Systematic paleontology

Dinosauria Owen, 1842

Theropoda Marsh, 1881

Coelurosauria von Huene, 1914

Tyrannosauroidea Osborn, 1905

Tyrannosauroidea indet.

Referred materials NCSM 33393, NCSM 33276 isolated tyrannosauroid premaxillary teeth.

Locality and horizon NCSM 33393 and NCSM 33276 were recovered from Suicide Hill (NCPALEOUT11) and the Cliffs of Insanity Microsite (NCPALEOUT18), respectively, lower Mussentuchit Member, upper Cedar Mountain Formation, Emery County, Utah, USA (Fig. 1). NCPALEOUT11 and NCPALEOUT18 are located on land administered by the US BLM and State of Utah, respectively; access to exact locality information is restricted by federal and state statutes and is available to qualified researchers via the NCSM. Fossils at Suicide Hill are entombed in a laterally discontinuous lenticular crevasse splay. Microvertebrate fossils from the Cliffs of Insanity Microsite were recovered from volcanilithic-rich, fine-grained muddy sandstone, and muddy siltstone (Supplementary Methods 1, Supplementary Fig. 1).

Fig. 1 Location of holotype locality for M. intrepidus (NCSM 33392). (a) Global view showing extent of Cedar Mountain Formation outcrop in central Utah, (b) generalized stratigraphic section outcrop of the Cedar Mountain Formation in area of discovery, and (c) silhouette of M. intrepidus showing recovered elements. Isolated indet. tyrannosauroid premaxillary tooth (NCSM 33393) recovered from nearby strata in (d) occlusal, (e) mesiodistal, and (f) lingual views. Holotype specimen of M. intrepidus (NCSM 33392) 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 Full size image

Diagnosis (autapomorphy denoted with asterisk) tyrannosauroid premaxillary teeth exhibiting a salinon cross section; sharp, sinuous mesial, and distal carinae lacking serrations; and a pronounced lingual ridge; some teeth bearing a deep, obliquely oriented groove incising the lingual ridge* (Fig. 1; Supplementary Methods 2, Fig. 2).

Fig. 2 Right femur of M. intrepidus (NCSM 33392). (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 (see Supplementary Fig. 5), 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 (sensu17), 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 bar (a–e) 5 cm; (g–i) 1 mm Full size image

Description and comparisons Two isolated premaxillary teeth bear lingually rotated mesial and distal carinae forming a salinon cross-section at mid-crown height, and a highly convex labial aspect as in tyrannosauroids generally10,11,12 (Fig. 1; Supplementary Figs. 2 and 3). In mesial/distal views carinae are sinuous, transitioning from lingually convex near the base to lingually concave approaching the occlusal surface (Supplementary Fig. 2d, f, h). Carinae terminate prior to reaching the root/crown juncture (Supplementary Fig. 2b). Mesial and distal aspects of the crown are depressed, yielding a weakly hourglass-shaped cross-section at the crown base (Fig. 1a; Supplementary Fig. 2). Crown height ranges from 6 mm (NCSM 33276) to 11.34 mm (NCSM 33393). Carinae lack serrations as in the Early Cretaceous tyrannosauroid Xiongguanlong9 (Supplementary Fig. 4) and an isolated tooth from the Cloverly Formation12. This feature has been interpreted as ontogenetically variable, yet serrations line the premaxillary teeth of Tarbosaurus beginning at 2-years of age13, suggesting it may be phylogenetically informative as suggested by Zanno and Makovicky12.

As in other tyrannosauroids, teeth exhibit a pronounced lingual ridge. The lingual ridge on the largest tooth is cleaved by a deep, obliquely oriented groove (Fig. 1d, g; Supplementary Fig. 2). However, the groove is not present on the smaller specimen (NCSM 33276) and may be an ontogenetic, or tooth position-dependent trait. To our knowledge, this feature has not been described on other tyrannosauroid premaxillary teeth and as such can currently be considered autapomorphic. However, given the dearth of premaxillary teeth in mid-Cretaceous taxa, damage to this region in teeth on other mid-Cretaceous tyrannosauroid teeth12, and difficulty in assessing premaxillary dentition in labial view in articulated skulls of tyrannosaurids, we consider it possible that the trait is more widespread.

Moros intrepidus gen. et sp. nov.

Holotype NCSM 33392, an associated right hind limb of a subadult individual including portions of the femur, tibia, fourth, and second metatarsals and phalanges of the fourth digit (Figs. 2, 3, and 4).

Fig. 3 Right tibia (a–f) and right fourth metatarsal (g–l) of M. intrepidus (NCSM 33392). (a, g) lateral, (b, h) cranial, (c, i) medial, (d, j) caudal, (e, k) proximal, and (f, l) distal views. Abbreviations: afMT2 articular facet for second metatarsal, afMT3 articular facet for third metatarsal, alc accessory lateral condyle, Ca caudal aspect, cc cnemial crest, clp collateral ligament pit, Cr cranial aspect, conc concave surface of the articular facet for the third metatarsal, conv convex surface of the articular facet for the third metatarsal, dfb distal fibular buttress, fbc fibular crest, ff articular facet for the fibula, L lateral aspect, lc lateral condyle, lg lateral groove, M medial aspect, mc medial condyle, nf nutrient foramen, smgl scar for M. gastroccnemius lateralis. Scale bar 5 cm Full size image

Fig. 4 Right second metatarsal (a–d), right pedal phalanges IV–III (superior) and IV–IV (inferior) (e-j) of M. intrepidus (NCSM 33392). (a, e) lateral, (b, g) medial, (c, i) proximal, (d, j) distal, (f) dorsal, (h) ventral views. Abbreviations: afMT3 articular facet for third metatarsal, Ca caudal aspect, cc cnemial crest, clp collateral ligament pit, Cr cranial aspect, L lateral aspect, lc lateral condyle, M medial aspect, mc medial condyle. Scale bar 5 cm Full size image

Etymology Moros, (Greek) the embodiment of impending doom, in reference to the establishment of the Cretaceous tyrannosauroid lineage in NA, and intrepidus, (Latin) for intrepid, in reference to the hypothesized intracontinental dispersal of tyrannosaurs during this interval.

Locality and horizon NCSM 33392 was recovered from the lower Mussentuchit Member (6–7 m above the Ruby Ranch contact), upper Cedar Mountain Formation, Emery County, Utah, USA (“Stormy Theropod” NCPALEOUT05; Fig. 1). NCPALEOUT05 is located on land administered by the State of Utah; access to this information is restricted by state statute and is available to qualified researchers via the NCSM. Recovered skeletal remains and co-occurring detrital zircons were hosted within volcanilithic-rich, intercalated drab gray to light gray silty-mudstones and muddy siltstones. Site-specific facies analysis and architectural reconstruction indicates that sediments and fossil materials were emplaced along a coastal mudflat (with associated minor ephemeral channels and lakes) dated to no older than 96.4 Ma (average youngest maximum depositional age) via LA–ICP–MS analysis of recovered co-occurring detrital zircon grains (120 grains in total) (Supplementary Fig. 1, Supplementary Table 1, Methods I), which approximates the youngest ages reported for the Mussentuchit Mbr. in previous studies (96.7 and 97 Ma [recalibrated in 2007])14,15.

Diagnosis Small-bodied, gracile-limbed tyrannosauroid (Supplementary Tables 2 and 3) exhibiting a semicircular tuberosity on the craniomedial femoral shaft originating at the distalmost extent of the lesser trochanter* (Fig. 2a); sinuous articular facet on medial aspect of fourth metatarsal for contact with third metatarsal* (Fig. 3i); transversely compressed, subtriangular distal articular condyle of the fourth metatarsal in distal view (Fig. 3l); and distal articular surface of fourth metatarsal exhibiting hypertrophied craniolateral aspect*, confluent with a deeply incised, striated extensor groove that grades indistinctly into the lateral collateral ligament pit* (Fig. 3g). (Autapomorphies denoted with asterisk.)

Description and comparisons

NCSM 33392 preserves a partial right hind limb including portions of the femur, tibia, second and fourth metatarsals, and phalanges of the fourth pedal digit (Figs. 2, 3 and 4; Supplementary Figs. 5–9). The proximal and distal aspects of the femur are poorly preserved—the femoral head, greater trochanter, and distal articular condyles have eroded—nonetheless, much of the femoral morphology can be discerned. Moros exhibits the base of an alariform, heavily striated lesser trochanter as in Alioramus16 and later diverging tyrannosaurids17. The base of the groove separating the lesser and greater trochanters and much of the intertrochanteric fossa is preserved (Fig. 2a), thus, although the relative height of the lesser and greater trochanters is indeterminate, it is clear they were separated by a deep, narrow cleft, as opposed to the broader condition of ornithomimosaurs (e.g., Gallimimus MPC-D 100/14 and Garudimimus MPC-D 100/13). A well-developed ridge rises from the lateral aspect of the lesser trochanter as in Alioramus16, Gorgosaurus (ROM 1247), and Tyrannosaurus17 and ornithomimosaurs (e.g., ROM 852, 797) (Fig. 2a). Caudal to this ridge and distal to the base of the lesser trochanter, a distinct, bulbous trochanteric crest extends from the caudomedial aspect of the shaft as in Dryptosaurus18, Bistahieversor (NMMNH P-25049), and Gorgosaurus (ROM 1247)(Fig. 2a,d). Together these features border a lateral depression as in tyrannosauroids generally (Dilong, Guanlong, Alioramus, Dryptosaurus18, Gorgosaurus [ROM 1247], Bistahieversor [NMMNH P-25049], Albertosaurus, Tarbosaurus, and Tyrannosaurus16), and to a lesser degree Late Cretaceous ornithomimids. A flange-like accessory trochanter is traceable extending from the distal-most lesser trochanter as in Guanlong, Dilong, and Xiongguanlong9 and an additional semicircular accessory tuberosity is present laterally, at the distal-most extent of the lesser trochanter (Fig. 2a). The medullary cavity is expansive and cortical bone relatively thin (Supplementary Methods 3).

Despite damage to the proximal aspect of the intertrochanteric fossa, the caudal margin of a relatively small nutrient foramen is preserved in this region (here termed an intertrochanteric nutrient foramina)(Fig. 2b; Supplementary Discussion 1.a) as observed in select eutyrannosaurs (e.g., Tyrannosaurus17 and Alioramus16 and some ornithomimids (e.g., Gallimimus [MPC-D 100/14]; Bissekty and Bostobe Fms taxa19,20). A second, diminutive nutrient foramen is also preserved piercing the femoral shaft distal to the lesser trochanter (the principle nutrient foramen, sensu Madsen21, Supplementary Discussion 1.a) as in coelurosaurians generally. On Moros, the principle nutrient foramen is located medial to the long axis of the lesser trochanter as in other tyrannosauroids (Supplementary Fig. 7).

A subtriangular fourth trochanter rises from the caudomedial shaft (Fig. 2. c, d), extending proximally to overlap vertically with the trochanteric shelf as in Bistahieversor (NMMNH P-25049) and creating a “D”-shaped cross-section, bearing a flattened caudal aspect as in tyrannosaurids (e.g., Gorgosaurus ROM 1247; FMNH PR2211, Albertosaurus sarcophagus ROM 807) and Ornithomimus (ROM 797, 852). This flattening of the caudal shaft continues distally in M. intrepidus, being emarginated by two intramuscular lines, one extending distally from the base of the trochanteric crest (linea intermuscularis caudalis)22, and one from the fourth trochanter (i.e., adductor ridge, crista supracondylaris medialis)22 (Fig. 2d). In lateral profile, the midpoint of the fourth trochanter is slightly concave giving it a bimodal outline (Fig. 2c); the proximal-most aspect gradually fades into the proximal femur.

The shaft is bowed cranially, as is common in theropods generally, including tyrannosauroids (e.g., Xiongguanlong9, Dryptosaurus18, and Tyrannosaurus17). Erosion to the caudal aspect of the shaft precludes identification of muscle scars in this region; however, based on better preservation of the lateral aspect, if present, the scar for the M. caudifemoralis longus was located toward the medial margin.

In cross-section, the distal femur is highly skewed, with the longest diameter angled obliquely in a craniomedial/caudolateral orientation as in tyrannosauroids generally, and to a lesser degree ornithomimosaurs; we term this a lens-shaped cross-section of the distal femur (Fig. 2f). There is a pronounced, sharp mesiodistal crest rising from the medial condyle proximally (Fig. 2b–d); it is unknown if this crest bifurcated distally as in Alioramus, Albertosaurus, Alectrosaurus, or Tarbosaurus16. Caudal to the crest the medial shaft is clearly concave, yet lacks the posterior crest bounding an autapomorphic fossa in this region on Dryptosaurus18 and ornithomimosaurs (ROM 851). The linea intermuscularis caudalis morphs into a large bulbous tuberosity, forming the proximal rim of the flexor groove, and extending further distally as a robust caudolateral crest that was likely confluent with the crista tibiofibularis (Fig. 2d). Among ornithomimosaurs, this condition is similar to that observed on ROM 852, yet is not well-developed on other taxa. It is difficult to judge the depth of the intercondylar fossa (the proximal-most extent of the extensor groove) as only a portion is preserved and may be distorted; however, it appears more developed than Dilong and Guanlong, and we interpret it as similar to the condition in Juratyrant23, Xiongguanlong9, and Gorgosaurus (ROM 1247).

The tibia is slender, longer than the femur, and missing its proximal and distal articular condyles (Fig. 3a–d; Supplementary Fig. 10). The proximal portion bears the base of a well-developed, cranially extensive cnemial crest. The preserved portion suggests a mediolaterally narrow condylar surface. A convexity bordering the caudal margin of the cnemial fossa is suggestive of an accessory condyle extending off the lateral condyle (Fig. 3e), although it is unknown if this was hook-like as in Xiongguanlong9 and tyrannosaurids. An elongate fibular crest is subrectangular, in contrast to the distally sloping crest of ornithomimids19 (e.g., TMP 1994.012.1010). A nutrient foramen pierces the lateral aspect, as in theropods generally. The cross-section at midshaft is cranially flatted, with a convex caudal aspect (Fig. 3f), producing a strongly semicircular cross-section, a feature characterizing Tyrannosaurus (BMRP 2002.4.1). The fibular shaft would have been tightly appressed to the tibia as in Bistahieversor (NMMNH P-25049). A pronounced articular facet for the fibular shaft indicates that it shifted abruptly from a lateral to cranial position on the tibia and remained cranially oriented bracing the ascending process of the astragalus (Fig. 3b) as in Tyrannosaurus (BMRP 2002.4.1; FMNH PR 2081) and Gorgosaurus (ROM 1247; FMNH PR 2211). A ridge rises from the craniolateral aspect of the shaft at the point where the fibula and ascending process of the astragalus contact as in ornithomimids and other tyrannosauroids16, here termed the distal fibular buttress. A clearly delineated facet for the ascending process of the astragalus indicates that it was proximally extensive, oriented proximolaterally, and covered the entire face of the distal tibia at its base (Fig. 3b).

Moros intrepidus bears an unusually gracile arctometatarsalian pes, most closely resembling Alectrosaurus (AMNH 6554) in morphology and proportions (Fig. 3g–j; Supplementary Fig. 11). Preserved portions of the right metatarsus include a fragmentary, midshaft section of metatarsal two exhibiting surface erosion (Fig. 4a–d) and a nearly complete fourth metatarsal, missing the proximal-most articular aspect (Fig. 3g–l).

A mid-shaft section of the second metatarsal and associated fragments are preserved (Fig. 4a–d). The shaft fragment is subtriangular in cross-section, flattened medially, and bears a raised longitudinal ridge laterally (Fig. 4c, d). A portion of the facet for MTIII is preserved. It trends toward the flexor surface proximally (Fig. 4a), consistent with the facet for MTIII on MTIV. The plantar surface of the shaft is rugose as in other tyrannosauroids.

The fourth metatarsal is elongate (~270 mm) and slender, exhibiting an estimated proximodistal length to midshaft transverse width ratio of 1:21. This value is intermediate between tyrannosaurids (e.g., 1:10–16; ROM 1247, 807, BMRP 2002.4.1) and ornithomimids (1:22–32; ROM 851, 1790, 757). The proximalmost two-thirds of the preserved shaft is straight, “D” shaped in cross section (Fig. 3k), and highly symmetrical, bearing sharp caudomedial and caudolateral margins and a height to transverse width ratio of 1:1.4. The medial surface in this region is concave (Fig. 3i). These features characterize the fourth metatarsal shaft of tyrannosaurids (e.g., Gorgosaurus (ROM 1247), Albertosaurus (ROM 807), Tyrannosaurus (BMRP 2002.4.1), Tarbosaurus). Shaft compression (dorsoventral/transverse width) falls within the range observed for other Late Cretaceous tyrannosaurids (1:1–1.9; ROM 1247, 807, BMRP 2002.4.1) as opposed to the highly compressed metatarsals of Late Cretaceous ornithomimids (1:1.7–2.4; ROM 851, 1790, 757). Distally, the shaft flares laterally and there is a distinct scar for the M. gastrocnemius lateralis on the plantar surface at the point of deflection as in tyrannosaurids generally (e.g., Gorgosaurus (ROM 1247), Albertosaurus (ROM 807); Fig. 3j). Taphonomic distortion to the fourth metatarsal deemphasizes the degree of lateral deflection in M. intrepidus; however, it is still significantly more pronounced than observed in ornithomimids (Supplementary Fig. 8).

As in tyrannosaurids, the articular facet for MTIII is extensive (half the length of MTIV) and pinches out proximally, trending toward the palmar surface, suggesting MTIII would have been visible in palmar, yet not dorsal view. The facet itself is convex toward the extensor surface and concave toward the flexor surface giving it a sinuous profile in cross-section (Fig. 3i); this is in contrast to the uniformly convex facet observed in tyrannosaurids (e.g., Gorgosaurus [ROM 1247], Albertosaurus [ROM 807], Tyrannosaurus FMNH PR 2081) and likely represents incipient development of this feature. The plantar surface of the shaft just proximal to the distal condyles is concave and distinctly compressed craniocaudally as in Gorgosaurus (ROM 1247), and Albertosaurus (ROM 807) (Fig. 3j, Supplementary Fig. 8). Its medial aspect is rimmed by a ridge originating from the junction point of the proximal–most point of the distal condyles as in tyrannosaurids (e.g., Gorgosaurus [ROM 1247], Albertosaurus [ROM 807]; Supplementary Fig. 8). In lateral view, the distal aspect arcs caudally and the ventral margin of the distal condylar surface extends well beyond the shaft (Fig. 3g; Supplementary Fig. 9). Together, with the dorsoventral compression of the shaft just proximal to the distal articular surface, these features create a backswept distal aspect to the fourth metatarsal in tyrannosauroids (Supplementary Fig. 9). In contrast, the ventral aspect of the distal articular surface is relatively in-line with the shaft in ornithomimids (Supplementary Fig. 8).

The distal aspect of MTIV is transversely narrow (i.e., the maximum transverse width of the distal articular condyles is subequal in width to the distal shaft) and subtriangular with a dorsally tapering apex as in Gorgosaurus (ROM 1247), Albertosaurus (ROM 807), and Coelurus (YPM 2010) (Fig. 3l). In dorsal view, the distal condyle forms a bulbous, hypertrophied lateral margin that is delineated by a deep, oblique groove (Fig. 3g, h; Supplementary Fig. 9). This groove trends craniomedially to caudolaterally, and bears proximodistally oriented striations as in Bistahieversor (NMMNH P-25049), as well as a subadult Tyrannosaurus (BMRP 2002.4.1), and grades into a nearly indistinct lateral collateral ligament pit (Fig. 3g). A transversely compressed, subtriangular distal metatarsal bearing a groove and indistinct lateral collateral ligament pit, contrasts with the morphology observed in ornithomimosaurs, which exhibit a laterally flaring lateral articular condyle, gradual condyle to shaft transition on the dorsal surface, dorsally rounded distal condyle, and distinct (e.g., Arkansaurus;24 YPM 542; MPC-D 100/13; MPC-D 100/29), yet not always deep (e.g., Gallimimus [MPC-D 100/14, MPC-D 100/29]) lateral collateral ligament pit (Supplementary Fig. 9). In plantar view, lateral and medial distal condyles are distinguishable, the former being more than twice as wide mediolaterally as the latter (Fig. 3j). The medial collateral ligament pit is deep and relatively large, occupying half the craniocaudal width of the distal MTIV, and emmarginated by a raised rim (Fig. 3i).

Pedal phalanges IV-3 and IV-4 from the right foot are elongate as in tyrannosauroids generally (Fig. 4e–j), unlike the shortened pedal phalanges on digit IV of ornithomimids25. Phalanges are subtriangular in cross-section (i.e., transversely pinched on the extensor surface, relative to the plantar surface), bear weakly developed intercondylar ridges, and26 deep medial, yet shallow lateral collateral ligament pits.