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Paleoecological implications of two closely associated types from the Upper St. Mary River Formation,

Authors: Frankie D. Jackson & David J. Varricchio

NOTICE: this is the author’s version of a work that was accepted for publication in Cretaceous Research. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Cretaceous Research, VOL# 79, (November 2017) DOI# 10.1016/j.cretres.2017.08.003

Jackson, Frankie D. , and David J. Varricchio. "Paleoecological implications of two closely associated egg types from the Upper Cretaceous St. Mary River Formation, Montana." Cretaceous Research 79 (November 2017): 182-190. DOI: 10.1016/j.cretres.2017.08.003.

Made available through Montana State University’s ScholarWorks scholarworks.montana.edu Paleoecological implications of two closely associated egg types from the Upper Cretaceous St. Mary River Formation, Montana

* Frankie D. Jackson , David J. Varricchio

Department of Earth Sciences, Montana State University, Bozeman, MT, 59717, USA

abstract

Two closely associated egg types occur at the same locality in the Upper Cretaceous () St. Mary River Formation in north central Montana. These specimens represent the first fossil described from this formation. At least fifteen small ovoid eggs or egg portions are scattered through a 25 cm interval of rock. Five significantly larger, round eggs overlie these smaller eggs and are in close proximity to one another on a single bedding plane. The best preserved egg of the smaller size measures 36 mm 62 mm and exhibits the prismatic, two-layered structure of a theropod egg. The dispersed distribution and inconsistent angles of these small eggs likely resulted from disturbance by subsequent nesting activity and/or possibly predation. At least twelve additional small prismatic eggs also occur at this site. We assign the small eggs as a new oogenus and oospecies, Tetonoolithus nelsoni, within the . The large round eggs measure 130 mm in diameter and the eggshell displays substantial diagenetic alteration. These eggs likely belonged to a hadrosaur due to their similarity in egg size, shape, and eggshell thickness to eggs from the stratigraphically lower . Eggs at different stratigraphic levels at this site indicate that conditions favorable to both species persisted for an extended period of time. However, determining whether these occupied the nesting site at the same or different remains beyond the resolution of the rock record.

1. Introduction locality. Jackson and Varricchio (2010) report prismatic and spherulitic eggs immediately adjacent to one another from the Mesozoic nesting localities occasionally preserve two or more lowermost Two Medicine Formation in Montana, and Fernandez ootaxa at the same stratigraphic level or at the same general and Garcia (2013) report clutches of faveoloolithid and filispher- nesting locality (Salgado et al., 2007; Selles et al., 2013). Russo et al. ulithid eggs from the Allen Formation of northern Patagonia. The (2014), for example, report crushed eggs belonging to the oofamily filispherulithid clutch also contains a single megaloolithid egg that Krokolithidae collected from a theropod nesting site in the Upper Fernandez and Garcia (2013) suggest may represent nest Lourinha~ Formation of . Horner (1984) also reports parasitism. two egg types that occurred together at three stratigraphic levels at Documentation of nesting sites facilitates a better understand- the Egg Mountain locality in Montana; these were later assigned as ing of possible association of dinosaur to specific lithologies in levis Zelenitsky and Hills, 1996 (i.e., ) and the rock record; permits recognition of past surfaces of subaerial Continuoolithus canadensis Zelenitsky et al., 1996 (an unidentified exposure; and allows assessment of environments favored by theropod). However, closely associated egg types that are separated various dinosaur species and other reproductive behaviors by a few centimeters or occur in the same nest are rare. Varricchio (Varricchio et al., 2015). However, some descriptions that mention et al. (1999) describe a hatched or destroyed clutch of C. canadensis the occurrence of different egg types at the same locality fail to next to a nesting trace containing Troodon eggs at the Egg Mountain discuss their spatial relationship and possible significance (e.g., Horner, 1982; Hirsch and Quinn, 1990; Jackson and Varricchio, 2010). These multiple egg occurrences provide additional infor- mation about paleoecology and nesting behavior beyond that obtainable from the eggs of either taxon alone. Here, we describe the occurrence of two closely associated egg types from the St. in the two plaster jackets. Preparation of the specimens included Mary River Formation, Teton County, Montana. We assign the eggs removal of matrix with small hand tools and stabilizing loose to ootaxonomy and discuss potential inferences of reproductive eggshell fragments with glue. Egg orientations were measured with behavior at this site. Below, we provide the geologic setting in order a Brunton compass. fragments were removed from both to compare ootaxonomic diversity within four Upper Cretaceous egg types and half of each fragment was prepared as a standard formations exposed in north central and . petrographic thin section (30 mm thick) and studied by transmitted and polarized light . The other half of each eggshell was 2. Geologic setting coated with (10 nm), mounted on an aluminum stub, and imaged at 15 kV under a JEOL 6100 scanning electron During time an Andean-style plate collision (SEM) with backscattered electron imaging (BEI). Structural attri- resulted in subduction of the oceanic Farallon plate beneath the butes (shell thickness, pore width) were measured with ImageJ western margin of North America. The resulting foreland basin analysis software (http://imagej.nih.gov/ij/). experienced marine incursions from the north and south that eventually joined to form the Western Interior Cretaceous Seaway 4. Systematic paleontology (Kauffman, 1977). Terrigenous clastic sediments resulting from erosion of older sedimentary rocks in the Cordilleran thrust belt Oofamily Prismatoolithidae Hirsch, 1994 were delivered eastward by fluvial systems to the western margin Oogenus Tetonoolithus oogen. nov. of the Cretaceous seaway (Horner et al., 2001). Together, the Two Diagnosis. As for type and only oospecies. Medicine and Judith River formations represent an eastward thin- Oospecies. Tetonoolithus nelsoni oosp. nov. ning clastic wedge of terrestrial sediments that accumulated pri- Figs. 2e3 marily during regression of the Claggett Sea and subsequent transgression of the Bearpaw Sea (Fig. 1A: R8 and T9, respectively). Holotype. Prismatic eggs from MOR 1144-2, a specimen that also The Sweetgrass arch truncates this formerly intact lithosome includes larger eggs with highly altered eggshell structure. (Horner et al., 2001). Regression of the Bearpaw Sea (Fig. 1A: R9) Derivation of name. Tetonoolithus for the nearby Teton River and produced the shoreline deposits of the Horsethief Sandstone and/ “oolithus” from the Greek word meaning “egg stone”. The specific or marine Bearpaw over the Two Medicine Formation, name “nelsoni” honors Joel Nelson, in recognition of his discovery whereas only the latter covers the (Lorenz, of the specimen and many contributions to the MOR field camp at 1981; Rogers et al., 2016). The clastic deposits of the St. Mary River Egg Mountain. and Hell Creek formations, respectively, overlie the Horsethief Type locality and age. MOR locality SM 110; Teton County, north Sandstone to the west, and the transitional marine-to-brackish Fox central Montana, U.S.A., Saint Mary River Formation, Upper Creta- Hills Sandstone to the east (Fig. 1; Rogers et al., 1993; Fastovsky and ceous (Maastrichtian). Bercovici, 2016). Referred specimens. MOR 1144, a cluster of 12e13 prismatoolithid Dawson (1884) reported the non-marine deposits of the St. eggs. Mary River Formation as Maastrichtian in age, whereas Cobban and Diagnosis (as combined characters). Smooth, ovoid eggs approxi- Reeside (1952) further refined the age as early Maastrichtian. More mately 33 mm by 62 mm with Elongation Index (EL) of approxi- recent work based on mammalian fossils from the St. Mary River mately 1.7. Eggshell 530e550 mm; two structural layers of , Formation in Montana provides support for an “Edmontonian” land with gradual transition from mammillary to prismatic layer; age (approximately 74 to 67 Ma; Hunter et al., 2010). The ML:CL ¼ 1:3.8; unevenly distributed squamatic texture within St. Mary River Formation crops out in a relatively small area of discernable prisms. northwestern Montana. The study area lies north and east of Description. The original field sketch gave the strike and dip of the Augusta, Montana on the edge of the fold and thrust belt (Fig. 1B). sandstones beneath the eggs as N20W and 45W, respectively. The The St. Mary River Formation varies from 300 to 400 m thick and sketch shows that most eggs were concentrated in an area of about records deposits of upland fluvial environments similar to the un- 500 cm2; however, the total number of eggs excavated from this derlying Two Medicine Formation (Cobban, 1955; Weishampel and site remains unclear. Nevertheless, MOR 1144-2 and MOR 1144 Horner, 1987). In contrast, lowland habitats characterize the Hell represent two jackets removed from this denser concentration of Creek Formation to the east (Weishampel and Horner, 1987). The eggs. Although both jackets were prepared upside down, the non-marine Willow Creek Formation overlies the St. Mary River following description depicts the eggs in their original orientations Formation, and the KeP boundary lies within this formation. The within the stratum. Willow Creek Formation is thought to correlate with the upper MOR 1144-2. The 51 cm by 58 cm block preserves 15 small smooth, parts of the Lance and Hell Creek formations and lower part of the black egg or egg portions dispersed within 25 cm green-gray, very Fort Union Formation (Russell, 1975). All four formations yield fossil fine sandstone to siltstone (Fig. 2A, B). These eggs occur at three or dinosaur eggs. possibly four levels and lie stratigraphically lower than the second egg type (described below). Only a few of the small eggs are in 3. Materials and methods contact with one another (Fig. 2A, B). Additional eggs may occur within the matrix that supports the specimen. Eggshell debris on Museum of the Rockies specimens MOR 1144-2 and MOR 1144 the edge of the plaster jacket near egg 4 suggests loss of an egg were collected from the same locality in 1991 by David Weishampel during excavation. Some of the small eggs exhibit continuity of and his crew from Johns Hopkins University. The Museum of the eggshell that curves into the surrounding rock, suggesting they may Rockies' Saint Mary River Formation locality, SM 110, lies approxi- represent complete, unhatched eggs. A few specimens appear more mately 24 km west of Augusta, Montana in Lewis and Clark County, fragmentary and disturbed. For example, eggs 3, 6, 12, and 10 USA (Fig. 1B). MOR 1144-2 includes small prismatoolithid eggs and appear badly crushed, and egg 10 may represent large fragments substantially larger eggs with altered eggshell, whereas the second from three nearby eggs. The most completely preserved egg bot- specimen, MOR 1144, contains only the former. The lack of a toms (eggs 5 and 8) measure approximately 36 mm 61 mm and measured section at the time of excavation limits stratigraphic in- 35 mm 62 mm, indicating an ovoid egg shape. Most eggs lie formation to data discernable from the rock surrounding the eggs nearly horizontal within the rock, whereas others (eggs 1, 2, 4, 14 Fig. 1. Geologic setting and nesting locality. A, Schematic cross section of Montana Group with restoration across the Sweetgrass arch. Modified from Rogers (1998: fig. 2); T and R represent marine transgressions and regressions, respectively. B, geologic map of the fold and thrust belt of Montana with black star to indicate the fossil egg locality in the St. Mary River Formation (Ksm). Fig. 2. Associated egg types. A, Specimen MOR 1044-2 containing prismatic theropod eggs (1e15) and larger eggs (L1eL5); the latter likely belonged to a hadrosaur; B, enlargement of upper left corner of A. Eggs were prepared upside down and therefore the small prismatic eggs are stratigraphically lower than the large eggs; C, MOR 1044 containing portions of 13 prismatic eggs. Gray-green sediment color appears tan due to artificial lighting. D, enlargement of egg 2 in C. Scale bars in AeC equal 10 cm; scale bar in D equals 2 cm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) and 15) vary from about 10e20. Some eggs are incompletely by 56 mm; it displays a smooth surface and ovoid shape (Fig. 2D). exposed and therefore prohibit assessment of egg trend and, Seven eggs (eggs 1, 2, 4, 6, 8, 9, and 11) occur at or near the same consequently, possible egg pairing. level, whereas three (eggs 3, 5, and 7), and the possible 13th egg, are slightly higher. These eggs lie about 2e3 cm horizontally from Five large, round, brown to dark rusty colored eggs overlie the the nearest adjacent egg. Two additional egg portions (eggs 10 and small eggs (Fig. 2A, B). These larger eggs show substantially greater 12) are a further 5e6 cm above this level. This vertical distance was lithostatic compaction than the small eggs and measure about measured without correction for the dip of the bed. Where the axes 130 mm in diameter. They occur on a single bedding plane and lie in of the eggs are discernable, the estimated orientation varies from close contact with one or more adjacent eggs of the same type. Four horizontal to about 45 . Trends are inconsistent, with eggs neither of these large egg portions are concave up, whereas a fourth egg parallel to one another nor centered about a given focal point. The portion (L3) lies concave down. The latter may represent an poor preservation and incomplete exposure of the eggs prohibits extremely flatten egg bottom or the top of an adjacent, possibly definitive assessment of possible egg pairing. hatched egg. Eggshell structure. The eggshells from the small eggs in both MOR MOR 1144. The 43 cm by 56 cm block contains at least 12 and 1144-2 and MOR 1144 are 530e550 mm thick, and exhibit moderate possibly 13 small, black eggs in approximately 10 cmethick, green- alteration (Fig. 3A, B). The eggshell consists of two structural layers: gray very fine silty sandstone (Fig. 2C). The quality of preservation a110mm mammillary layer (ML) and 418 mm continuous layer (CL), and exposure of the eggs vary and two eggshell concentrations with a ML:CL ratio of about 1:3.8 (Fig. 3A, B). Nucleation sites at the (eggs 10, 11) may represent crushed eggs. One well-exposed and inner eggshell surface are absent, likely due to recent . largely intact egg (egg 2) would likely have measured about 33 mm The blocky, barrel-shaped and tightly packed mammillary cones Fig. 3. Eggshell from two egg types. A, Thin section of eggshell under plain polarized light from small egg in MOR 1044-2; outer shell surface is at top of image. Triangles show widths of two adjacent blocky mammillary cones and white arrow points to the outer shell surface. Secondary calcite overlies the eggshell. White bar shows approximate transition between the mammillary (ML) and continuous layers (CL); B, same as A under SEM. Black bar indicates ML and CL transition; note prisms in the continuous layer. White arrow indicates outer shell surface; C, enlargement of a mammillary cone; triangles indicate cone width and white bar show marks upper boundary of the ML; D, eggshell from large egg (L1) in thin section showing highly altered structure; outer surface is at top of the image. Arrow points to eroded inner eggshell surface; D, SEM image of eggshell from L1 showing substantial alteration of calcite structure. Ornamentation on the outer shell surface consists of ridges. Scale bar in A equals 250 mm, B equals 200 mm, and C equals 100 mm; D and E equal 1000 mm. exhibit tabular structure and measure about 100e130 mmin 4.1. Comparisons diameter (Fig. 3C). A gradual transition occurs from the mammil- lary to the continuous layer (Fig. 3B, C). The latter exhibits tightly Small eggs. Eggs with prismatic structure in the second layer are interlocking prisms with faint margins (Fig. 3B); irregular squa- represented by two oofamilies, Arriagadoolithidae Agnolin et al., 2012 matic texture and 2e4 mm-wide vesicles are recognizable in some and Prismatoolithidae Hirsch, 1994, whereas expression of this but not all SEM images, in the latter case due to alteration of the feature appears less distinct in a third oofamily, Montanoolithidae calcite eggshell. Secondary calcite deposits cover some portions of Zelenitsky and Therrien, 2008 (Table 1). The lack of a complex third inner and outer surface of the eggshell (Fig. 3A, B). layer and absence of anastomosing ridges excludes the small eggs from MOR 1044-2 and MOR 1044 from the oofamilies Arriagadooli- Eggshells from the large eggs measure about 850e920 mm thick thidae and Montanoolithidae, respectively. However, the eggs and sometimes display irregular ridges on the outer surface conform to the emended diagnosis of the Prismatoolithidae (Moreno- (Fig. 3D, E). Diagenetic alteration almost completely obliterated the Azanza et al., 2014) in possessing a two layered eggshell structure, eggshell structure. Some irregular calcite-filled spaces midway tabular structure of inner layer, and gradual transition between layers. through the eggshell may represent obliquely oriented pores but The smooth surface differs from Prismatoolithus eggs with ornamen- alteration prohibits definitive identification. tation, namely, P. caboti Garcia et al., 2000; P. tenuis Vianey-Liaud and Table 1 Comparison of ootaxa with prismatic eggshell structure.

Oofamily Oogenus Ootaxa Eggshell thickness Egg size Eggshell surface Reference

Arriagadoolithidae Arriagadoolithus A. patagoniensis 900e1200 70 ? Nodes and ridges Agnolin et al., 2012 Triprismatoolithus T. stephensi 525e850 30 75 Nodes Jackson and Varricchio, 2010 Montanoolithidae Montanoolithus M. labadousensis 600e700 e Anastomosing ridges Vila et al., 2017 M. strongorum 700e850 60 125 Anastomosing ridges Zelenitsky and Therrien, 2008 Prismatoolithidae Preprismatoolithus P. coloradoensis 700e1000 60 110 Smooth Hirsch, 1994 Prismatoolithus P. caboti 500e600 30 70 Dispersituberculate Garcia et al., 2000 P. gebiensis 700e900 40e70 120 Smooth Zhao and Li, 1993 P. hanshuiensis 1000e1200 140e150 Weathered Zhou et al., 1998 P. heyuanensis 600 (520) 48.6 ?115 Smooth Lü et al., 2006 P. hirschi 500e560 e Smooth Jackson and Varricchio, 2010 P. hukouensis 700e1000 56 140 Smooth Zhao, 2000 P. jenseni 830e1160 75 140 Smooth Bray, 1999 P. levis 700e1000 50e57 130e150 Smooth Zelenitsky and Hills, 1996 P. matellensis 1060e1220 e Smooth Vianey-Liaud and Crochet, 1993 P. tenuous 240e600 e Dispersituberculate Vianey-Liaud and Crochet, 1993 P. trempii 250e530 e Flat nodes Selles et al., 2014 Protoceratopsidovum P. fluxuosum 600e700 130e150 50e57 Smooth Mikhailov, 1994 1400 P. minimum 300e700 40e?60 100e110 Smooth Mikhailov, 1994 P. sincerum 300e1200 40e50 110e120 Smooth Mikhailov, 1994 S. pirenaica 190e340 40 70 Smooth Lopez-Martínez and Vicens, 2012 Spheruprismatoolithusa S. condensus 660e940 e Ridges and nodes Bray, 1999 Tetonoolithus T. nelsoni 530e550 33 53, 35 62 Smooth This paper Trigonoolithus T. amoae 330e1040 e Round-triangular Moreno-Azanza et al., 2014 Unnamed Pseudogeckoolithus P. nodosus 330e350 e Nodes Vianey-Liaud and Lopez-Martinez, 1997

a Prismatic eggshells (Jackson and Varricchio, 2010).

Crochet, 1993;andP. trempii Selles et al., 2014 (Table 1). Prismatooli- terms are used to describe this behavior: communal and colonial thus levis Zelenitsky and Hills, 1996 and P. gebiensis Zhao and Li, 1993 nesting. Espinoza and Lobo (1996) define communal nesting as differ in their larger size and strongly asymmetrical egg shape. Pris- eggs laid with those of conspecifics under or within structures such matoolithus hirschi Jackson and Varricchio, 2010 also differ in their as rocks, logs, and vegetation. In contrast, colonial nesting refers to slender elongate mammillary cones, distinct transition between the use of common nesting areas where buried eggs are typically not mammillary and continuous layers, and evenly distributed squamatic visible to other animals. This represents an important distinction texture. Lü et al. (2006) describe a possible external layer in because colonial nesting females often inadvertently disrupt or dig P. heyuanensis and their size exceeds that of Tetonoolithus nelsoni. up the eggs of conspecifics. A review of the literature by Doody et al. Further, the new eggs are smaller than all Prismatoolithus oospecies (2009) reveals that descriptions of conspecific aggregations among known from intact eggs (Table 1). extant are relatively common, whereas reports of inter- Other oogenera. Two additional oogenera in the Prismatoolithidae specific nesting are rare. exhibit ornamentation and therefore differ from Tetonoolithus nel- Recognition of either type of egg-laying strategy in the fossil soni: Trigonoolithus Moreno-Azanza et al., 2014 and Spher- record presents a challenge for paleontologists. As with most uprismatoolithus Bray, 1999 (Table 1). Jackson and Varricchio (2010) reproductive behaviors (e.g., site fidelity, colonial nesting, egg suggest that the latter may represent spherulitic, rather than pris- brooding), paleontological definitions may differ from those of bi- matic eggshells. The oogenus Pseudogeckoolithus Vianey-Liaud and ologists, and time resolution in the rock record poses a difficult Lopez-Martinez, 1997 is unassigned to an oofamily and possesses problem (Varricchio et al., 2015). For example, Horner (1982) hy- ornamentation (Table 1). The eggs described here also differ from pothesized colonial nesting for hadrosaurs belonging to the genus Sankofa pirenaica Lopez-Martínez and Vicens, 2012 and Proto- Maiasaura at the Willow Creek anticline in Montana; he noted that ceratopsidovum Mikhailov, 1994 in their absence of unusual texture the nests occurred at approximately equal vertical distances from a in the continuous layer in the former and the strongly elongate and paleosol horizon, with absence of nest overlap. However, this and asymmetric shape of the latter. Preprismatoolithus coloradonensis subsequent articles (e.g., Horner and Gorman, 1988; Horner, 2000) (sensu Zelenitsky and Hills, 1996) are larger, with thicker eggshell lack a stratigraphic section, taphonomic study, and detailed and oblique pores. Because of the differences discussed above, we description of the paleosol horizon, which are essential for sup- assign the small prismatic eggs as a new oogenus and oospecies, porting interpretations. Further, the formation of carbonate nod- Tetonoolithus nelsoni, within the oofamily Prismatoolithidae. ules (caliche) requires hundreds to thousands of years (Chen and Large eggs.The large eggs are similar in size, shape, ornamentation and Pollach, 1986; Küçükuysal et al., 2011) and, thus, the Maiasaura eggshell thickness to eggs of the oofamily Spheroolithidae (Zhao, nests may represent a time-averaged assemblage. 1979)and,morespecifically, those of the hadrosaurine Maiasaura The homogeneous nature of sedimentary rock at fossil egg lo- peeblesorum Hornerand Makela,1979 from the stratigraphicallylower calities often prohibits precise determination of the stratigraphic Two Medicine Formation. However, the highlyaltered condition of the position of clutches (Jackson, 2007). Many if not most of these eggshell prohibits positive identification of the eggshell structure. nesting sites occurred in overbank deposits on the flood plain (Jackson et al., 2013 and references therein). Sediment accumula- 5. Discussion tion rates in these environments are typically low, resulting pri- marily from vertical accretion by suspension settling of fine grained 5.1. Nesting ecology sediment (Walling et al., 1992; Aalto et al., 2003; Pizzuto et al., 2008). Estimating the amount of time represented by the few Aggregation of extant reptilian taxa often occurs for the purpose centimeters of rock separating eggs at the fossil egg localities re- of oviposition (Doody et al., 2009 and references therein). Two mains problematic. Alterations to biological patterns of egg-laying by other adults during subsequent nest construction may further represents an important component of fitness, especially in animals contribute to the difficulty of interpreting the reproductive without parental care (Doody et al., 2009). A number of hypotheses behavior of extinct animals (Jackson et al., 2013). attempt to explain communal and colonial nesting in extant taxa: Despite these difficulties some conclusions are possible inadequate nesting sites, social behavior, mediation of predation regarding the nesting site in the St. Mary River Formation. The eggs risk, environmental conditions (e.g., shade, food, soils), and previ- clearly represent the nesting activities of two species of dinosaurs. ous reproductive success as evidenced by eggshell accumulations The small prismatic eggs represent those of theropods, whereas the (Doody et al., 2009). Aggression and interspecific competition may size, shape, shell thickness and ornamentation of the large eggs also lead to late comers driving out earlier-nesting females, thereby strongly resemble Maiasaura eggs, suggesting that the eggs likely providing a nest, as well as eggs for adult consumption. belonged to a hadrosaur. The size disparity between the two egg The aggregation of eggs at the St. Mary River Formation locality types also suggests that the theropods were substantially smaller. in Montana likely reflects factors such as those reported for extant These small theropod eggs are widely dispersed and occur on three taxa. Further, the multiple egg levels indicate that favorable con- or possibly four levels, ranging in depth from a few to 25 cm below ditions persisted over an extended period of time, and the desir- the larger eggs. In contrast, the five large eggs form a tight cluster ability of this site was not limited to one dinosaur species. However, on a single stratum. The scattered arrangement and inconsistent it remains beyond the resolution of the rock record to determine if angles of the small theropod eggs suggest disruption by the same this locality meets the biological definitions of interspecific colonial species and/or other animals as they prepared the soil and exca- nesting or if nesting events were separated by years, decades, or vated their nests. Alternatively, nest predation reported at some possibly even centuries. extant nesting localities (e.g., Jackson et al., 2015) may offer a plausible explanation for the scattered distribution of the small 5.2. St. Mary River Formation vertebrate fauna eggs at this site. We propose the following sequence of events to explain the egg Although Dawson (1884) mapped the St. Mary River Formation distribution as this locality: 1) one or more small theropods exca- more than a century ago, reports of vertebrate fossils from expo- vated nests and laid the lowermost Tetonoolithus nelsoni eggs; 2) at sures in Montana are rare. Barnum Brown collected a ceratopsian least some of the eggs failed to hatch for unknown reasons; 3) dinosaur, Montanoceratops cerorhynchus (Brown and Schlaikjer, during the same and/or subsequent nesting seasons other thero- 1942), from a locality near Buffalo Lake in Glacier County, Mon- pods of the same species excavated nests, disrupting these deeper tana and subsequent work yielded additional specimens eggs; and finally 4) a larger animal such as a hadrosaur also selected (Weishampel and Horner, 1987; Chinnery and Weishampel, 1998). this nesting site, producing the stratigraphically higher egg clutch. Witmer and Weishampel (1993) describe the frontal of an orni- Because of the large number of eggs in other hadrosaur nest sites thomimid (cf. Dromiceiomimus) and the basicranium of an inter- (e.g., 16 eggs; Horner, 1999), the specimen may preserved only part mediate maniraptora from the Glacier County;Martineau (2014) of this original clutch. also mentions four undescribed dinosaur bonebeds in Lewis and Nesting site selection in modern taxa exerts a profound influ- Clark County near Augusta, Montana, and Hunter et al. (2010) ence on environmental conditions critical to the developing em- describe mammal remains from Shell Hell (MOR Locality 703), bryo and impacts the probability of egg predation (Howard, 1978; which lies along the Two Medicine River in Glacier County south of Deeming and Ferguson, 1991). The choice of an egg-laying locality Browning, Montana. The Buffalo Lake site and Shell Hell localities

Table 2 Fossil eggs and eggshells from four formations in Montana.

Lower Two Medicine Formation Mid to Upper Two Medicine Formation Judith River Formation Hell Creek Formation St. Mary River Formation

Tetonoolithus nelsoni/theropod (this paper) Spheroolithus albertensis Maiasaura aSpheruprismatoolithus Spheroolithus sp., Types I ?Spheroolithus/hadrosaur choteauensis/hadrosaur (10) peeblesorum/hadrosaur; (2,7) condensus/theropod (6) and II (12)/hadrosaur (this paper) aSpheruprismatoolithus Oospecies unknown/Lambeosaurine Oospecies condensus/theropod (6) stebingeri/(8) unknown/Lambeosaurine Continuoolithus Montanoolithus strongorum/theropod (14) Hypacrosaurus stebingeri/(9) canadensis/theropod (10) Prismatoolithus levis/Troodon (1e4, 11) Prismatoolithus Continuoolithus canadensis/theropod (2,13) levis/Troodon (5, 11) Triprismatoolithus stephensi/theropod (10) Prismatoolithus hirschi/theropod (10) Tubercuoolithus tetonensis/theropod (10) Belonoolithus garbani/theropod (12) Dimorphoolithus bennetti/theropod (12) Krokolithes oosp./crocodilian (10) ?Krokolithes wilsoni/crocodilian (2) Krokolithes sp./crocodilian (12) Testudoolithus Testudoolithus sp./ (9,15,16, 17) sp./turtle (12)

References:1,Horner (1982);2,Hirsch and Quinn (1990);3,Varricchio et al. (2002);4,Grellet-Tinner et al. (2006);5,Jackson et al. (2010);6,Bray (1999);7,Horner and Makela (1979);8,Horner and Currie (1994);9,Clouse (2001); 10, Jackson and Varricchio (2010); 11, Zelenitsky and Hills (1996); 12, Jackson and Varricchio (2016); 13, Zelenitsky et al. (1996); 14, Zelenitsky and Therrien (2008); 15, Jackson and Schmitt (2008); 16, Lawver and Jackson (2014); Lawver and Jackson (2017). a Spheroolithus choteauensis and Spheruprismatoolithus condensis are likely the same oospecies and belonged to a hadrosaur rather than a theropod (Jackson and Varricchio, 2010). produced lizard, turtle, champsosaurid, crocodilian, and mamma- application of such terms to fossil localities requires caution. lian remains (Weishampel and Horner, 1987; Hunter et al., 2010). Finally, T. nelsoni are unique to the St. Mary River Formation, when More importantly for our study, Weishampel and Horner (1987) compared to other Upper Cretaceous formations of the Montana mention hadrosaurine and ceratopsian , three types of foreland basin. eggshell, as well as fragmentary dinosaur eggs at the Buffalo Lake locality. Hunter et al. (2010) also report a large eggshell concen- Acknowledgements tration at Shell Hell, whereby the site derived its name. However, the eggs and eggshell fragments at both localities have not been We thank J. Nelson and D. Weishampel for discovering and studied. The eggs reported here, therefore, represent the first fossil excavating the eggs, respectively, and J. Horner, Museum of the eggs described from the St. Mary River Formation, further Rockies, for access to specimens. Montana Wildlife and Parks contributing to our understanding of taxonomic diversity of this provided access to the locality and the Department of Earth Sci- formation, as well as the Montana portion of the Upper Cretaceous ences and the Image and Chemical Analysis Laboratory, Montana foreland basin. State University, Bozeman, Montana provided use of laboratory Transgressive/regressive cycles of the Western Interior Creta- equipment. Finally, we thank two anonymous reviewers for their ceous Seaway have long been considered responsible, at least in helpful suggestions that improved the manuscript. This research part, for distinct biozones within the Upper Cretaceous formations was funded by a grant (EAR-9219035) from the National Science of central and (Horner, 1982, 1984; Weishampel Foundation to D. Varricchio. and Horner, 1987; Horner et al., 2001). Each formation is said to have its own distinct dinosaur fauna that are not present in pre- References ceding or succeeding formations (Weishampel and Horner, 1987). Differences also occur within the same formation. For example, Aalto, R., Maurice-Bourgoin, L., Dunne, T., Montgomery, D.R., Nittrouer, C.A., fi Guyot, J.-L., 2003. Episodic sediment accumulation on Amazonian flood plains Jackson and Varricchio (2010) describe ve new ootaxa from the influenced by El Nino/Southern Oscillation. Nature 425, 497. lowermost Two Medicine Formation that are unreported from the Agnolin, F.L., Powell, J.E., Novas, F.E., Kundrat, M., 2012. New alvarezsaurid (Dino- middle to upper portion of the same formation. Comparison of sauria, ) from uppermost Cretaceous of north-western Patagonia e ootaxa described here to those of the Two Medicine, Judith River, with associated eggs. Cretaceous Research 35, 33 56. Bray, E.S., 1999. Eggs and eggshell from the Upper Cretaceous North Horn Forma- and Hell Creek formations in Montana also seem to support the tion, Central Utah. Vertebrate Paleontology in Utah 99, 361e375. biozone hypothesis to some extent (Table 2; Jackson and Varricchio, Brown, B., Schlaikjer, E.M., 1942. The skeleton of Leptoceratops with the description e 2010, 2016). The small theropod eggs Tetonoolithus nelsoni are of a new species. American Museum Novitates 1169, 1 15. Chen, Y., Pollach, H., 1986. Validity of 14C ages of carbonates in sediments. Radio- unique to the St. Mary River Formation. However, the presumed carbon 28 (2A), 464e472. hadrosaur eggs (Spheroolithus) also occur in the Two Medicine, Chinnery, B.J., Weishampel, D.B., 1998. Montanoceratops cerorhynchus (Dinosauria: Judith River, and Hell Creek formations. Generally, these Spher- ) and relationships among basal neoceratopsians. Journal of Verte- brate Paleontology 18, 569e585. oolithus eggs share similar morphology. 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