The Alvarezsauroids are an enigmatic group of Maniraptoran Theropod Dinosaurs that were first described in the early 1990s. They share a number of features with Birds, including a keeled sturnum, fused carpometacarpus and a Bird-like arrangement of the pubis and ischium. The forelimbs of Alvarezsauroids are highly derived, with an enlarged manus with a single functional claw and reduction of the other digits, an enlarged radius with powerful muscle attachments and an enlarged olecranon process (elbow joint on the ulna). At least one species possessed an extensive covering of feathers.
This led to the Alvarezsauroids initially being considered to be either early Birds, or very close relatives of the Birds, though they are now recognised to be more distant relatives within the Maniraptoran group, less closely related to Birds than are the Troodontids, Dromaeosaurs, Oviraptors, and Therizinosaurs.
The difficulty in classifying the position of the Alvarezsauroids within the Maniraptora was due to the rarity of specimens for this group, and complete absence of early, less highly derived members of the group. Then in 2010 a team of scientists led by Jonah Choiniere of the the Department of Biological Sciences at George Washington University (now also of the Evolutionary Studies Institute at the University of the Witwatersrand and the Division of Paleontology at the American Museum of Natural History) described an early Alvarezsauroid, Haplocheirus sollers, from the Late Jurassic Shishugou Formation of northern Xinjiang Province, China, in a paper in the journal Science. The specimen comprised an almost complete skeleton lacking only parts of the tail and pelvis, found in close association with the articulated skeleton of a Crocodyliform.
In a paper published in the American Museum Novitates on 22 October 2014, Jonah Choiniere, along with James Clark of the Department of Biological Sciences at George Washington University, Mark Norell of the Division of Paleontology at the American Museum of Natural History and Xing Xu of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute for Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, describe the skull of Haplocheirus sollers and discuss the conclusions they make from this about the origins of the Alvarezsauroids and the lifestyle of Haplocheiru ssollers in particular.
(A) Skull and mandible of Haplocheirus sollers in right lateral view. (B) Line drawing of (A). Abbreviations: ang, angular; aof, antorbital fenestra; art, articular; d, dentary; emf, external mandibular fenestra; en, external naris; for, foramen; j, jugal; ljr, jugal ramus of lacrimal; lmr, maxillary ramus of lacrimal; map, maxilla anterior process; mf, maxillary foramen; mlr, lateral maxillary ridge; mp, maxillary pila; n, nasal; o, orbit; oc, occipital condyle; pf, prefrontal; pm, premaxilla; pmf, promaxillary foramen; pnr, pneumatic recess; pop, paroccipital process; pt, pterygoid; pvp, vomeropterygoid process of palatine; q, quadrate; qj, quadratojugal; rap, retroarticular process; sa, surangular; snf, subnarial foramen; spl, splenial; sq, squamosal. Choiniere et al. (2014).
The skull of Haplocheirus sollersis almost complete, and shows very little flattening or distortion, having only a slight displacement of the bones on the back of the right part of the skull. However many of the bones are poorly preserved and some missing, particularly on the upper part of the snout and right part of the face. Many of the teeth on the left side are missing, while many of those on the right are obscured by the matrix.
The snout is long, and somewhat low, similar to that seen in Ornithomimosaurs, the Parvicursorine Alvarezsaurid Shuvuuia and some Troodontids. The maxillary fenestra is located approximately at midlevel in the antorbital fossa unlike the dorsally displaced maxillary fenestrae of many Dromaeosaurids. The external naris is ovoid, anteroposteriorly long and dorsoventrally low, with a long axis orientated roughly horizontally, as seen in the basal Tyrannosauroids Guanlong and Proceratosaurus, Ornithomimosaurs such as Gallimimus, Troodontids such as Sinovenator and Byronosaurus and the Alvarezsaurid Shuvuuia. The supratemporal fenestrae are large, and are separated by a low narrow ridge along the midline of the parietals, which differs from the wide, smooth pariatals which separate the supratemporal fenestrae of many basal Theropods, Ornithomimosaursand Shuvuuia. The infratemporal fenestra is high on its dorsovental axis and short anteroposteriorly, as in Ornithomimosaurs such as Garudimimus and the Therizinosaurid Erlikosaurus.
(A) Skull and mandible of Haplocheiru ssollers in dorsal view. (B) Line drawing of (A) Abbreviations: bc, braincase; f, frontal; fns, frontonasal suture; j, jugal; lac, lacrimal; lsp, laterosphenoid; m, maxilla; n, nasal; o, orbit; oc, occipital condyle; p, parietal; pf, prefrontal; pm,premaxilla; pop, paroccipital process; psp, squamosal process of postorbital; spp, paroccipital process of squamosal; stf, supratemporal fenestra. Choiniere et al. (2014).
Prior to the discovery of Haplocheirus sollers the earliest and basalmost (phylogeneticly closest to the common ancestor of the whole group) Alvarezsauroids all came from South America, leading to the belief that the group was South American in origin. A new phylogenetic analysis including Haplocheirus sollers suggests that this is not just the earliest but the basalmost member of the group (this is not always the case). The new analysis keeps the Alvarezsauroids as a monophyletic group (everything currently classified as an Alvarezsauroid descends from a common ancestor, and nothing not currently classified as an Alvarezsauroid is descended from that ancestor), with all members of the group except Haplocheirus sollers and the South American Patagonykus and Alvarezsaurus belonging to the monophyletic but poorly resolved Parvicursorinae.
The Alvarezsauroids are retained as the most basal group of Maniraptorans, with the other five groups (Therizinosaurs, Oviraptors, Dromaeosaurs, Birds and Troodonids) all more closely related to one another than to the Alvarezsauroids. The closest outgroup to the Maniraptorans (i.e. the group of Dinosaurs thought to be most closely related to the Maniraptorans without being included in the group) remain the Ornithomimids.
Phylogenetic relationships of Haplocheirus sollers. Tree is strict consensus of 80 MPTs, length = 3045, CI = 0.213, RI = 0.611. Open circle denotes Coelurosauria; open triangle denotes Maniraptora; open star denotes Paraves; closed triangle denotes Alvarezsauridae; closed circle denotes Parvicursorinae. Numbers below nodes are Bremer support values. Choiniere et al. (2014).
Haplocheirus sollers appears to have been a very unusual Theropod. Its skull was very lightly built, with enlarged fenestrae (gaps in the skull) and reduction of the supporting bones around these, strongly suggesting that it was incapable of delivering a particularly hard bite. The long snout and slender jawbone also indicate a low bite-force, but probably a rapid jaw closing ability, and its teeth were small, and weekly recurved, and show reduced serration. This suggests that Haplocheirus sollers was feeding on small vertebrate prey, which it could seize with rapid bites and probably swallowed whole.
The orbit (eye-socket) of Haplocheirus sollers is particularly large, and the preserved sclerotic ring (a ring of small bones inside the eyes of many vertebrates, but not Mammals) is also large, with an inner ring diameter estimated to be 15 mm and an outer ring diameter of 24 mm. This suggests that both the eye and the lens of the eye were exceptionally large, and significantly larger than even other Dinosaur species theorized to have been nocturnal, such as Velociraptor. This would suggest that Haplocheirus sollers was also adapted to very low light levels, and almost certainly nocturnal.
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