Large-bodied ornithomimosaurs inhabited Appalachia during the Late Cretaceous of North America

Reconstructing the evolution, diversity, and paleobiogeography of North America’s Late Cretaceous dinosaur assemblages requires spatiotemporally contiguous data; however, there remains a spatial and temporal disparity in dinosaur data on the continent. The rarity of vertebrate-bearing sedimentary deposits representing Turonian–Santonian ecosystems, and the relatively sparse record of dinosaurs from the eastern portion of the continent, present persistent challenges for studies of North American dinosaur evolution. Here we describe an assemblage of ornithomimosaurian materials from the Santonian Eutaw Formation of Mississippi. Morphological data coupled with osteohistological growth markers suggest the presence of two taxa of different body sizes, including one of the largest ornithomimosaurians known worldwide. The regression predicts a femoral circumference and a body mass of the Eutaw individuals similar to or greater than that of large-bodied ornithomimosaurs, Beishanlong grandis and Gallimimus bullatus. The paleohistology of MMNS VP-6332 demonstrates that the individual was at least 11 years of age (similar to B. grandis [∼375 kg, 13–14 years old at death]). Additional pedal elements share some intriguing features with ornithomimosaurs yet suggest a larger-body size closer to Deinocheirus mirificus. The presence of a large-bodied ornithomimosaur in this region during this time is consistent with the relatively recent discoveries of early-diverging, large-bodied ornithomimosaurs from mid-Cretaceous strata of Laurasia (Arkansaurus fridayi and B. grandis). The smaller Eutaw taxon is represented by a tibia preserving seven growth cycles, with osteohistological indicators of decreasing growth, yet belongs to an individual with near reaching somatic maturity of the larger taxon, suggesting the co-existence of medium- and large-bodied ornithomimosaur taxa during the Late Cretaceous Santonian of North America. The Eutaw ornithomimosaur materials provide key information on the diversity and distribution of North American ornithomimosaurs and Appalachian dinosaurs and fit with broader evidence of multiple cohabiting species of ornithomimosaurian dinosaurs in Late Cretaceous ecosystems of Laurasia.

. 170 171 172 S1 . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made

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As previously discussed, close proximity of the superjacent, well-dated Tombigbee Sand 278 allows a minimum age of late Santonian for the Lux lag. The lack of a bone-bearing subjacent 279 facies as a potential clast-contributor to the Lux lag suggests temporal mixing is minimal to 280 nonexistent, unless of course any vertebrate-bearing facies were removed earlier in the lowstand.
. CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made   398 The border of the articular surface of calcaneum is convex anteriorly and straight 399 anterodistally in lateral view (Fig 3C), similar in form to the tyrannosauroids A. montgomeriensis, . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made   422 These appear to belong to different individuals (Figs 4-6).

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. CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The second metatarsal (MT-II) 425 MMNS VP-6332 is essentially complete yet missing the proximal-most articular surface 426 and most of the shaft is deformed due to a pathology ( Fig 4A). MMNS VP-6332 is identified as a 427 second metatarsal of the right foot based on the subtriangular proximal end with a nearly flat 428 articular surface for the articulation of the third metatarsal, medially deviated distal end with a 429 transversely unconstricted, quadrangular distal articular surface relative to its height, and a less 430 flared medial condyle (Fig 4).   (Fig 4A). The buttressing surface, which is located on the lateral surface of the . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted March 25, 2022. ; https://doi.org/10.1101/2022.03.25.485782 doi: bioRxiv preprint 448 distal half of the second metatarsal, is one of the differentiating characteristics between 449 ornithomimosaurs and tyrannosauroids (e.g., [57][58][59]). Based on architecture of the non-450 pathological cortical bone periosteal surface segmented via CT reconstruction (Fig 4B-D), there is 451 no evidence of the buttressing surface on MMNS VP-6332, indicating that this metatarsal is not 452 referable to tyrannosauroids.

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Proximally, MMNS VP-6332 is expanded anteroposteriorly and medially relative to the 454 shaft as in most theropods (Fig 4A and D). However, this expansion is less than that of (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made  (Fig 4E).

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The proximal and distal ends are slightly rotated clockwise in relation to the main axis, 474 resulting in a weakly twisted metatarsal shaft (Fig 4).

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The articulation for the metatarsal-phalangeal joint of the distal articular surface is non-491 ginglymoid; rather, it has a smooth and bulbous articular surface as in other ornithomimosaurs 492 (Fig 4A and F). Transversely, the distal articular caput is slightly broader than the width of the 493 shaft in anterior view like O. velox and the large Gansu ornithomimid (Fig 4F). The height/width 494 ratio of the distal articular caput is subequal, with equally developed two distal condyles, forming . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The third metatarsal (MT-III) 535 The distal half of a right third metatarsal (MSC 13139) is only preserved (Fig 5; Table 1), 536 but the preserved portion is damaged at the mid part of the shaft (Fig 5A). The shaft is 537 mediolaterally broad anteriorly with a flat surface that transitions proximally to become extremely 538 thin and narrow, forming a splint bone with a slightly convex anterior surface. The cross-section 539 of MSC 13139 is wedge-shaped and transversely wider than deep.  In anterior view, the medial border of the proximal-most preserved portion of the shaft is 556 straighter when compared to the lateral one, which trends proximomedially, constricting the shaft 557 at midlength for reception of the fourth metatarsal (Fig 5A). The latter feature is common in both

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The distal articular surface of MSC 13139 is smooth and non-ginglymoid with unevenly 593 developed lateral and medial condyles (Fig 5A). The lateral and the medial distal condyles are 594 mediolaterally subequal, but the medial condyle is posteriorly more extended and 595 anteroposteriorly slightly taller than the lateral condyle in distal view (Fig 5E).  . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made

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The medial collateral ligament fossa of MMNS VP-6183 is circular and larger than the 715 proximodistally elongated lateral fossa (Fig 6C-D). Whereas the medial collateral fossa is . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made   Anteriorly, the shaft of MMNS VP-4949 is straight (Fig 7C1), which is slightly 874 constricted mediolaterally and wider than tall, forming an oval shape in cross-section. The oval-875 shaped cross-section of MMNS VP-4949 is like those of ornithomimosaurs, but unlike large . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted March 25, 2022. ; https://doi.org/10.1101/2022.03.25.485782 doi: bioRxiv preprint 948 proximal articular surface is slightly inclined laterally, exhibiting laterally and medially convex, 949 and posteriorly concave borders in proximal view (Fig 7D5) [63,87].

950
The distal condyles are only slightly wider transversely in dorsal view compared to the 951 shaft (Fig 7D3-D4)  968 fragilis, G. libratus, and T. rex, as well as those of medium-sized theropods, such as D.
969 antirrhopus. The mediolateral width of the condyles is approximately the same; however, the 970 medial condyle is slightly larger and taller anteroposteriorly than the lateral condyle. In addition, 971 whereas the axis of the lateral condyle is relatively perpendicular to the transverse width, the . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted March 25, 2022. ; https://doi.org/10.1101/2022.03.25.485782 doi: bioRxiv preprint . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted March 25, 2022. ; https://doi.org/10.1101/2022.03.25.485782 doi: bioRxiv preprint . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted March 25, 2022. ; https://doi.org/10.1101/2022.03.25.485782 doi: bioRxiv preprint . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted March 25, 2022. ; https://doi.org/10.1101/2022.03.25.485782 doi: bioRxiv preprint . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted March 25, 2022. ; https://doi.org/10.1101/2022.03.25.485782 doi: bioRxiv preprint . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint this version posted March 25, 2022. ; https://doi.org/10.1101/2022.03.25.485782 doi: bioRxiv preprint