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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