Idealized skull of Allosaurus jimmadseni in lateral (A), dorsal (B) and posterior (C) views. Skeletal reconstructions of DINO 11541 (D) and MOR 693 (E). Missing elements in indicated in gray. A–C original artwork by Samantha Zimmerman; D and E are modified from artwork by Scott Hartman. Scale bar equals 10 cm for A–C; one m for D and E.

This paper describes both DINO 11541 and MOR 693 ( Fig. 3 ) as a new species of Allosaurus and assigns other specimens from the Morrison Formation to the new taxon. The present description focuses on the head skeleton of the new taxon. We also differentiate this new species Allosaurus jimmadseni from the other two valid species of Allosaurus , Allosaurus fragilis and Allosaurus europeaus . Other previously named species of Allosaurus are invalid, including the recently named Allosaurus lucasi ( Dalman, 2014 ), and are referable to either Allosaurus fragilis or are Allosaurus species indeterminate. These findings will be addressed in a subsequent review of species of Allosaurus , which is in preparation. The objective of this study is to provide a detailed description of the skull, mandible, dentition, atlas, and axis in a comparative context and to discuss the major cranial differences between the two species of Allosaurus in the Morrison Formation. Descripiton of the postcranial skeleton of Allosaurus jimmadseni will be the subject of a future paper.

In 1934, Barnum Brown and a field crew from the American Museum of Natural History collected over 30 tons of sauropod bones from the Howe Ranch Quarry near Shell, Wyoming ( Brown, 1935 ; Colbert, 1968 ). Brown’s field crews excavated remains of multiple sauropods—including Barosaurus , Diplodocus , Apatosaurus , and Camarasaurus —along with elements of the ornithopod Camptosaurus ( Ayer, 1999 ). The only theropod remains recovered during this period were of Allosaurus . During the 1990s a commercial fossil collecting company Siber + Siber, Ltd., from Switzerland began digging at the Howe Quarry, located on private land adjacent to land administered by the U. S. Bureau of Land Management (BLM). During this effort, the commercial company found limited numbers of specimens in the original Howe Quarry and subsequently began to prospect nearby for sites nearby ( Ayer, 1999 ). In 1991 they discovered an associated Allosaurus skeleton that became known as “Big Al.” The skull was still articulated with the axial column and much of the skeleton itself was in articulation ( Fig. 2 ). Thereafter, the BLM recognized that this new site was located on public land and the excavation of the specimen (MOR 693) was taken over by a field crew from the Museum of the Rockies in Bozeman, Montana ( Breithaupt, 1996 ). Undeterred, the Swiss found another, slightly larger individual (SMA 0005) on private land at Howe Ranch and dubbed it “Big Al II” ( Ayer, 1999 ; Foth et al., 2015 ). This second Howe Ranch Quarry Allosaurus is housed in the Saurier Museum of Atahal in Switzerland. SMA 0005 is currently being described by scientists at Ludwig-Maximilians-University in Munich, Germany. Pathonogies in this specimen were described by Foth et al. (2015) .

Photograph of a painted cast of parts of the skeleton and skull of DINO 11541 in their original positions with respect to each other (A) and an explanatory line drawing taken from original quarry photos (B). Photos by Dan Chure. Scale bar equals one m.

After the articulated and nearly complete postcranium was removed, excavation continued for another 2 weeks in an attempt to find the skull, but work ceased when the quarry wall became vertical and there was no sign of it. During the summer of 1996, Ray Jones of the University of Utah came to the monument and used his recently developed radiological surveying techniques to locate a high gamma emission source in the quarry wall. Excavations began again and the skull was found just below the surface ( Jones & Chure, 1998 ; Jones, McDonald & Chure, 1998a , 1998b , 1998c ). The spatial relationship between the skull and skeleton are shown in Fig. 1 . Collection of the skull was completed in 1996 and the DINO 11541 was prepared by Scott Madsen and Ann Elder at Dinosaur National Monument during 1996 and 1997 ( Chure, 2000a ).

Here we describe two specimens of Allosaurus from the lower part of the Morrison Formation: DINO 11541 from Dinosaur National Monument of Utah and MOR 693 from the Howe Quarry in Wyoming. DINO 11541 was found by Dr. George Engelmann (University of Nebraska, Omaha) on July 15, 1990 ( Hubert & Chure, 1992 ) during a contracted paleontological inventory of the Morrison Formation of Dinosaur National Monument (National Park Service contract CA-1463-5-0001). The surface material consisted of several articulated pedal phalanges of the right pes and several articulated midcaudal vertebrae. The specimen was located about six m off the ground in a sandstone face dipping approximately 70° south. Excavation of DINO 11541 by staff of the National Park Service’s Dinosaur National Monument started in the late summer of 1990 and continued through the summer of 1994. The tilt of the beds and the weight of the block required the judicious use of explosives to remove overburden and the development of innovative solutions to getting the block horizontal on a palette ( Elder & Madsen, 1994 ; Elder, Madsen & Chure, 1994 , 1997 ). The postcranial skeleton was jacketed primarily in a single 2,700 kg block and flown out by helicopter ( Chure, 2000a ).

Over the past 20 years, the authors have conducted a hands-on, detailed morphological study of virtually all North American Allosaurus material, including several new and remarkably complete specimens that shed important light on the morphology of this dinosaur. Given the abundance of data we now possess on Allosaurus , we will present our analyses over a series of publications; this present study describing skull morphology, is the first. A postcranial description and a revision of genus Allosaurus will be the subject of a future publication.

Allosaurus is the most common genus of theropod in the Late Jurassic of North America. It is widespread both geographically and stratigraphically and the most abundant theropod in virtually all quarries ( Turner & Peterson, 1999 ; Foster, 2003 ). Nonetheless, well-preserved complete skeletons are rare, and most occurrences are represented by scattered elements. A major concentration of Allosaurus material is preserved in the Cleveland-Lloyd Dinosaur Quarry, where disassociated bones of dozens of individuals over a wide ontogenetic range occur by the thousands ( Madsen, 1976 ; Miller, Horrocks & Madsen, 1996 ).

We employ traditional, or “Romerian” anatomical and directional terms over veterinary alternatives ( Romer, 1956 ; Wilson, 2006 ). For example, “anterior” and “posterior” are used as directional terms in lieu of the veterinary alternatives “rostral”, “cranial” and “caudal.” English equivalents of standard Latin terms are used, except for the musculature system, and directional terms follow Clark (1993) . Terminology for pneumatic features is that of Witmer (1997a , 1997b) .

Exhaustive examination of Allosaurus fragilis for purposes of comparison included the proposed neotype USNM 4734 ( Paul & Carpenter, 2010 ; Carrano, Loewen & Evers, 2018 ) and material from the CLDQ quarry including: UMNH VP 1251, 3113, 5316, 5326–5328, 5470, 5480, 6317, 6340, 6365, 6400, 6408, 6473, 6475, 6499, 6502, 7190, 7408, 7411, 7794, 7880, 7882, 7884–7885, 7889–7891, 7895, 7898, 7908, 7922, 7926–7930, 7932, 7934, 7937–7938, 7957, 7966, 8102, 8123, 8142, 8151, 8229, 8240–8241, 8355, 8397, 8484, 9103, 9147, 9149, 9162, 9168, 9180, 9191, 9201, 9212, 9323, 9327, 9366, 9376, 9401, 9470, 9473, 9480, 9500, 9502, 9505, 9514, 9709, 10360, 10386, 10779, 11031, 11463, 12231, 16584–16585, and other UMNH CLDQ material. Significant other specimens of Allosaurus fragilis excavated from CLDQ at other institutions were examined including specimens at: BYU (not to be confused with the Dry Mesa Quarry material); CEU; FMNH (P1505 and P25114); ROM (12868); and YPM. Other materials examined include AMNH 275, 287, 290, 324, 408,496, 600, 666, 680, 813, 851, 5750, 5753, 5767 (holotype, Epanterias amplexus ), 6125, and 6128 from BCQ. Allosaurus material from the DNMCQ was examined including: CM 11844, and DINO 3984 and 2560 (previously catalogued as UUVP 6000). Articulated skulls MCZ 3897 R; YPM 1893; BYU 2028 (“Easter Allosaurus ”) and BYU 571-8901 (“Hinkle Allosaurus ”) were also examined.

The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the ICZN, and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/ . The LSID for this publication is: urn:lsid:zoobank.org:pub:DF37FD14-171C-4C02-8A5B-D2FCE929AABF. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS. The LSID for Allosaurus jimmadseni is: urn:lsid:zoobank.org:act:4D577308-64BC-4F87-A1F6-EE0467CF1A2F.

The specimens that are the focus of the descriptions in this paper (DINO 11541, and MOR 693) are reposited in the public repositories of Dinosaur National Monument and The Museum of the Rockies respectively. Both specimens were collected under permits obtained from the United States Department of the Interior and remain public property of the citizens of the United States. Other referred specimens come from lands administered by the BLM (USMN 544100 and SDSM 30510) and National Forest Service (BYU 4861, 5164, 5268, 5292, 5583, 11936, 13621, 16942, 17106, 17281, and other Dry Mesa Quarry allosaur materials) and were collected under research and excavation permits by other researchers. SMA 0005 was collected on private land and is reposited in the collection of the Sauriermuseum of Aathal in Switzerland. Locality information for each specimen is available from the specific repository institutions as per institutional policy. All necessary permits were obtained for the described study, which complied with all relevant regulations.

Atlantoaxial complex of Allosaurus jimmadseni in posterior view (A) with explanatory drawing (B), in right lateral view (C) with explanatory drawing (D); in anterior view (E), with explanatory drawing (F); in ventral view (G) with explanatory drawing (H); and in dorsal view (I) with explanatory drawing (J). Photos by Sorjosha Evers. Scale bar equals 10 cm. Osteological abbreviations : c1, atlantal centrum or odontoid; c2, axial centrum; dp, diapophysis; ep, epipophysis; ic1, atlantal intercentrum; na1, atlantal neural arch; nat, axial neural arch; nc, neural canal; ns, neural spine; pzp, postzygapophysis.

Medial view of disarticulated posterior right mandible of MOR 693 (A) and explanatory drawing (B) showing the relative position of the articulated antarticular. Oblique posteromedial view of the articulated left mandible of MOR 693 (C) and (D) explanatory drawing illustrating the position of the antarticular on the prearticular. Photos by Mark Loewen. Scale bar equals 10 cm. Osteological abbreviations : a, articular; an, angular; ant, antarticular; j, jugal; pa, prearticular; q, quadrate; qj, quadratojugal; rap, retroarticular process.

(A) Left jugals representing an ontogenetic series of Allosaurus fragilis from the Cleveland-Lloyd Dinosaur Quarry in left lateral view, note the highly sigmoidal ventral margin. (B) Right jugals representing an ontogenetic series of Allosaurus fragilis from the Cleveland-Lloyd Dinosaur Quarry that have been photoreversed for comparison. (C) Jugals of Allosaurus jimmadseni from top to bottom in order of descending size: BYU 13807 (photoreversed), BYU 5122, SMA 0005, MOR 693, DINO 11541, and UMNH VP C481 (photoreversed). (D) The left jugal of Allosaurus europeaus ML 415. The jugals of Allosaurus jimmadseni have a much flatter ventral margin. Photos by Mark Loewen. Scale bar equals 10 cm.

Nasals of Allosaurus jimmadseni including: MOR 693 (A), DINO 11541 (B), and SMA 0005 (C) in oblique and dorsal views illustrating the pinched nature of the nasal crest along the lateral margin of the nasal. Jugals of Allosaurus fragilis from USNM 4734 (D) and DINO 2560 (E). Photos by Mark Loewen.

Photograph of the skull of DINO 11541 (A) and MOR 693 (C) and explanatory line drawings (B) and (D). Matrix shown as stippled; dark grey represents skull mounting armature. Scale bars equals 10 cm. Photos by Mark Loewen and Serjoscha Evers. Osteological abbreviations : a, articular; an, angular; ant, antarticular; bo, basioccipital; bp, basipterygoid; bt, basal tubera; bs, basisphenoid; bsr, basisphenoid recess; fm, foramen magnum; l, lacrimal; oc, occipital condyle; ot, otoccipital; p, parietal; pa, prearticular; po, postorbital; pop, paroccipital process of the otocippital; pt, pterygoid; q, quadrate; qf, quadrate foramen; qj, quadratojugal; sa, surangular; sq, squamosal; so, supraoccipital; st, stapes.

Photograph of the braincase of DINO 11541 (A) by Serjoscha Evers and explanatory line drawing (B). Matrix shown as stippled. Scale bars equals 10 cm. Osteological abbreviations : bo, basioccipital; bp, basipterygoid process; bt, basal tubera; bs, basisphenoid; cfp, cultriform process of the parasphenoid; CN V, trigeminal foramen; CN VII, facial nerve; l, lacrimal; ls, laterosphenoid; oc, occipital condyle; ot, otoccipital; p, parietal; pfa, prefontal articulation; pop, paroccipital process of the otocippital; ppf, postorbital process of the frontal; ppl, postorbital process of the laterosphenoid; pr, prootic; pt, pterygoid.

Ventral oblique photograph of the skull of DINO 11541 (A) and interpretative line drawing (B). Ventral photograph of the skull of MOR 693 (C) and explanatory line drawing (D). Matrix shown as stippled, dark grey represents skull mounting armature. Photos by Dan Chure and Serjoscha Evers. Scale bars equals 10 cm. Osteological abbreviations : a, articular; an, angular; ant, antarticular; bo, basioccipital; bp, basipterygoid; bs, basisphenoid; bsr, basisphenoid recess; bt, basal tubera; cfp, cultriform process of the parasphenoid; d, dentary; ect, ectopterygoid; f, frontal; ic, internal choanae; imf, internal mandibular fenestra; iof, infraorbital fenestrae; j, jugal; l, lacrimal; ls, laterosphenoid; m, maxilla; n, nasal; oc, occipital condyle; p, parietal; pa, prearticular; pl, palatine; pm, premaxilla; pop, paroccipital process of the otocippital; pr, prootic; pt, pterygoid; q, quadrate; qj, quadratojugal; sa, surangular; sd, supradentary; sq, squamosal; sp, splenial; sq, squamosal; st, stapes; v, vomer.

Photograph of the skull (A) and explanatory line drawing (B) of the holotype of Allosaurus jimmadseni (DINO 11541). A CT surface scan of a cast of the skull of MOR 693 (C) and explanatory line drawing (D). Also included is a photograph of MOR 693 (E) that includes lense parallax when compared to the CT surface scan (C). Matrix shown as stippled. CT surface scan courtesy Eric Snively and Larry Witmore. Photos by Mark Loewen. Scale bars equal 10 cm. Osteological abbreviations : a, articular; ant, antarticular; aof, antorbital fossa; bs, basisphenoid; ect, ectopterygoid; ep, epipterygoid; f, frontal; j, jugal; l, lacrimal; ls, laterosphenoid; lv, lacrimal vacuity; m, maxilla; mf, maxillary fenestra; n, nasal; nf, narial fossa (external naris); o, orbit; oc, occipital condyle; ot, otoccipital; p, parietal; pa, prearticular; pf, prefrontal; pm, premaxilla; pnf, perinarial fossa; po, postorbital; pop, paroccipital process of the otocippital; pr, prootic; pt, pterygoid; q, quadrate; qj, quadratojugal; sa, surangular; sq, squamosal; so, supraoccipital; stf, supratemporal fenestra; stfo, suprtemporal fossa.

Diagnosis — Allosaurus jimmadseni is distinguished from other basal tetanurans by the following unique combination of characters: (1) in lateral view, a row of neurovascular foramina pierce the medioventral wall of the maxillary antorbital fossa; (2) straight posteroventral jugal ramus of maxilla where it articulates with jugal; (3) laterodorsal margin of nasal “pinched” into low crest continuous from premaxilla to lacrimal; (4) posterior portion of dorsal surface of nasal cup-shaped, producing a median peak in region of nasofrontal contact; (5) relatively taller lacrimal horns than in Allosaurus fragilis ; (6) jugal with relatively straight ventral margin and straight-to-slightly-curved outline in dorsal view; a well-developed distinct antarticular, and (7) axial intercentrum is rotated dorsally and has a flared rim in lateral view.

Regional horizon — Allosaurus jimmadseni was found in the Salt Wash Member of the Morrison Formation in Utah and lower part of the Brushy Basin Member of the Morrison Formation in Wyoming and South Dakota. Allosaurus jimmadseni occurs below the “clay change” of Turner & Peterson (1999) , except for at DMQ, which occurs only two m above the “clay change”.

Referred material —Referred specimens include: MOR 693 (“Big Al”), a nearly complete associated skeleton, including an articulated skull ( Figs. 2 , 3 , 5 , 6 , 8 , 10 – 12 and 14 – 16 ); SMA 0005 (“Big Al II”), a nearly complete associated skeleton, including disarticulated skull and skin impressions on the base of the tail; USMN 544100; SDSM 30510, a juvenile partial skeleton and other disarticulated adult material from the Little Houston Quarry, Wyoming; all allosaurid material from the Dry Mesa Quarry, CO curated at BYU including: BYU 4861, 5164, 5268, 5292, 5583, 11936, 13621, 16942, 17106, 17281; and unpublished material from the Meilyn Quarry reposited as casts at the NHMU (UMNH VPC 481).

Holotype —DINO 11541 is a nearly complete and articulated skeleton, including: the left half of the skull with an occluded left mandible, an articulated vertebral column from cervical 2 through caudal 8, an isolated midcaudal vertebra, an articulated string of 16 distal caudal vertebrae from near the tip of the tail, cervical and dorsal ribs, a complete gastral basket, right and left scapulae, coracoids and articulated furcula, right and left humeri, left radius and ulna, four left carpals (two proximal (radiale and intermedium) and two distal), complete left tridactyl hand, complete pelvic girdle, right and left femora, tibiae, and fibulae, right astragalus and calcaneum, right and left distal tarsal III, left distal tarsal IV, right metatarsals I–IV, proximal half of left metatarsals II–IV, right pedal phalanges II and III-1 through 2, and right pedal phalanges IV-1 through IV-5 ( Figs. 1 , 3 , 4 , 6 – 13 and 16 ).

Etymology —In honor of the late James H. Madsen, Jr and in recognition of his outstanding contributions to our knowledge of Allosaurus through his herculean efforts of protecting, excavating, preparing, and curating of many thousands of Allosaurus bones from the Cleveland-Lloyd Dinosaur and his masterful monograph ( Madsen, 1976 ) of that collection.

Description and Comparisons of Allosaurus jimmadseni

General description of the skull and lower jaw The skull of DINO 11541 was found approximately two m downstream from the axis, resting on its left side (Fig. 1). The skull is separated along the midline suture of the premaxillae and nasals and lacks the right side of the skull and the palate. The braincase, frontals and parietals are preserved slightly rotated out of their natural position. The prepared areas of the skull include: the dorsal and left side of the skull, the occipital, the right side of the braincase, and the lateral and medial surfaces of the left hemimandible. The skull of MOR 693 was found in articulation on its left side, with the right premaxilla and mandible slightly disarticulated and displaced. The skull was still attached to the first six cervical vertebrae and included sclerotic ring segments on the left side and the hyoid bones in place (Figs. 1 and 4). As is typical in theropods and many predatory archosaurs, the premaxillary and maxillary teeth project lateral to the dentary, obscuring the dorsal margin of the dentary and its dentition when the jaws were occluded. The lower jaw is shorter than the skull, resulting in an overbite as present in almost all ornithodires. Distinct external and internal mandibular fenestrae are present as in most theropods.