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Fossil Palms (, ) Associated with Juvenile Herbivorous Dinosaurs in the Upper Aguja Formation, Big Bend National Park,

Article in International Journal of Sciences · July 2010 DOI: 10.1086/653688

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Steven R Manchester Elisabeth Wheeler Florida Museum of Natural History North Carolina State University

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Available from: Steven R Manchester Retrieved on: 23 July 2016 Int. J. Plant Sci. 171(6):679–689. 2010. Ó 2010 by The University of Chicago. All rights reserved. 1058-5893/2010/17106-0009$15.00 DOI: 10.1086/653688

FOSSIL PALMS (ARECACEAE, CORYPHOIDEAE) ASSOCIATED WITH JUVENILE HERBIVOROUS DINOSAURS IN THE UPPER CRETACEOUS AGUJA FORMATION, BIG BEND NATIONAL PARK, TEXAS

Steven R. Manchester,1,* Thomas M. Lehman,y and Elisabeth A. Wheelerz

*Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611, U.S.A.; yDepartment of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, U.S.A.; and zDepartment of Forest Biomaterials, North Carolina State University, Raleigh, North Carolina 27695-8005, U.S.A., and North Carolina State Museum of Natural Sciences, Raleigh, North Carolina 27601-1029, U.S.A.

Seeds of two palm species conforming to the extant have been recovered from the Campanian (Upper Cretaceous) Aguja Formation of Big Bend National Park, Texas: Sabal bigbendense sp. nov. and Sabal bracknellense (Chandler) Mai. These remains, found together with anatomically preserved palm stems, augment previous reports of Sabalites ungeri (Lesq.) Dorf leaves from the same formation. The co-occurrence of palm seeds with numerous juvenile hadrosaur and ceratopsian bones indicates that palms closely related to modern cabbage palms may have provided fodder and shelter for young herbivorous dinosaurs. The distribution of these and other palm is reviewed.

Keywords: Arecaceae, Campanian, ceratopsians, dinosaur herbivory, fossil, leaves, Sabal, seeds, stems, Texas.

Introduction phytogeography of this genus and the ecological co-occurrence with herbivorous dinosaurs. Big Bend National Park in southwestern Texas preserves a continuous sequence of Upper Cretaceous (Campanian- Geologic Setting ) through Lower Tertiary (Paleocene) continental strata. These strata contain a varied terrestrial and aquatic The Aguja Formation contains two terrestrial intervals re- vertebrate fauna of Judithian age (;79–74 Ma; Lehman and ferred to informally as the lower and upper shale members Busbey 2007) and a diverse assemblage of dicotyledonous (Lehman and Busbey 2007). Both the lower shale member and and coniferous woods (Wheeler 1991; Wheeler et al. 1994; the lowermost part of the upper shale member consist primar- Wheeler and Lehman 2000, 2005, 2009; Lehman and Wheeler ily of dark organic-rich shale, lignite, and coal that accumu- 2001). , represented by leaves, seed casts, and lated in poorly drained brackish-water marshes or swamps petrified stems and roots, also occur commonly in the Campa- close to the shoreline. These deposits interfinger with coastal nian Aguja Formation and include some of the oldest records strand plain and marine shelf deposits of the Pen Formation. for coryphoid palms. The seeds and stems co-occur with bones The upper part of the upper shale member consists of drab of juvenile hadrosaur and ceratopsian dinosaurs at multiple fluvial floodplain deposits, primarily olive, yellow, and gray localities. mudstone interbedded with lenticular stream channel sand- In this article we describe the Aguja palm seed and stem re- stone. These sediments accumulated in better-drained fresh- mains, which, along with the leaf remains already described water alluvial environments some distance landward of the by Dorf (1939), allow recognition of the modern coryphoid shoreline. Most of the diverse fossil wood assemblage thus far palm genus Sabal in the Upper Cretaceous. Sabal Adanson ex documented for the Aguja Formation has been recovered Guersent, with 15 extant species, is now confined to Mexico, from the upper shale member, including a unique association the Caribbean Islands, Bermuda, and the southeastern United of in situ tree stumps (Agujoxylon olacaceoides and Metcal- States (Zona 1990), but fossil records indicate that the genus feoxylon kirtlandense) that demonstrates that dicot trees were was more widespread in the Northern Hemisphere, occurring dominant canopy-forming elements in some Late Cretaceous in midlatitudes of North America and Europe and possibly forests at lower latitudes of western North America (Lehman also in Asia during the Tertiary (reviewed in Manchester and Wheeler 2001). 1999; Dransfield et al. 2008). Although Sabal was previously Palm leaf impressions and permineralized stem fragments recognized based on seeds as old as Early (;52 Ma), are found throughout the Aguja Formation, as well as in the the Aguja occurrence extends the minimum age of this genus overlying Javelina Formation. Baghai (1996) reported pollen back to the Campanian (;77 Ma). The significance of fossil having suggested affinities with the palm family (Arecaceae; Sabal from the Aguja Formation is considered in relation to the e.g., Arecipites Wodehouse, Palmaepollenites Potonie´, Mono- colpopollenites Thomson & Pflug, Sabalpollenites Thiergart in Raatz) from throughout the same stratigraphic interval. 1 Author for correspondence; e-mail: steven@flmnh.ufl.edu. However, the palm seed collection sites described herein and Manuscript received April 2010; revised manuscript received April 2010. the best-preserved in situ palm stems are confined to the lower

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50 m of the Aguja upper shale member (fig. 1). Vertebrate bio- 2007). Exact locality information and maps for the collection stratigraphy (Rowe et al. 1992) and radiometric age determi- sites are available at the Florida Museum of Natural History nations (Befus et al. 2008) indicate that the upper shale Paleobotanical Collection, University of Florida, Gainesville, member of the Aguja Formation is Middle to Late Campa- and at the Vertebrate Paleontology Laboratory of the Texas nian in age. Memorial Museum in Austin.

Description of Fossil Seeds Material and Methods

The fossil seeds and stems described herein were collected at Sabal bigbendense sp. nov. (Fig. 2A–2H) four sites near the western border of Big Bend National Park. Diagnosis. Seeds subglobose-oblate, rounded dorsally; prox- Two localities are on the southern side of Rattlesnake Moun- imal surface with a broad circular depression that is domed tain, about 1 km apart and 2 km north of Maverick Road. in the center. Seeds 12.0–13.9 mm in diameter and 11.0– These two sites are Texas Memorial Museum (TMM) verte- 11.1 mm in height. Surface smooth to wrinkled. Germination brate fossil localities TMM 42534 (20 m above the base of operculum small (2.0 mm), circular, positioned equatorially the upper shale member) and TMM 43163 (40 m above to supraequatorially. the base of the upper shale member), equivalent to Florida Specimens. Holotype, here designated: UF403-53791 (fig. Museum of Natural History Paleobotanical Collection (UF) 2E–2G). Paratypes: UF402-53789 (fig. 2A,2B), UF403- localities 402 and 403, respectively. Two additional sites 53792 (fig. 2C,2D), UF403-53793 (fig. 2H). are TMM 41917 (UF401), near Bruja Canyon, and WPA-1 Localities. TMM 42534 and TMM 43163. (UF404), a site originally excavated as part of a Works Prog- These suboblate seeds conform to Sabal in having a circular ress Administration (WPA) project in the 1930s (Lehman outline; a circular rim surrounding a basal scar including the

Fig. 1 Geographic and stratigraphic location of selected palm-bearing sites in the Aguja Formation of Big Bend National Park, Texas. The map shows positions of measured stratigraphic sections (1–8), also shown on a restored cross section of the intertonguing Aguja and Pen formations. Sites discussed in the text near Rattlesnake Mountain (RM) are shown, as is the approximate stratigraphic level of Dorf’s (1939) palm collection site. The stratigraphic section of the Aguja Formation at Rattlesnake Mountain shows informal subdivisions and levels of palm-bearing Texas Memorial Museum (TMM) vertebrate fossil sites and the WPA-1 ceratopsian bone bed described by Lehman (2007). Fig. 2 Fossil and extant Sabal seeds. A–H, Sabal bigbendense sp. nov. from the Aguja Formation. A, B, Basal view showing circular outline and hilar scar and lateral view showing protruding hilum and supraequatorial circular germination operculum scar; UF402-53789. C, D, Basal and lateral views of seed with longitudinally ruptured seed coat; UF403-53792. E–G, Basal, lateral, and apical views, with circular germination operculum. Arrow in G and subsequent illustrations indicates the germination operculum; UF403-53791. H, Specimen with longitudinal fractures; UF403-53793. I–S, Sabal bracknellense (Chandler) Mai from the Aguja Formation. I–K, Basal, apical, and lateral views of seed with fully intact seed coat, showing irregular indentations; UF403-53795. L, M, Basal and lateral views of seed; UF404-53798. N, O, Basal and lateral views of seed; UF403-53794. P, Q, UF401-53784. R, S, Lateral and apical views of seed; UF401-53785. T–V, Extant , Gainesville, Florida; basal and lateral views and longitudinal section intercepting the embryo; UF Modern Ref. Coll. 1437. W, X, Basal and lateral views, extant Sabal bermudana Bailey, Bermuda; Bailey sn. Y, Extant Sabal pumos , Mexico; Moore 8113. 682 INTERNATIONAL JOURNAL OF PLANT SCIENCES hilum; a thin, smooth seed coat; and a small circular plug of resent distinct species if other parts of the , such as the embryo protrusion in equatorial to supraequatorial position. flowers and frond tips, were available for direct comparison. These are the largest of all fossil Sabal seeds known, the next biggest being those of the Eocene London Clay species Sabal Taxonomic Placement grandisperma Reid & Chandler (1933; 10 mm in diameter). Among extant species of Sabal, seeds of a similarly large size Both of these species of fossil seeds are indistinguishable in occur in Sabal pumos (Kunth) Burret, Sabal rosei (Cook) morphology from those of extant Sabal. The shared charac- Beccari, and Sabal uresana Trelease (fig. 3; Zona 1990). ters include thin testa, subglobose to oblate (not elongate) shape, rounded distal surface, domed proximal truncation with chalaza and raphe, and small circular outline of the em- Sabal bracknellense (Chandler) Mai (Fig. 2I–2S) bryo radicle protrusion located at the equator or offset to- Basionym. Palmospermum bracknellense Chandler 1961, ward the distal pole. These are all features that Mai (1976) p. 125, pl. 13, figs. 15, 16, Sabal bracknellense (Chandler) Mai indicated as characteristic of the genus. Descriptions of the 1976, p. 104, pl. 2, figs. 5–9. position of the embryo scar have varied in the literature as Description. Seeds subglobose, rounded dorsally; proxi- a result of conflicting interpretations of ‘‘which way is up.’’ mal surface with a broad circular depression that is domed in Paleobotanists have referred to the position of this scar as the center, 5.1–7.0 mm in diameter and 5.1–6.0 mm in equatorial to subequatorial, treating the prominent hilar cav- height. Surface smooth to wrinkled. Outline of the germina- ity as the apical end of the seed, whereas neobotanists refer tion operculum circular (0.9–2.0 mm diameter), positioned to the position as equatorial to supraequatorial or subapical, equatorially, subequatorially, or supraequatorially. treating the hilar scar as basal (Zona 1990; Dransfield et al. Specimens. UF401-53784 (fig. 2P,2Q), 401-53785 (fig. 2R, 2008). Despite apparent differences from the descriptions, 2S), 403-53795 (fig. 2 I–2K), 403-53794 (fig. 2N,2O), 404- when orientations are treated consistently, the position of the 53798 (fig. 2L,2M). embryo corresponds in the fossil and extant Sabal seeds. Localities. TMM 41917, 42534, WPA-1 (Lehman 2007; Here we treat the hilum as basal and the embryo position as UF404). equatorial to supraequatorial. These seeds are less than half the size of those of S. bigben- Being concerned that identical seed morphology might oc- dense (fig. 3). We cannot distinguish them morphologically cur in other genera of palms, we examined seeds of a wide or in size from seeds of S. bracknellense from the Eocene of range of extant genera in the palm collection of the Bailey Europe and Oregon (Mai 1976; Manchester 1994), a species Hortorium, Cornell University, with the latest systematic with seeds resembling extant Sabal minor (Jacq.) Persoon treatments (Asmussen et al. 2006; Dransfield et al. 2008) as (fig. 3). Because we lack any clear morphological distinction a guide. The latter reference provides descriptions and dia- from the Eocene representatives, we are hesitant to coin a new grammatic illustrations of seeds throughout the family. In the species name for the Aguja representatives and have assigned revised classification of palms provided by Asmussen et al. them to the same species as the Eocene European specimens. It (2006), including relationships inferred from molecular is likely, however, that the populations would be found to rep- sequence data, Sabal is treated as the only member of tribe

Fig. 3 Plot of seed diameter ranges for selected extant and fossil species of Sabal. Data from Mai (1976), Zona (1990), and Manchester (1994). Superscripts 1 and 2 indicate Aguja and Eocene specimens, respectively. MANCHESTER ET AL.—FOSSIL PALMS AND JUVENILE DINOSAURS 683

Sabalinae (tribe ) and sister to the subtribe Thrina- seed size within or between separate trees of the same species cinae (including , , , in extant Sabal. , , , , , ). These two groups are sister to all remaining Coryphoideae, Aguja Palm Stems which is now considered to be a morphologically diverse group including not only but also, for example, The palm stems that co-occur with the Sabal seeds and juve- , , , , , nile dinosaur remains are relatively uniform in anatomy (fig. , , , , , 4). The stems typically range from 10 to 20 cm in diameter, , , , , , Mede- and their preserved length varies from less than 20 up to 150 mia, , , , , Corypha, cm. Cortical and dermal zones are absent in the samples we and . The seed morphology of Sabal appears to be examined. The collateral vascular bundles have fibrous bundle unique among these and other clades of the palm family. Seeds sheaths that are much larger than the vascular bundles. Some of the sister subtribe, Thrinaceae, are globose to slightly prolate, bundles have only protoxylem; more commonly, the bundles without the prominent basal depression, and in some genera are have one, two, or three large metaxylem vessels. There is a sin- prominently grooved (Dransfield et al. 2008). Among the other gle phloem strand per bundle. The is not lacu- Coryphoidieae, seeds of Washingtonia are similarly smooth and nose and includes tabular parenchyma, sometimes encircling shiny with a prominent concavity, but the concavity is lateral the bundles and sometimes radiating from the bundles. rather than basal, and the seed is ellipsoidal rather than oblate. Most fossil palm stems have been assigned to the genus Given the apparent uniqueness of Sabal seeds and the phyloge- Schenk. Uhlia allenbyensis Erwin and Stockey netic separation of this genus from other Coryphoid palms, it (1994) is an exception. This species is based on stem frag- seems reasonable to place these Cretaceous seeds in the extant ments with attached petioles, which showed features of sys- genus. tematic value, allowing assignment of this Eocene palm to Aside from the size differences, Sabal bigbendesnse and subfamily Coryphoideae, tribe Corypheae. The anatomy of Sabal bracknellense seeds are morphologically very similar. palm stems varies considerably even within a single individ- Indeed, there could be some concern that these two sizes of ual, both horizontally and vertically, and is affected by stem seeds from the Agua Formation merely represent variant age (Tomlinson 1990). This variability makes it difficult to seeds from a single species. However, the size ranges do not assign isolated palm stem fragments to individual genera. It overlap, and the disparity in size would be sufficient to dis- has long been recognized that assigning fossil palm stems to tinguish among different extant species (fig. 3). In addition, extant genera or groups of genera is difficult because system- we have not observed obvious bimodality in distribution of atically diagnostic characters have not been found for many

Fig. 4 Palm stems and leaves from the Aguja Formation. A, Palm base preserved in growth position (source of thin section RM-27) at TMM 42534; preserved height is 30 cm. B, Palm base preserved in prone position at TMM 4163; preserved length is 130 cm. C–E, Palmoxylon; RM-27. C, Stem section showing disposition of vascular bundles. D, E, Vascular bundles with large fibrous bundle sheaths. F, G, Sabalites leaf impression in sandstone. 684 INTERNATIONAL JOURNAL OF PLANT SCIENCES groups. Kaul (1960), who reviewed earlier work on palm Fossil Record of Sabal anatomy, remarked that there were no workable natural or artificial classifications for fossil palm stem anatomy. Nam- Although confined to the New World in modern distribu- budiri and Tidwell (1998) reviewed the characteristics used tion, Sabal was more widespread in the Northern Hemi- in classifying Palmoxylon species. They found that some of sphere during the Tertiary, with records confirmed by seeds the features previously used to distinguish Palmoxylon spe- from the Early Eocene of southern England (Reid and Chan- cies were features of dubious taxonomic value because they dler 1933) and the Middle Eocene of Germany (Mai 1976). were affected by position within the stem. Dransfield et al. (2008) reviewed the fossil record for Sabal For these Aguja palm stems our objectives were to deter- and Sabalites. The oldest verified occurrence of Sabalites is mine (1) whether their anatomy is consistent with the anat- Sabalites carolinensis leaves from the Coniacean-Santonian omy of Sabal and other coryphoid palms and (2) whether of eastern North America (Berry 1914; Kvacek and Herman they resemble palms from the nearby late Campanian–early 2004); these are the oldest unequivocal fossils of Arecaceae Maastrichtian Olmos Formation of northern Mexico (Cevallos- (Harley 2006). Other Late Cretaceous records include palm Ferriz and Ricalde-Moreno 1995). According to Tomlinson leaves referred to as Sabalites longirhachis (Unger) J. Kvacek (1961) the sabaloid palm group has well-developed fibrous et Herman from the Campanian of Austria (Kvacek and Her- bundle sheaths, undivided phloem bundles, and xylem sur- man 2004). Sabal and sabaloid fossils are also known from rounded by parenchyma; within the group the number of the Tertiary to Holocene from a variety of sites that are not metaxylem vessels per bundle is variable. The Aguja palms only within but also well outside the current range of the ge- also have these features. These traits are not exclusive to the nus, e.g., Great Britain (Reid and Chandler 1933), Oregon sabaloid palms; nevertheless, they do not contradict the evi- (Manchester 1994), Kentucky, Tennessee, Texas (Daghlian dence from seeds and foliage for the recognition of Sabal in 1978), China, and Japan (Zona 1990). The Asian occurrences the Aguja flora. The Olmos Formation shares some dicot are based only on leaves without epidermal anatomy and have genera with the Aguja and Javelina formations of Big Bend not been confirmed by seeds and should be treated in the mor- (Wheeler and Lehman 2009), so it is possible that there also phogenus Sabalites, but we accept the European seed records are shared monocots. Four Palmoxylon species have been as valid representatives of the extant genus Sabal. Its range di- reported from the Olmos Formation (Cevallos-Ferriz and minished after the terminal Eocene cooling, and the genus was Ricalde-Moreno 1995): Palmoxylon longum, Palmoxylon eradicated from Europe after the . commune, Palmoxylon fibrosum,andPalmoxylon polymor- Because of the fossil evidence and relationships within the phum. The size of fibrous bundle sheaths and some features genus, Zona (1990) hypothesized that Sabal probably had of ground tissue parenchyma in P. commune and P. fibr o- a Laurasian origin. More recent work also suggests a Laura- sum are similar to those of the Aguja palms. sian origin for coryphoid palms (Bjorholm et al. 2006).

Aguja Palm Fronds Discussion

The Aguja palm leaves (fig. 4F,4G) fit the concept of Saba- Lehman (1985) interpreted the palm-bearing part of the lites Saporta as summarized by Read and Hickey (1972, p. 133) Aguja upper shale member as consisting of deltaic interdistri- in a review of morphogenera for fossil palm leaves: ‘‘Unarmed, butary marsh and coastal floodplain deposits, accumulated palmate leaf fossils exhibiting a condition wherein the petiole immediately landward of the shoreline. This sedimentary fa- appears to extend into the blade (costapalmate), with the fused cies association is compatible with the coastal environments segments and their midveins emerging at an acute angle and ex- favored by some modern Sabal species, e.g., tending straight away from the costa toward the leaf apex. Ad- (Kunth) Burret (Wade and Langdon 1990; Zona 1990). The axial surface of the leaves may exhibit a ‘hastula’ or ligule-like palm seeds and stems are found in association with two of appendage at the junction of petiole and blade.’’ Among extant the typical Aguja wood assemblages recognized by Wheeler palms, leaves fitting this description occur in various other pre- and Lehman (2000, 2005): the platanoid-icacinoid assem- dominantly coryphoid genera, in addition to Sabal (Dransfield blage, which consists predominantly of small, slender axes of et al. 2008), so the morphogenus Sabalites should not be low-growing shrubs or scrambling vines, and the cupressoid- equated with Sabal. Dorf (1939) assigned the Aguja fossils to podocarpoid ‘‘acme’’ zone, which is dominated by ‘‘logjam’’ the genus Sabalites and indicated relationships with the subfam- accumulations of large conifer trunks. The occurrence of ily Sabaleae rather than direct assignment to the extant genus palm stems and seeds in association with both of these wood Sabal because he considered that other genera of the subfamily assemblages suggests that palms may have grown in areas are also similar. Daghlian (1978) recognized the extant genus bordering both vegetational facies, for example, in the same Sabal, as well as the morphogenera Sabalites, Costapalma way that modern Sabal palmetto (Walt.) Lodd. ex J. A. & Daghlian, and Palustrapalma Daghlian, from Eocene fronds J. H. Schulz. in Florida grows in ‘‘hammocks’’ between salt with well-preserved epidermal anatomy. However, these Cre- marshes and stands of cypress or pine forest (Wade and taceous Aguja leaf fossils, known only from impressions, Langdon 1990). A similar setting was envisioned by Wing without epidermal anatomy, should be retained in the mor- et al. (1993), who documented palmetto-dominated vegeta- phogenus Sabalites. Nevertheless, we hypothesize that these tion based on an in situ assemblage of diverse leaf remains leaves were produced, along with the Palmoxylon stems, by from a delta plain environment in the early Maastrichtian or the same plants that shed the Sabal seeds described here. late Campanian Meeteetse Formation of Wyoming. MANCHESTER ET AL.—FOSSIL PALMS AND JUVENILE DINOSAURS 685

Palms and Dinosaur Herbivory there may have been an ecological relationship between palms and juvenile dinosaur herbivores. Some of these fossil It is significant that juvenile certatopsian and hadrosaurian Sabal seeds show apparent pre- or paradepositional break- dinosaur skeletal material occurs in association with these age, but there are no obvious bite marks, and it is unclear Sabal seeds, at all four sites. For example, one of the Sabal whether the breakage was biologically or physically induced. bracknellense seeds was found at the WPA (UF404) quarry Barrett and Willis (2001) and Butler et al. (2009) reviewed site, described by Lehman (2007), that preserves a mass mor- evidence for coevolutionary interaction between angiosperms tality assemblage of the ceratopsian Chasmosaurus marisca- and herbivorous dinosaurs but found little support in either lensis Lehman. The skeletal material consists mostly of the times of appearance or the diversification patterns of ma- disarticulated limb and girdle elements and vertebral centra jor plant and dinosaur clades to indicate that dinosaurs (fig. 5). Apart from their small size, the isolated bones are played a significant role in the adaptive radiation of flower- identifiable as juvenile on the basis of their cortical bone tex- ing plants. Lecky and Tiffney’s (2002) review of the North ture and open neurocentral sutures. The skeletal elements be- American record found that hadrosaurians tend to co-occur long to individuals of varied sizes. For example, identifiable with floras with a high percentage of angiosperms. Whether elements recovered at TMM 42534 represent individuals by intensity of browsing, environmental disturbance (‘‘dino- from 26% to 70% adult size (fig. 5). Hence, young animals turbation’’), or acting as seed dispersal agents, it seems cer- of varied ages inhabited or were preserved at these sites. Ju- tain that large dinosaurian herbivores exerted some influence venile dinosaurs are otherwise quite rare in the Aguja Forma- on the vegetative structure and composition of Mesozoic tion. This co-occurrence led Lehman (2002) to suggest that plant communities. The wide taxonomic variety of dinosaur

Fig. 5 Typical juvenile ceratopsian and hadrosaur skeletal elements from TMM 42534. The length of each element expressed as a percentage of adult length (in parentheses) is determined on the basis of comparison with the largest specimens from the Aguja Formation attributed to Chasmosaurus mariscalensis (Lehman 2007) and to ?Kritosaurus sp. (Davies 1983). A, Ceratopsian right scapula (26%) in lateral view. B, Ceratopsian right scapula (29%) in lateral view. C, Hadrosaur right astragalus (36%) in proximal and anterior views. D, Hadrosaur right astragalus (50%) in proximal and anterior views. E, Ceratopsian proximal caudal vertebral centrum (42%) in anterior, left lateral, and dorsal views. F, Ceratopsian proximal caudal vertebral centrum (70%) in anterior, left lateral, and dorsal views. G, Hadrosaur anterior dorsal vertebral centrum (45%) in anterior, left lateral, and dorsal views. H, Hadrosaur anterior dorsal vertebral centrum (59%) in anterior, left lateral, and dorsal views. 686 INTERNATIONAL JOURNAL OF PLANT SCIENCES herbivores and their varied dentition and jaw occlusion 1987). In Late Cretaceous environments and before the ap- mechanisms further suggest that each may have specialized in pearance in Paleogene time of large fruits, nuts, and seeds processing different sorts of plant food. adapted for biotic dispersal in tropical forest dicots, palm Because of their great stature, large gut capacity, and high fruits could have been an important food resource (Tiffney dietary demands, adult dinosaurian herbivores are generally 1984). In modern tropical ecosystems, palm fruits are con- considered to have been ‘‘bulk feeders’’ and rather indiscrimi- sumed by a wide variety of arboreal herbivores (birds, bats, nant browsers (Farlow 1987; Tiffney 2004). However, direct squirrels, and monkeys), but so many fruits are produced evidence of the kinds of plant tissues they consumed is scant. that they are also eaten by terrestrial herbivores (rodents, Putative gut contents preserved within articulated skeletons pigs, deer, cattle, tapir; Corner 1966; Kahn and de Granville of several Late Cretaceous hadrosaurs contain wood, leaf cu- 1992). A nutritional analysis of Sabal seeds determined that ticle, and seeds of both gymnosperms and angiosperms, but they are suitable for feeding pigs and ruminants (Arellano it is possible that these tissues instead represent detritus et al. 1992). There were few, perhaps no, arboreal herbivores washed into the abdominal cavities with sediment during in Late Cretaceous environments, and juvenile herbivorous burial (Currie et al. 1996). More definitive gut contents pre- dinosaurs were probably the most abundant small terrestrial served within an ankylosaur skeleton include vascular tissue, herbivores. The beaks and shearing teeth of ceratopsians and foliage, and seeds of angiosperms (Molnar and Clifford hadrosaurs would have been well suited for gaining hold of 2000). Coprolites attributed to hadrosaurs on the basis of and crushing the relatively tough palm fruits. Cretaceous pal- their size, form, and spatial association with skeletal remains metto palms, if they grew in dense ‘‘thickets,’’ as in some contain fragments of gymnosperm wood, foliage, and seeds modern settings, would also have provided protective cover (Chin and Gill 1996; Baghai-Riding and DiBenedetto 2001; for the juvenile dinosaur herbivores. Tweet et al. 2008). Some coprolites possibly attributable to Modern palm seeds are also spread by floodwaters (Corner dinosaurian herbivores contain leaf cuticle, stem fragments, 1966), so it is possible that accumulations of palm seeds with and abundant small angiosperm seeds (Nambudiri and Binda bones of juvenile dinosaurs could have resulted from hydrau- 1989; Rodriguez de la Rosa et al. 1998). Such evidence may lic concentration. Indeed, at two of the sites (TMM 43163 support the notion that dinosaurs were indiscriminant browsers and 41917) the seeds and bones occur in a fluvial sandstone and only facultative seed dispersal agents (Tiffney 2004). bed, along with abraded tooth, bone, and wood fragments Zona (1990) summarized observations on the ecology of indicative of transport. However, at the other sites (TMM modern Sabal, and several of the attributes seem well suited 42534 and WPA-1) the sedimentary matrix consists of fine- for an environment inhabited by large herbivores. Sabal spe- grained organic-rich mudstone, accumulated in quiescent envi- cies have a high tolerance for disturbance and are widespread ronments. At all four of these sites, bones of adult dinosaurs in disrupted lowland areas, thriving as an invasive ‘‘weed’’ to- are also found, which together with the small seeds and juvenile day in anthropogenic habitats. They can withstand burning dinosaur remains do not comprise a group of hydraulically and even persist after clearing for agriculture (Zona 1990). equivalent objects. Hence, concentration by running water The aerial stem arises only after many years of underground alone cannot explain these accumulations. We infer, there- growth; the underground stem may be vital in allowing Sabal fore, that the association of palms with juvenile ceratopsians to withstand disturbance and to rapidly colonize open areas. and hadrosaurs cannot be attributed simply to sedimentary The high tolerance for disturbance exhibited by Sabal may sorting. have initially evolved as an adaptation to the presumably large ecological ‘‘footprint’’ of dinosaurian herbivores, in the Palms and Paleoclimate manner hypothesized by Bakker (1978). However, most such observations and discussion of dinosaur- The frost sensitivity of modern palms limits their worldwide plant interactions emphasize the stature of adult dinosaurian distribution to climates with mean annual temperature (MAT) ‘‘megaherbivores’’ and their likely high or low browsing greater than 10°C, coldest month mean temperature (CMMT) ranges without considering the much greater numbers of juve- greater than 5°C, and yearly minimum temperature greater nile dinosaurs that must have fed directly on or very near the than À10°C (Greenwood and Wing 1995). More specifically, ground. In addition, given the communal nesting behavior ev- the distribution of Sabal spp. along the Gulf Coast of North ident in some dinosaurian herbivores and the absence of other America is limited to areas with MAT at least 15°C, having small ground-dwelling herbivores in Mesozoic ecosystems, in- less than a 4–6-d duration of longest annual frost and where fant dinosaur ‘‘rookeries’’ may have placed quite a heavy de- CMMT is no colder than 5°–10°C (Greenwood and Wing mand on plant materials close to the ground. And, if young 1995). These limitations make the distribution of fossil palms dinosaur herbivores had high metabolic rates and rapid useful as paleoclimate proxy data. Hence, the palm fossils de- growth rates, their plant food diets must have been of high scribed here may provide insight into the climatic conditions energy value compared to bulk foliage (Lehman 2007). There- of the Aguja flora and fauna. fore, juvenile dinosaur herbivores were likely more specialized The northernmost report of palm foliage during the in their feeding than adults, and fallen seeds and fruits such Campanian-Maastrichtian in the North American interior is as those of palms described herein might have comprised an from northern Montana (Crabtree 1987), which would have important food resource for them. been ;55°N paleolatitude, based on the paleogeographic map To sustain their presumed high growth rates, young herbiv- of Scotese (1991; fig. 6). Although palm foliage has also been orous dinosaurs, with their limited gut capacities, must have described from the Late Cretaceous of Vancouver Island, Brit- fed selectively on plant parts with high energy value (Farlow ish Columbia (Bell 1957), at ;60°N paleolatitude, this region MANCHESTER ET AL.—FOSSIL PALMS AND JUVENILE DINOSAURS 687

palms produce structurally identical simple pollen types (Up- church et al. 1999). If the northernmost Late Cretaceous limit of preserved palm foliage approximates their actual limit, then that was between 55° and 58°N paleolatitude, and the ;15°C MAT isotherm must have been at about that latitude (fig. 6). The northernmost limit of Late Cretaceous crocodilians in North America is somewhat higher than that inferred for palms, between 58° and 65°N, which should be near the 14°C MAT isotherm (Markwick 1998). Together, the distribution of palms and crocodilians indicates that the Late Cretaceous MAT at ;60°N was no colder than 15°C.

Conclusions

Palm seeds found in the Aguja Formation are referable with some confidence to the extant genus Sabal. Two distinct size classes among the seeds indicate that two species are rep- resented. This remarkable occurrence reveals that the modern palmetto palms originated over 77 million years ago, shortly after the earliest appearance of palms in the fossil record. Fig. 6 Campanian-Maastrichtian paleogeographic map of North Palm leaves and stems morphologically consistent with Sabal America (light gray, shallow marine; dark gray, terrestrial environ- also occur within the same stratigraphic interval and at the ments; modified from Scotese 1991; Hay et al. 1999) showing same sites that yield the seeds. The four seed-bearing sites are paleolatitude of sites where palm leaves and stems have been reported particularly notable because they also preserve remains of (filled circles): Aguja (Ag; reported herein), Almond (Al; Johnson juvenile ceratopsian and hadrosaurian dinosaurs. Juvenile 2003), Eagle (Ea; Van Boskirk 1998), Fruitland (Fr; Tidwell et al. specimens are rare elsewhere in the Aguja Formation. This 1981), Hell Creek (He; Johnson 2002), Magothy (Ma; Berry 1914, co-occurrence likely reflects an ecological association. Palm 1916), McRae (Mc; Upchurch and Mack 1998), Nanaimo (Na; Bell fruits are an important food resource for arboreal and terres- 1957; asterisk shows present location, and arrow indicates likely Cretaceous position before translation based on Wyld et al. trial herbivores in modern tropical ecosystems, and fallen 2006), Olmos (Ol; Cevallos-Ferriz and Ricalde-Moreno 1995), Two fruits would have provided juvenile dinosaurian herbivores Medicine (Tw; Crabtree 1987). Leaf megaflora sites lacking palms with high-energy-value fodder necessary to sustain high (open circles) are Patoot/lowerAtanekerdluk (At; Heer 1883), Chignik growth rates with their limited gut capacities. (Ch; Hollick 1930), Dinosaur Park Group (Di; Koppelhus 2005), Eureka Sound (Eu; Felix and Burbridge 1973), Prince Creek (Pr; Acknowledgments Spicer et al. 1987). We thank the Science and Resource Management staff of Big Bend National Park, in particular D. Corrick and S. is part of an accreted terrane that has been displaced north- Wick, for their assistance with our research in the park and ward tectonically from its Late Cretaceous position (Wyld J. Browning for discovering several of the palm seeds de- et al. 2006). Palm foliage has not been reported from well- scribed herein. David Greenwood and Scott Wing contrib- studied strata of this age in the Dinosaur Provincial Park area uted helpful discussion on the climatic requirements of of southern Alberta (Koppelhus 2005), which was at ;58°N extant palms. Kevin Nixon, Cornell University, provided ac- paleolatitude, or from higher latitudes in the interior of North cess to the Bailey Hortorium for comparative study of extant America (Upchurch and Wolfe 1993). Although pollen with palm fruits and seeds, and Hongshan Wang accessioned and possible palm affinities (e.g., Arecipites) has been reported curated the cited fossil Sabal seeds at UF. Scott Wing, Edith from southern Alberta (Braman and Koppelhus 2005), some Taylor, and Bruce Tiffney provided helpful review com- have argued that these pollen genera may not be useful for ments. This work was supported in part by NSF grant BSR- paleoclimatic interpretation because plant families other than 0743474 to S. R. Manchester.

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