Canadian Journal of Earth Sciences Revisiting Russell’s troodontid: autecology, physiology, and speculative tool use Journal: Canadian Journal of Earth Sciences Manuscript ID cjes-2020-0184.R1 Manuscript Type: Article Date Submitted by the 12-Jan-2021 Author: Complete List of Authors: Varricchio, David; Dept of Earth Sciences Hogan, Jason; Dept of Earth Sciences Freimuth, William; North Carolina Museum of Natural Sciences, Paleontology; North Carolina Community College System, Biological Sciences Draft Keyword: Dinosaurs, Traces, Paleobiology, Troodontids, Troodon, gastric pellets Is the invited manuscript for consideration in a Special Tribute to Dale Russell Issue? : © The Author(s) or their Institution(s) Page 1 of 65 Canadian Journal of Earth Sciences 1 2 3 Revisiting Russell’s troodontid: autecology, physiology, and speculative tool use 4 5 6 D. J. Varricchio, J. D. Hogan, and W. J. Freimuth 7 8 D.J. Varricchio and J.D. Hogan. Earth Sciences, Montana State University, Bozeman, MT 9 USA 59717, [email protected], [email protected] 10 W.J. Freimuth. [1] Paleontology, North Carolina Museum of Natural Sciences, 11 W Jones St, 11 Raleigh, NC USA 27601 and [2] DepartmentDraft of Biological Sciences, North Carolina 12 State University, 100 Brooks Ave., Raleigh, NC USA 27607 [email protected] 13 Corresponding author: David Varricchio (e-mail: [email protected]), 406-994-6907, fax 406- 14 994-6923. 15 16 17 18 19 20 21 22 23 1 © The Author(s) or their Institution(s) Canadian Journal of Earth Sciences Page 2 of 65 24 Abstract: Dale Russell described the osteology, morphology, and ecology of the small theropod 25 “Stenonychosaurus inequalis” in two papers, speculating on its life habits, brain-power, vision, 26 movement, feeding and hand capabilities. Russell even pondered a tool-using dinosauroid, the 27 hypothetical troodontid descendant if the lineage survived the K/Pg extinction. We revisit the life 28 habits of the North American troodontids, Troodon formosus and Latenivenatrix mcmasterae, by, 29 in part, reviewing various trace fossils of T. formosus discovered in Montana. These include egg 30 clutches, a nest, and recently discovered regurgitalites. We also ruminate on the possibility of 31 dinosaur tool use. Troodon likely constructed earthen nests as ratites and other birds create their 32 nesting scrapes, through backward hindlimb kicks. The more complex clutch architecture 33 suggests dexterous movement of the eggs, potentially requiring manual manipulation. 34 Functionally, reproductive traces supportDraft elevated body temperatures and a metabolic output that 35 approached but did not equal that of modern birds. Brooding would require very high energy 36 investment from the adult. The regurgitalites largely contain multi-individual aggregations of the 37 marsupialiform Alphadon and support Russell’s hypotheses of troodontids as nocturnal, 38 intelligent, small game hunters with elevated metabolism and enhanced vision. Tool use in a few 39 crocodilians and widely among extant birds suggests a reasonable possibility of this behavior in 40 non-avian dinosaurs. Whether avian-comparable encephalization quotient and freed forelimbs 41 would make North American troodontids good candidates to exhibit such behavior remains an 42 open and speculative question. However, given the minimal modification made to tools by 43 modern archosaurs, recognition of fossil tools poses a challenging problem. 44 45 Keywords: Troodon, troodontid, reproduction, encephalization quotient, regurgitalites, gastric 46 pellets, trace fossils, tool use, physiology 2 © The Author(s) or their Institution(s) Page 3 of 65 Canadian Journal of Earth Sciences 47 Introduction 48 Dale Russell summarized the osteology, morphology, and ecology of the troodontid 49 dinosaur “Stenonychosaurus inequalis” in two papers (Russell 1969; Russell and Séguin 1982). 50 The first paper (Russell 1969) describes several troodontid specimens collected in the Upper 51 Cretaceous beds of Alberta, Canada from what at the time were considered part of the Oldman 52 Formation. In “Reconstruction of the small Cretaceous theropod and a hypothetical 53 dinosauroid,” Russell and Séguin (1982) document their building of a three-dimensional model 54 of the dinosaur and the reasoning behind the proportions and shape of their reconstruction. In 55 conjunction with this project, they also speculate on its life habits including brain-power, vision, 56 movement, hand capabilities, and feedingDraft (topics also covered in Russell 1969) (Fig. 1). 57 Additionally, and somewhat surprisingly, they ponder what the descendants of this dinosaur 58 might have looked like if they had survived the K/Pg extinction event. 59 Here we revisit the life habits of the North American troodontids, Troodon formosus (a 60 senior synonym of S. inequalis, see below) and Latenivenatrix mcmasterae, by reviewing the 61 various trace fossils discovered in Montana over the last 40 years. These include egg clutches, a 62 nest, and regurgitalites (fossil regurgitates) that have implications for troodontid ecology, 63 feeding, hand use, and metabolism. Finally, with a nod to the ruminations of Russel and Séguin 64 (1982), we speculate on possible tool use in Mesozoic dinosaurs. 65 In 1969, small theropods of any clade, from anywhere, were exceedingly rare; 66 surprisingly, the year saw osteological description of three important taxa: “Stenonychosaurus 67 inequalis” (Russell 1969), Dromaeosaurus albertensis (Colbert and Russell 1969), and 68 Deinonychus antirrhopus (Ostrom 1969). These descriptions were important contributions to the 3 © The Author(s) or their Institution(s) Canadian Journal of Earth Sciences Page 4 of 65 69 revitalization of the dinosaur origins of birds and in the re-thinking of Mesozoic dinosaurs as 70 active, intelligent and complex vertebrates (Russell 1969; Ostrom 1969). 71 Russell (1969) identifies his “Oldman” troodontid material as Stenonychosaurus 72 inequalis, a taxon named by Sternberg (1932) on a foot and additional associated material. 73 However, in 1993, the upper portion of the Oldman Formation was recognized as a distinct unit, 74 the Dinosaur Park Formation (Eberth and Hamlin 1993) and Russell (1969) includes specimens 75 from both the Oldman and Dinosaur Park Formations. Since 1969, there have been several 76 taxonomic revisions of S. inequalis. Currie (1987) synonymizes S. inequalis into the earlier 77 named Troodon formosus from the Judith River Formation of Montana. Although Currie (2005) 78 introduces a new combination, Troodon inequalis in 2005, T. formosus has largely remained in 79 use since 1987. More recently, van der ReestDraft and Currie (2017), abandon the earlier 80 synonymization of Currie (1987) and recognize two species within T. formosus of Currie (1987): 81 S. inequalis and the newly named Latenivenatrix mcmasterae for troodontids of the lower and 82 upper portion of the Dinosaur Park Formation, respectively. Russel’s 1969 description of “S. 83 inequalis” thus incorporates specimens representative of both taxa. Varricchio et al. (submitted) 84 argue, as Russell (1969) also notes, that T. formosus has priority, and thus is the proper name for 85 S. inequalis as defined by van der Reest and Currie (2017), a usage we follow here. 86 Russell (1969) documents a number of taxonomically and likely functionally important 87 osteological features in troodontids. Among these are a sizeable brain with large cerebral 88 hemispheres, spacious orbits with a strong anterior orientation, bowed ulna, a semilunate carpal 89 with a mesocarpal wrist articulation, long hind limb proportions, fused calcaneum and astragalus, 90 and an arctometatarsalian and highly asymmetric metatarsus. Based on these observations, 91 Russell (1969, 1981; Russell and Séguin 1982) proposes several hypotheses concerning the 92 physiology, behavior, and intelligence of these Late Cretaceous troodontids of Alberta and 4 © The Author(s) or their Institution(s) Page 5 of 65 Canadian Journal of Earth Sciences 93 Mongolia. These include: 1) a brain with an avian level of intelligence and organization 94 requiring a high metabolism with potentially endothermy (Béland and Russell 1979), 2) excellent 95 vision with stereoscopic and nocturnal capabilities, 3) mobile lower arm, 4) an opposable and 96 grasping manual digit III providing manual dexterity, 5) rapid and agile gait, 6) occupation of 97 terrestrial environments distal from depositional centers, and 7) predation upon small crepuscular 98 to nocturnal mammals (Fig. 1). Noting that Late Cretaceous troodontids possess a large brain, 99 stereoscopic vision, opposable digits, and a bipedal gait, attributes also found within the earliest 100 hominids (Jones et al. 1995a), Russell and Séguin (1982) speculate on the course of evolution for 101 these theropods if the lineage survived the end-Cretaceous extinction. They propose that 102 selection would favor an ever-increasing brain size, tool-use, live birth, and a rather hominid-like 103 body plan in the subsequent “dinosauroid”.Draft 104 A number of these hypotheses of Russell were more fully demonstrated by later research 105 and proved to be important in the re-imagining of dinosaurs as more active and endothermic 106 vertebrates. For example, recognition of the large brain and brain to body size ratio in “S. 107 inequalis” (Russell 1969; Russell and Séguin 1982; Russell and Dong 1993) figured prominently 108 in subsequent discussions on the evolution of brain size in dinosaurs and its implications for