slender-framed, and thus it's unlikely that was as heavy as two elephants: perhaps it was even lighter than T.rex. The way-of-life of Deinocheirus has been even more enigmatic, and still remains misterious to this day. Early reports described it as a gigantic and fearsome predator, but we now know such an image is highly unlikely. Deinocheirus was either a basal toothed ornithomimosaur or a derived toothless ornithomimid. If the first is true, Deinocheirus could have been an active hunter, and someone could even imagine titanic battles againts the contemporaneous T. rex relative Tarbosaurus or even Therizinosaurus (see below). But wait: even with sharp-toothed jaws, Deinocheirus shouldn’t be seen as such a powerful killer. Its jaws and teeth would be much smaller and weaker than tyrannosaurs', carnosaurs', or even spinosaurids'. Furthermore, its claws seem too blunt to be able to rip the tough skin of a hadrosaur or a sauropod. Today, the best guess is that Deinocheirus was a sort of giant omnivore, which could have eaten from tree-tops using its forelimbs to pull down branches, and at the same time could have scavenged carrion of large herbivores, hunted small dinosaurs that could be swallowed whole, and maybe even chased Tarbosaurus away from their kills using its “terrible hands” as a scaring device. To resolve the mystery, we dino-fans are still patiently waiting for a complete Deinocheirus skeleton.
 * Most bird-like dinosaurs were small and unimpressive in Real Life compared to most other dinosaurs. This definitively couldn’t be said for the following examples, expecially Deinocheirus (“terrible hands”, not to be confused with Deinonychus, “terrible claw”). This is indeed one of the largest known theropods, and at the same time, one of the most mysterious. Discovered in the 1970s in Mongolia in Late Cretaceous rocks, only its complete forelimbs are known, along with shoulder-blades and some other fragments from the rest of the skeleton. These forelimbs were similar in shape to those of an ornithomimid… only, they were two times taller than a fully grown human. To give you an idea of the scale, several drawings have shown these immense “arms” encircling an adult man, with the three-fingered hands (each as wide as a TV-set) shown like they’re going to grasp and then lift him. The drawings usually don’t show the whole body, because its shape is totally unknown.

Converted to Veganism: Segnosaurus, Alxasaurus, and relatives

Segnosaurus used to be the best example of this in the recent past. When its incomplete remains were discovered in the 1970s, hailing from Late Cretaceous Mongolia, this 7 m long dinosaur made the scientists' eyes roll in their sockets: how could a dinosaur have the body-shape of a prosauropod, the hands and feet of a theropod, and an Iguanodon-like skull with a round bill at the front and grinding teeth behind? And, even though its pelvis was clearly saurischian in its overall structure, why did it have the pubis uniquely pointing backwards? Taxonomists were totally confused, and placed Segnosaurus in its own group, the “segnosaurs”, along with some less-known relatives. These were believed a separate evolutive branch which arose early in dino-evolution, and were classified in between theropods and sauropodomorphs or sometimes even saurischians and ornithischians. Science Marches On, however, and at the beginning of the 1990s, a much smaller relative, the 3.5 m long Alxasaurus (from Early Cretaceous China) clearly showed a coelurosaurian anatomy. This meant that segnosaurians were not only true theropods, but also members of the Maniraptoriformes (the subject of this page). Not only this: thanks to a more accurate comparison, it was discovered that the enigmatic Therizinosaurus was another member of the same group. Today, Therizinosaurus, being far cooler-looking, is much more frequent in books than Segnosaurus, and the whole group is now more frequently called “therizinosaurians”. Therizinosaurus too has had its own Science Marches On story, independent from that of Segnosaurus. We'll get to that in a minute. The diet of Segnosaurus used to be just as problematic as its classification. One theory made it a fish-eater like Baryonyx, but slippery fish could have easily escaped from its round beak; some paintings have even shown it with webbed feet, based on footprints. Another unlikely hypothesis made segnosaurians termite-eaters because of their large handclaws apparently apt to dig into termite-mounds; but again, these dinosaurs hadn't the typical tubular muzzle of a mammalian anteater, and such lage creatures couldn't have lived on insects alone. Today we think therizinosaurs were plant-eating theropods. This also explains their backward-pointing pubis: its function was to give space to the massive gut of a herbivore without losing the bipedality of a theropod. . Today, Segnosaurs are a rather well-known group, includinsg both large and small genera. Among the large ones (all Late Cretaceous), most have been found in Asia, just like Segnosaurus and Therizinosaurus (ex. Erlikosaurus), but one is known from North America: Nothronychus. Among small therizinosaurs (all Early Cretaceous), other than Alxasaurus and the feathered Beipiaosaurus, worthy of note is the North American Falcarius -- found in 2006, this one has left us with a whole graveyard containing hundreds of specimens.
 * Most dinosaurs would appear as a bunch of Mix and Match Critters if alive today, with traits resembling those of mammals, bird, and crocodiles. But the Mix and Match Critter trope can also be applied in a more subtle way. Some relatively unknown dinos actually resembled strange mixes of Stock Dinosaurs, rahter than modern animals:

Wolverine Claws: Therizinosaurus

Discovered in the 1950s but recognized as a dinosaur only in the 1970s, its forelimbs were as long and powerful as those of the giant ornithomimosaur. But Therizinosaurus had an additional curiosity, one that made it even more awesome: three scythe-like claws on each hand (hence its name, “scythe lizard”), some as long as a human arm. In short, it had the biggest nails known so far within the entire Animal Kingdom. One of these oversized claws was in fact the first known remain, and for the longest of time, scientists thought it was the rib of a giant marine turtle. With such poweful weapons, Therizinosaurus has received in the past the same treatment as Deinocheirus. Some old drawings went as far as to show our “scythe-dino” as a giant deinonychosaur with sickle-claws on each foot (if Therizinosaurus was really shaped that way, it would really have been the most Badass dinosaur one can imagine…). More accurate researches made in the beginning of the 1990s definitively debunked these fantasies: we now know with a good level of sureness that Therizinosaurus was a bulky-bodied, round-bellied, and quite slow-moving animal, that used its claws mainly to pull down branches. Furthermore, its jaws were arguably weak with a rounded horny tip and small grinding teeth similar to those seen in its relative Segnosaurus. This obviously doesn't lessen its general coolness: even with this new shape, Therizinosaurus remains an odd-looking, powerful beast, and thanks its massive body, it might even be the biggest and heaviest theropod ever discovered, weighing even more than the famous Spinosaurus. Just like with Deinocheirus, we dino-fans are patiently waiting for new exciting remains of our "Wolverine Claws-osaurus" being excavated. Meanwhile, a special spinoff of Walking With Dinosaurs from 2002 temporarily recreated our imagination in CGI: in the episode titled “The Giant Claw” Nigel Marven talks about Therizinosaurus, lampshading its whole Science Marches On story from a mighty carnivore to a Gentle Giant. Nigel is in Late Cretaceous Mongolia searching for the possessor of the eponymous “giant claw”, which the zoologist believes to have pertained to a fearsome predator. After goung through several adventures with other famous dinosaurs of the habitat (Protoceratops, Velociraptor, etc.), Nigel witnesses a fight between Therizinosaurus and Tarbosaurus: even though the former unexpectedly reveals itself to be a herbivore, it easily defeats the tyrannosaur by slapping it in the face with its scythe-claws, obligating the predator to flee. Finally, the therizinosaur licks Nigel’s face. Really!
 * Therizinosaurus could be considered the Non Identical Twin of Deinocheirus: it was a colossal yet awfully bird-like theropod, just like Deinocheirus; specialized to a non-big-prey-based diet, just like Deinocheirus; was discovered in Late Cretaceous rocks from Mongolia, just like Deinocheirus; is known mainly from forelimbs and few other bits, just like Deinocheirus; entered the dinosaur list around the same time as Deinocheirus; and, last but not least, it is another candidate for “the biggest theropod” title, like Deinocheirus! But, unlike Deinocheirus, Therizinosaurus was not a giant ornithomimosaur -- it was even more bizarre animal.

Gigant-[ic] O-[vi]-raptor: Gigantoraptor

Gigantoraptor grew up to 25ft in length, almost as big as the neighboring tyrannosaur Tarbosaurus, but with the anatomy of the classic oviraptorosaurs: and if their way of life was hard to decipher, imagine what kind of headscratching Gigantoraptor causde. It's all cool, though: three, generally small-sized lineages of non-avian coelurosaurs have at least one oversized member within their ranks: Deinocheirus the giant ornithomimosaur, Utahraptor the giant dromaeosaur, and Gigantoraptor the giant oviraptorosaur. On the other hand, tyrannosaurs and therizinosaurs include many gigantic species, while other coelurosaurs, such as the troodonts, have none. But who knows? Maybe one day a “Gigantroodon” would be discovered… Gigantoraptor recenly recevied some mild media attention, appearing in paleo-documentaries like Planet Dinosaur and Dinosaur Revolution. It could well be on its way to becoming a new member of the Stock Dinosaurs.
 * Most Oviraptor relatives were small-sized like their group's namesake, except for one: Gigantoraptor. Discovered in Asia only in 2007, this dinosaur, despite its name, is not an overgrown dromaeosaur: its name means “gigantic thief” (an evident reference to Oviraptor).