Iani Smithi: A new species of Rhabdodontomorph Iguanodontian Dinosaur from the Cretaceous Cedar Mountain Formation of Utah.

The Cenomanian (Earliest Late Cretaceous) Mussentuchit Member of the Cedar Mountain Formation, which outcrops in central Utah, is one of the most abundant Late Cretaceous bonebeds known from anywhere in the world, with more than 100 species described to date, including Tyrannosaurids, Dromaeosaurs, Velociraptors, Troodontids, Therizinosaurs, Titanosauromorphs, Thescelosaurids, Pachycephalosaurids, Neoceratopsians, Nodosaurids, Hadrosaurids, and non-Hadrosaurid Iguanodontians, as well as a number of less well defined specimens.

In a paper published in the journal PLoS One on 7 June 2023, Lindsay Zanno, Terry Gates, and Haviv Avrahami of Paleontology Research at the North Carolina Museum of Natural Sciences, and the Department of Biological Sciences at North Carolina State University, Ryan Tucker of the Department of Earth Sciences at Stellenbosch University, and Peter Makovicky of the Department of Earth and Environmental Sciences at the University of Minnesota, describe a new species of Rhabdodontomorph Iguanodontian Dinosaur from the Cedar Mountain Formation.

The new species is described upon the basis of a single partial skeleton (specimen NCSM 29373) comprising a largely complete, disarticulated skull; cervical, dorsal, sacral, and caudal vertebrae; associated ribs and haemal arches; and portions of the right and left pectoral girdle, left pelvic girdle, right forelimb, and right hindlimb. The elements of the briancase are incompletely fused, leading Zanno et al. to conclude that the specimen is immature. The specimen was recovered from an exposure of the Mussentuchit Member in Emery County, Utah, 1 m above the boundary with the Ruby Ranch Member, and 7 m below Mussentuchit Ash Zones 1, which has been dated to 99.490 million years before the present. The new species is named Iani smithi, where 'Iani' refers to the Roman god Ianus (or Janus), who presides over boundaries, in reference to the fact that the mid-Cretaceous was a time of biological transition in western North America, and 'smithi' honours Joshua Aaron Smith for his contributions to the discovery and conservation of paleontological resources in the region, particularly early explorations by the North Carolina Museum of Natural Sciences.

Location of holotype locality for Iani smithi (NCSM 29373). (A) Global map showing location of Mussentuchit Member outcrop in central Utah, western North America, and a stratigraphic section at the quarry with dated ash horizons; and (B) graphical representation of preserved skeletal elements of the holotype specimen (NCSM 29373). Preserved elements are coloured on the left facing skeletal whether they derive from the right or left side of the body. Exact positions of chevrons and ribs unknown due to poor preservation. See text and figures for specific positioning and completeness of elements. Abbreviations: MAZ1–4, Mussentuchit Ash Zones 1–4. Zanno et al. (2023).

Most of the skull is present, but the missing elements include the nasal and maxilla bones, so that the length and shape of the face of Iani smithi can only be estimated from the length of the mandible. This mandible comprises fragmentary predentary, complete pair of dentaries, right surangular, partial splenial, and several isolated dentary teeth. The rear part of the premaxila is thickened and shows the alveoli (sockets) of three premaxilary teeth. Premaxilary teeth are unusual in Ornithopod Dinosaurs, with most groups having lost them early in their history, but they are known in several other Rhabdodontomorphs.

Three-dimensional reconstruction of the skull of Iani smithi (NCSM 29373). (A) right lateral and (B) medial views of skull with left facial bones removed; skull reconstruction with all preserved elements (some mirrored) in (C) caudal, (D) dorsal, (E) ventral, and (F) rostral views. Scale bar 5 cm. Zanno et al. (2023).

The dentition of Iani smithi appears similar the other Rhabdodontomorphs, and non-Rhabdodontomorph early-branching Ornithopods such as Tenontosaurus and Qantassaurus. All of the teeth on the left dentary are still present, although the crowns of the middle teeth are damaged. Five teeth are still present in the left dentary, and thirteen loose teeth were found with the skeleton, interpretted as eight maxillary teeth, five dentary teeth, and a tooth of indeterminate origin. A single predentary tooth is present.

Predentary, dentary, and surangular of Iani smithi (NCSM 29373). Predentary in (A) right lateral, (B), rostral (right half mirrored and combined to show approximate shape of complete element), (C) ventral, and (D) dorsal views. Right dentary in (E) lateral view; left (pathological) dentary in (F) medial, (G) lateral, (H) dorsal, and (I) ventral views; right surangular in (J), lateral and (K), medial views. Abbreviations: a.co, estimated coronoid articulation; ad, accessory denticle; a.pd, predentary articulation; a.sr, surangular articulation; de, dentary; dp, dorsal process; dpa, dentary parapet; f, foramen; lg, lateral groove; mkc, Meckelian canal; ml, midline; path, pathological bone; pd, primary denticle; pathological bone; pr?, prearticular?; prd, peripheral denticle; sb, surangular boss; te, teeth; tr, exposed tooth root; vp, ventral process. Colour annotation: white, depressions/fossae/grooves; green, articular surfaces; blue circles, foramina; light blue lines, marginal contours; peach dashed lines, ridges/internal contours; light purple, dentition. Scale bar 5 cm. Zanno et al. (2023).

Portions of at least seven cervical (neck) vertebrae are present, although the atlas vertebra (which connects the spine to the skull) is absent. The axis (second vertebra) is present, and is compressed dorsoventrally, somewhat eroded ventrally, and approximately 150% longer than wide. At least eight unfused neural arches from the dorsal part of the spine are preserved, along with two partially fused dorsal vertebrae are preserved in variable states of damage and distortion. The dorsal vertabrae appear to be shorter and more gracile towards the head. Six sacral neural arches and four isolated sacral centra are present, showing the sacrum to have been unfused. The first seven caudal (tail) vertebrae are also present. 

Sacral vertebrae of Iani smithi (NCSM 29373). Neural arch of S1 in (A) dorsal and (D) cranial views; (B) sacral neural arches 1–6 in left lateral view; (C) neural arch of S6 in caudal view; (E) neural arch of S2 in cranial view; (F) neural arch of S4 in cranial view; (G) neural arch of S5 in cranial view; (H) neural arch of S4 in caudal view; Sacral centrum S1? in (I) left lateral, (J) caudal, and (L) cranial views; Sacral centra S2?–5? in (K) left lateral and (N) ventral views; (M) sacral centrum S2? In cranial view; (O) sacral centrum S5? in caudal view. Abbreviations: ipodf, infrapostdiapophyseal fossae; ns, neural spine; pop, postzygapophysis; prz, prezygapophysis; vk, ventral keel. Colour annotation: white, depressions/ fossae/grooves; green, articular surfaces; blue circles, foramina; light blue lines, marginal contours; peach dashed lines, ridges/internal contours. Scale bar 5 cm. Zanno et al. (2023).

At least fourteen dorsal ribs are present, these being more robust and strongly bowed than in other Rhabdodontomorphs. Both scapulae are preserved, these being stout and robust, but within the range of other Rhabdodontomorph species. An incomplete right humerus is present, with most of the proximal portion preserved, with the exception of most of the deltopectoral crest, but the distal portion is missing. The right ulna is well preserved and undistorted. A portion of a radius is also present, though this is highly eroded and lacks both ends, making it unclear which side it came from. Four elements of a manus (hand) are present; these are poorly preserved and somewhat compressed, making identification difficult, although they may be two metacarpals and penultimate and ultimate (ungual) phalanges of digits 1, 2, or 3. Few early-branching Ornithopods have well preserved manus, making comparison between these elements difficult. The shaft and distal end of the left ischium are preserved; the shaft is generally straight, but has a twist towards the distal end. The right femur is preserved, generally in good condition, but with some erosion in the region between the greater and lesser trochanters. 

Pelvic girdle and hind limb of Iani smithi (NCSM 29373). Right ischium in (A) dorsal, (B) ventral, and (C) medial views. Right femur in (D) medial, (E) lateral, (F) proximal, and (G), distal views. Abbreviations: fh, femoral head; ft, fourth trochanter; lc, lateral condyle; mc, medial condyle; op, obturator process. Colour annotation: white, depressions/fossae/grooves; green, articular surfaces; blue circles, foramina; light blue lines, marginal contours; peach dashed lines, ridges/internal contours. Scale bar 10 cm. Zanno et al. (2023).

Zanno et al. carried out several phylogenetic analyses, using a number of different matrices, which consistantly found that Iani smithi is closely related to the Rhabdodontomorphs and the non-Rhabdodontomorph Tenontosaurus, with all analyses bar one placing Iani smithi within the Rhabdodontomorphs.

Hypothesized evolutionary relationships of Iani smithi. Strict consensus tree of the Poole Matrix using (A) maximum parsimony optimality, and (B) Bayesian inference, and strict consensus tree of the Dieudonné et al. Matrix using (C) maximum parsimony optimality, and (D) Bayesian inference. Rhabdodontomorph taxa in green. Bremer support values (left) and posterior probabilities (right) shown. Grey names reflect taxa outside the illustrated clade for a particular analysis, or in the case of Dryosauridae (an ad hoc combined Operational Taxonomic Units with variable composition using different analytical approaches). Zanno et al. (2023).

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Moros intrepidus: A small Tyrannosauroid Theropod from the early Late Cretaceous of Utah.

Tyrannosauroids were the top predators in latest Cretaceous ecosystems in Asia and North America, reaching sizes of up to nine metres and having adaptive features such as rapid growth, specialised bone-crushing jaws and apparently well-developed senses of smell and vision. However, their time as top predators appears to have been limited to the last 15 million years of the Cretaceous, with the group having a much longer history as smaller predators, first appearing in the Middle Jurassic but being overshadowed by larger groups such as the Allosaurs. How the Tyrannosauroids made this transition from small to large predators is difficult to understand, particularly as the transition appears to have happened in North America, where there is a long gap in the Tyrannosauroid fossil record, between the small species of the Jurassic and Early Cretaceous, and the large species that appeared at the End Cretaceous. 

In a paper published in the journal Communications Biology on 21 February 2019, Lindsay Zanno of Paleontology at the North Carolina Museum of Natural Sciences, the Department of Biological Sciences at North Carolina State University, and the Section of Earth Sciences at the Field Museum of Natural History, Ryan Tucker of the Department of Earth Sciences at Stellenbosch University, Aurore Canoville, Haviv Avrahami, and Terry Gates, also of Paleontology at the North Carolina Museum of Natural Sciences and the Department of Biological Sciences at North Carolina State University, and Peter Makovicky, also of the Section of Earth Sciences at the Field Museum of Natural History, describe a new species of small Tyrannosauroid from the earliest Late Cretaceous Cedar Mountains Formation of Utah. 

The new species is named Moros intrepidus, where ‘Moros’ means ‘impending doom’ and ‘intrepidus’ means ‘intrepid’. The species is described from a partial right hindlimb, comprising portions of the femur, tibia, fourth, and second metatarsals and phalanges of the fourth digit, excavated from the ‘Stormy Theropod’ exposure of the Cedar Mountains Formation in Emery County, Utah, the precise location of which is restricted by Utah state statute. Analysis of zircons from the same horizon suggests that the specimen is no more than 96.4 million years old. 

Right femur of Moros intrepidus. (a) Lateral, (b) cranial, (c) medial, (d) caudal, (e) proximal, and (f) distal views. Partial mid-diaphyseal cross-section of the femur shown in (g) polarized light with lambda filter, (h) natural light with numbered arrows and tracings indicating seven growth cycles, and (i) polarized light. Abbreviations: ar adductor ridge, at accessory trochanter, Ca caudal aspect, Cr cranial aspect, ft fourth trochanter, if intercondylar fossa, inf intertrochanteric nutrient foramen, L lateral aspect, L2 lobe on lesser trochanter, lic linea intermuscularis caudalis. lt lesser trochanter, M medial aspect, mdc mesiodistal crest, pf popliteal fossa, pld lateral depression, proximal. pnf principle nutrient foramen, sat semicircular accessory tuberosity, ts trochanteric shelf. Scale bars (a)–(e) 5 cm; (g)–(i) 1mm. Zanno et al. (2019). 

As well as the hindlimb, Zanno et al. describe two isolated premaxillary teeth from separate exposures of the Cedar Mountains Formation at Suicide Hill and the Cliffs of Insanity. These are flattened in aspect with one concave edge, interpreted as the inner surface, and distinct carinae (grooves) on their front and back surfaces, all features typical of Tyrannosauroids. 

 (c) Silhouette of Moros intrepidus showing recovered elements. Isolated indet. tyrannosauroid premaxillary tooth recovered from nearby strata in (d) occlusal, (e) mesiodistal, and (f) lingual views. Holotype specimen of Moros intrepidus composed of (g) femur, (h) tibia, (i) fourth metatarsal, (j) second metatarsal, and (k) pedal phalanges of the fourth digit. Scale bar (c) 1 m, (g)–(k) 5 mm. (d)–(f) Enlarged to show detail, not to scale. Zanno et al. (2019). 

Based upon the size of the known bones, Moros intrepidus is estimated to have had a mass of about 78 kg. Examination of the microstructure of the bones of the specimen suggests that it was at least six-to-seven years old when it died (Tyrannosauroids are known to have had seasonal growth, resulting in bands of denser material within their bones that form growth rings similar to those seen in trees; because bone is sometimes re-absorbed by the body during growth, this cannot provide as an absolute age, but counting the rings does give a rough minimum age for a specimen). This implies a growth rate similar to that seen in Jurassic Asian Tyrannosauroids such as Guanlong wucaii, a specimen of which with a similar size to Moros intrepidus has been estimated to have been about seven years old when it died, and is in marked contrast to later North American Tyrannosaurids such as Gorgosaurus ibratus, which would have been around three times as large at a similar age. This suggests that the emergence of large Tyrannosauroids really was restricted to the last few million years of the Cretaceous, and may have been closely linked to the disappearance of the earlier large Allosaurs.

Phylogenetic relationships, chronostratigraphic, and palaeoecological implications of Moros intrepidus. (a) Graphic illustrating temporal range of North American Tyrannosauroids including species-level range prior to the discovery of Moros intrepidus, extension of current range, and hypothesized range based on isolated teeth. The current gap in the North American Tyrannosauroid record spans from the Tithonian to the Aptian. Faunal composition of Late Cretaceous ecosystems was established between the Albian and Turonian, as recognized by the stratigraphic appearance of major clades. (b) generalized phylogenetic relationships of Tyrannosauroidea, showing the appearance of select traits related to cursoriality in Tyrannosaurs that are newly optimized as a result of the discovery of Moros intrepidus. (c) Stratigraphic distribution of Allosauria in North America (including Megaraptora) documents overlap with Moros intrepidus in early Late Cretaceous ecosystems leading to (d) refined calibration on the origin of late diverging Tyrannosauroids and clade-level faunal turnover within apex predator roles throughout the Late Jurassic–Late Cretaceous of North America. Coloured polygons are stylized call-outs and are not intended to reflect two dimensional data. Zanno et al. (2019). 

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