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Extinct Pterygoboa (Boidae, Erycinae) from the Latest Oligocene and Early Miocene of Florida
Jim I. Mead and Blaine W. Schubert

Southeastern Naturalist, Volume 12, Issue 2 (2013): 427–438

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2013 SOUTHEASTERN NATURALIST 12(2):427–438 Extinct Pterygoboa (Boidae, Erycinae) from the Latest Oligocene and Early Miocene of Florida Jim I. Mead1,* and Blaine W. Schubert1 Abstract - Members of the Erycinae are small to medium-sized, semi-fossorial snakes in the family Boidae (Squamata, Serpentes) known today from Africa, southwestern Asia, India, and western North America. Erycines were the predominate snakes in faunas of North America during the Paleogene. In North America, only the minute, extinct erycine Pterygoboa is known to have additional wing-like processes situated on the postzygapophyses of the vertebrae. Here we report on Pterygoboa from one latest Oligocene (25–24 Ma) (White Springs) and two early Miocene (≈20–19 Ma) localities (Miller, Thomas Farm) in Florida. These specimens represent a significant chronological and southern geographic range extension for the genus and permit an amendment to the morphological description of this unusual snake. Introduction Members of the monophyletic subfamily Erycinae (sand-snakes) are small to medium-sized, semi-fossorial snakes (Squamata, Serpentes) in the family Boidae (Macrostomata, Booidea). Today members of the subfamily live in Africa, southwestern Asia, India, and western North America (Charina, Lichanura). During the Paleogene (early portion of the Cenozoic; Paleocene to Oligocene), erycine snakes were the predominate snakes in faunas of North America (Holman 2000). Many fossil specimens in North America are attributed to the Erycinae; however, Szyndlar and Rage (2003) state that such referral to this subfamily is “… groundless and the fossils in question actually may have represented other groups of the Booidea.” Their statement aside, a number of North American researchers believe that during the latest Oligocene and early Miocene (Arikareean North American Land Mammal Age [NALMA]) the abundance and diversity of erycine snake species began to wane at about the same time that the colubroid snakes began to expand (Holman 2000, Parmley and Holman 1995). Here we report on an unusual and minute, extinct erycine snake, Pterygoboa, recently recognized in two early Miocene (Hemingfordian NALMA) localities (Miller local fauna, Thomas Farm) and one latest Oligocene (middle Arikareean NALMA) locality (White Springs) in northern Florida. Erycine snakes Overview of erycine. Erycine vertebrae differ from all other snakes in having additional apophyses (small, bony outgrowths), known as pterapophyses, located on the neural arch between the neural spine and the postzygapophyses of the 1Department of Geosciences and Don Sundquist Center of Excellence in Paleontology, East Tennessee State University, Johnson City, TN 37614. *Corresponding author -MEAD@mail.etsu.edu. 428 Southeastern Naturalist Vol. 12, No. 2 caudal vertebrae (Fig. 2: pt; Hoffstetter and Rage 1972, Szyndlar 1994, Szyndlar and Schleich 1994). Trunk vertebrae within the clade (which characteristically lack these additional apophyses) differ little from one another and therefore create taxonomic and identification issues. European and North American researchers habitually differ in their approach to identifying fossil erycine snake vertebrae; “fossil trunk vertebrae should not be interpreted as belonging to the Erycinae unless they are accompanied by caudal vertebrae displaying complex morphology characteristic” of the subfamily (Szyndlar and Rage 2003). North American Neogene erycine species are diagnosed based on trunk vertebrae (see discussion and views in Szyndlar and Rage 2003). In the view of these authors, the extinct Calamagras (including the possibly congeneric Ogmophis) and the extant Charina and Lichanura are considered the only true erycines in North America. Holman (1976a) assigned the extinct Pterygoboa to Erycinae; however, Szyndlar and Rage (2003) disagreed because “additional vertebral apophyses in [Pterygoboa] are not restricted to the caudal portion of the column … which cast doubts on [its] assignment” to the subfamily (see amendment below). Pterygoboa miocenica Holman is diagnosed, in part, as “trunk and caudal vertebrae with winglike elaborations of the postzygapophyses”. In naming the taxon, and again in 1977, Holman referenced these wing-like apophyses (Holman 1976a, 1977). For whatever reason, in his description of a second species, P. delawarensis Holman, he termed the diagnostic wing-like structures as “pterapophyses”—“… pterapophyses present on postzygapophysis; …”; this is an error in terminology. Hoffstetter and Rage (1972) used the term “pterapophyses” for the supplementary apophyses located on the neural arch between the neural spine and the postzygapophyses; Sood (1941) used the term “accessory lateral process” for these same apophyses. Pterapophyses on the neural arch are not equivalent to the wing-like structures observed on the lateral edges of the postzygapophyses as described by Holman (1976a, 1977, 1998) to diagnose the genus Pterygoboa and in helping to diagnose the two species. Rage (1984) referred to the wing structures as “additional processes above the postzygapophyses.” Szyndlar (1994) named the high wing-like process above the postzygapophysis on caudal vertebrae of the European Eryx a “postzygapophyseal wing” (see also Szyndlar and Schleich 1994); this nomenclature will be followed here. We suspect that these postzygapophyseal wings are not homologous to Soods’s (1941:figs. 2–4) “winglike plate (alar plate), extending upwards and backwards from the middle of the zygapophysial ridge.” Here we state that the postzygapophyseal wings used by Szyndlar (1994) for caudal vertebrae is homologous to the wing-like elaborations above the postzygapophyses observed on trunk vertebrae of Pterygoboa. We also recognize on Pterygoboa the apophysis termed pterapophysis and use the definition of Hoffstetter and Rage (1972). The revised osteological terms as described above are used below and in place of Holman’s (1976a, 1977, 1998, 2000) original terminology. General morphology of species. There are two species described in Pterygoboa, with P. miocenica the genotype (Holman 1976a) and secondarily 2013 J.I. Mead and B.W. Schubert 429 P. delawarensis (Holman 1998) (Fig. 1). Although the genus is distinct among the booids, the two species seem near-identical in morphological description (see below). A large number of vertebrae are assigned to P. miocenica, yet only a single trunk vertebra is known for P. delawarensis. Original diagnoses and descriptions of the species are provided in citations above; revised diagnoses and descriptions for both species are provided by Holman (2000) along with amended osteological terminology discussed below. Pterygoboa miocenica is known from the type locality in the Black Bear Quarry II of the Rosebud Formation, Bennett County, SD (Holman 1976a; Fig. 1), and is of the earliest Hemingfordian NALMA (approximately 19 Ma; Tedford et al. 2004). The snake is also recovered from the Myers Farm fauna, Webster County, NE (Fig. 1) which is middle Barstovian NALMA (approximately 13.5 Ma; Holman 1977, Tedford et al. 2004). Holman (2000) found in analyzing the 76+ trunk and 28 caudal vertebrae from the Myers Farm locality that there is a lot of variation in the overall extent of the development of the postzygapophyseal wing, which he related to intracolumnar variation. Morphological information comes from the revised description of the type, paratype, Figure 1. Map of localities containing Pterygoboa. Specimens identified as P. miocenica: BBQ II (open circle), type locality at Black Bear Quarry II, Rosebud Formation, early Hemingfordian LMA, South Dakota; MF (open circle), Myers Farm Site, middle Barstovian NALMA, Nebraska. Pterygoboa from the Miller local fauna (black square), earliest Hemingfordian NALMA, Florida; Thomas Farm (open triangle) early Hemingfordian NALMA, Florida; White Springs local fauna (asterisk), middle Arikareean NALMA, Florida. The single locality of P. delawarensis: PF (black dot), type locality of at Pollack Farm site, early Hemingfordian NALMA, Delaware. 430 Southeastern Naturalist Vol. 12, No. 2 and other vertebrae assigned to the species, and is based on a minimum of 4 cervical, 79 trunk, and 29 caudal vertebrae. Morphological characters of Holman (2000) include: 1) vertebrae are “much” wider than long, 2) neural arch is moderately depressed (flattened), 3) postzygapophyseal wing is approximately two-thirds the length of the centrum, 4) anterior and posterior ends of postzygapophyseal wings are squared-off to blunt-pointed, 5) synapophyses are slightly divided into dia- and parapophyses but not distinct from one another, 6) deep subcentral grooves are present, and 7) neural spine is higher than it is long along the base, yet there is variation from moderately high to low as shown in Rage (1984), and it is typically swollen dorsally (Holman 1998, 2000). Pterygoboa delawarensis is known only from the Pollack Farm locality, Calvert Formation, Kent County, DE (Fig. 1), and is of the early Hemingfordian NALMA (approximately 18 Ma; Benson 1998). The description is based on a single trunk vertebra (Holman 1998). The following morphological information comes from the revised description of the type and includes: 1) vertebra is slightly wider than long, 2) neural arch is highly depressed (flattened), 3) postzygapophyseal wing relatively short compared to length of centrum (based on illustration of holotype (Holman 1998:fig. 1), 4) postzygapophyseal wings are pointed at both anterior and posterior ends, 5) it is unknown if the synapophyses are divided into dia- and parapophyses, 6) distinct subcentral ridges and grooves present, and 7) the neural spine is low and dorsally swollen (Holman 1998, 2 000). Study Area Faunal remains from Thomas Farm (TF; one of the most systematically excavated and analyzed fossil localities in the southeast US; Fig. 1) indicate that the fossils belong to the early Hemingfordian NALMA, dating about 19–18 Ma (specifically sub-event He1; R. Hulbert, Florida Museum of Natural History, Gainsville, FL, February 2012 pers. comm.). Located in Gilchrist County, the deposit is localized, and most of the layers cannot be traced laterally over a distance greater than 20 m (Pratt 1990). Geologic features preserved at the locality imply that the site was a large sinkhole (see Pratt 1990:fig. 15), with the recovered fauna representing an autochthonous assemblage. Faunal remains entered the sinkhole 1) as victims of a natural trap scenario, 2) as skeletal elements that accumulated on the surface surrounding the sinkhole and subsequently washed in, 3) as skeletal remains on talus slopes along the walls of the sinkhole such as from roosting behavior, and/or 4) as skeletal remains from animals that inhabited a cave (Morgan and Czaplewski 2003, Pratt 1990). Modification of bone by agents such as weathering or carnivores was not a significant factor (Pratt 1990). The amount, if any, of raptor activity and how it may have influenced the microfaunal species in the deposit is not known at this time. Pratt (1990) suggests that the environment around the sinkhole was decidedly tropical and forested rather than open terrain. The Miller Local Fauna (MLF) contains an extensive diversity of vertebrates, but to date most fossil groups have not been studied in any detail. Specimens 2013 J.I. Mead and B.W. Schubert 431 were collected from the Suwannee River in Dixie County, FL (Fig. 1). Recovered borophagine canid, hesperocyonine canid, and undescribed mustelids support an age assignment of early Hemingfordian NALMA (≈20 Ma, about 1–2 million years older than TF; Baskin 2003; R. Hulbert, May 2011 pers. comm.). The White Springs 3B local fauna (WS) occurs in sediments of the Parachucla Formation exposed along the banks of the Suwannee River near the town of White Springs, Columbia County, FL (Fig. 1.). The local fauna contains nearshore marine and terrestrial vertebrates (MacFadden and Morgan 2003, Morgan 1989). Biochronological placement here follows Albright et al. (2008) and Tedford et al. (2004) in placing WS in middle Arikareean (Ar3) NALMA of the latest Oligocene, 25–24 Ma. Methods All Florida specimens are cataloged into the collections of the Florida Museum of Natural History, University of Florida, Gainesville (UF). Eight vertebrae were recovered from MLF: 3 nearly complete trunk (UF 268750, 268751, 272799), 2 complete caudal (UF 268754, 272797), and 3 fragmented trunk (UF 268752, 268753, 272798). Six trunk vertebrae were recovered from TF (UF 268702-268705, 271969) and 1 juvenile caudal (UF 271968). Seventeen trunk vertebrae (of a single individual; UF 125435) were recovered from WS. Terminology and measurements of vertebrae follow Auffenberg (1963), Hoffstetter and Gasc (1969), LaDuke (1991), and Szyndlar (1994). Only a few of the vertebrae listed above will be described in detail. Results MLF trunk vertebra UF 272799 (Fig. 2A, B), the most complete trunk vertebra described here, is 2.2 mm in centrum length. The neural arch width is 2.1 mm, and is highly flattened in anterior view and slightly arched in posterior view. The neural spine is complete (1.5 mm long), showing a slightly swollen and flattened crest (but not to the extent of a neural plate [Sood 1941] or as observed on many snakes species; e.g., see Bogert 1964, Szyndlar and Rage 2003) and overhangs posterior to the neural arch. Immediately below the neural spine apex and separate from the crest are two accessory spines that develop at the posterior end of the neural spine and run the length of the crest, diverging slightly laterally (Fig. 2A, B). Postzygapophyseal wings are distinct, sweeping dorsally and form ridges that are square to blunt-pointed at both ends, 1.3 mm long. Prezygapophysis processes are extremely reduced to protruding barely beyond the edge of the prezygapophysis. The haemal keel is slight and minutely projecting; there are two subcentral foramina. Two pterapophyses are observed. MLF trunk vertebra UF 268750 is 2.1 mm in centrum length. The neural arch width is 1.9 mm. The neural arch is highly flattened in anterior view and only slightly arched in posterior view. The neural spine is damaged, which allows no information of its structure. The neural arch posterior 432 Southeastern Naturalist Vol. 12, No. 2 Figure 2. Trunk and caudal vertebrae identified as Pterygoboa from Florida. A) dorsal and B) anterior: Miller Local Fauna, trunk vertebra, UF 272799. C) dorsal and D) anterior: Thomas Farm, trunk vertebra, UF 271969. E) dorsal and F) anterior: Miller Local Fauna, caudal vertebra, UF 268754. G) dorsal and H) anterior: Miller Local Fauna, caudal vertebra, UF 272797. Abbreviations: h = haemapophysis, np = neural plate at crest of neural spine, ns = neural spline, pl = pleurapophysis as blade, po = postzygapophysis, pow = postzygapophysis wing, pt = pterapophysis, prp = prezygapophysis process. Scale bars equal 1 mm; A–F same scale; G–H same scale. 2013 J.I. Mead and B.W. Schubert 433 notch is U-shaped as viewed dorsally. A postzygapophyseal wing on the postzygapophysis is a dorsally oriented blade, straight-topped and 1.1 mm long with square-blunt ends. The prezygapophyses are slightly dorsally oriented, with the surface obovate in shape. There is slight evidence of a prezygapophysis process. Synapophyses (paradiapophyses) are present but damaged; thus, no indication of a parapophysis projection. The haemel keel is an arched, slightly projecting keel oriented the full length of the centrum. There are two subcentral foramina in addition to two minute pterapophyses positioned closer to the postzygapophyses than to the neural spine. MLF trunk vertebra UF 268751 has a centrum length of 1.8 mm; the neural arch width is 1.7 mm. The neural spine is flattened in anterior view and slightly arched in posterior view. The neural spine is ≈0.7 mm high, slightly swollen at the crest, and does not protrude (overhang) posteriorly to the neural arch. The neural arch posterior notch is U-shaped as viewed dorsally. The neural spine is 0.8 mm long at the base (45% of the centrum length) and low in height, with evidence of slight swelling at the crest. The prezygapophysis is slightly dorsally oriented with an obovate shape. There is no evidence of a prezygapophysis process. The postzygapophyseal wing is dorsally oriented, straight-topped, and 0.9 mm long with blunt-pointed ends. The synapophysis is present with slight evidence of a division into a diapophysis and a parapophysis; there is a slight parapophysis projection. A paracotylar notch is present, but there is no evidence of a paracotylar foramen. The haemel keel is arched, slightly projecting, and oriented most the length of the centrum length. There are two pin-point subcentral foramina. There is no evidence of pterapophyses, which we assume may indicate that the vertebra is more anteriorly positioned along the vertebral column. MLF caudal (postcloacal) vertebra UF 268754 (Fig. 2E, F) has a centrum length of 1.8 mm and a neural arch width of 1.8 mm. The neural arch is highly flattened in anterior view and only slightly arched in posterior view. The neural spine length is 1.3 mm, or about 72% the centrum length, and is situated along the posterior portion of the neural arch. The neural spine is relatively low, expanded dorsally, highly bifurcated at the apex, and has minute accessory apophyses immediately ventral to the apex (Fig. 2E, F). The prezygapophysis is dorsally directed with a minute prezygapophysis process that projects anteriorly. The postzygapophysis wing combines with the postzygapophysis to form a dorsally directed blade that is 1.2 mm long (67% of the centrum length) and blunt-pointed at both ends. Pleurapophyses occur ventral to prezygapophyses and are large expansions that are laterally directed. The area of the haemal keel is wide, short, mostly flattened, and shows incipient morphing into a haemapophysis. Pterapophyses are situated between the neural spine and postzygapophyseal wings and are oriented toward the anterior. MLF caudal vertebra UF 272797 (Fig. 2G, H) has a centrum length of 1.4 mm and a neural arch width of 1.4 mm. The vertebra is slightly incomplete due to etching along the anterior edge of the neural arch and zygosphene. The neural spine occupies most of the length of the neural arch and splays out at the apex 434 Southeastern Naturalist Vol. 12, No. 2 forming a neural plate (Sood 1941) with a shallow groove running the length. A V-notch occurs at both ends of the neural plate. Postzygapophyseal wings are 1.1 mm long with blunt-pointed ends. Two paracotylar foramina are present. Pleurapophyses form blades. The prezygapophysis process is positioned well ventral to the angled prezygapophyses. The hypapophysis is bulbous at the apex, with incipient haemapophyses forming. Trunk vertebra UF 271969 (Fig. 2C, D) and others from TF have centrum lengths ranging from 1.8 to 2.2 mm; neural arch widths range from 1.8 to 2.2 mm. All vertebrae from TF show relatively short postzygapophysis wings, with the one on UF 268705 being complete and 0.5 mm long. Those on UF 271969 are incomplete but are approximately 1.0 mm long and have one complete anterior end and one complete posterior end with both ends blunt-pointed. Two vertebrae (UF 268703, 268704) have pterapophyses on the neural arch between the neural spine and the postzygapophysis wings indicating that the vertebrae are probably more caudally positioned. UF 271968 is a juvenile caudal vertebra (based on size of the neural canal relative to the rest of the vertebra) with small but clearly developed postzygapophysis wings. The 17 trunk vertebrae recovered from WS are highly fragmented but are all about the same overall size. Characters of the vertebrae and the pattern to the breakage imply that the vertebrae are from the same individual. All vertebrae show some preservation of a postzygapophyseal wing. Two vertebrae have a relatively high neural spine that occupies the posterior portion of the neural arch; one has the conspicuous swollen crest. Pterapophyses occur on some of the vertebrae along the neural arch. Discussion and Conclusions Here we present the first records of the erycine boid Pterygoboa from the North American Gulf Coast region (Auffenberg 1963, Holman 2000). Previous records indicate that the genus is known from the early Miocene (early Hemingfordian NALMA and middle Barstovian NALMA; ≈20.0 Ma–13.5 Ma) fossil deposits in South Dakota, Nebraska, and Delaware (Fig. 1). This distribution makes the MLF and TF records in Florida presented here of biogeographic importance in showing a southern distribution at the same time the taxon occurred further north and west. Vertebrae from WS indicate that Pterygoboa was known in at least the Florida region as early as the middle Arikareean NALMA, latest Oligocene, which extends the earliest age for the taxon back another 4–5 million years to about 24–25 Ma. These three new records from Florida illustrate that the geographic distribution and chronological extent of Pterygoboa was far greater, more southern in extent, and older than previously known (Fig. 1). The morphological characters observed on the 8 vertebrae from the MLF, the 7 vertebrae from TF, and the 17 vertebrae from WS exhibit many characters found only on Pterygoboa miocenica and P. delawarensis. Vertebrae from MLF, TF, and WS are characteristically longer than wide with only about 0.1 2013 J.I. Mead and B.W. Schubert 435 mm difference; whereas, most Pterygoboa miocenica vertebrae and the single vertebra from P. delawarensis are slightly wider than long. MLF, TF, and WS vertebrae all exhibit a neural arch that is highly to well-flattened in anterior view; this characteristic is similar to both P. miocenica and P. delawarensis. The neural spine, when preserved, is typically low with some evidence of swelling at the crest to the extent of developing a flattened neural plate; this morphology is similar to the observed range on both P. miocenica and P. delawarensis. The postzygapophyseal wings on MLF, TF, and WS vertebrae are not all preserved but those that are have more blunt-pointed to square-blunt anterior and posterior ends. This character is one of the major distinctions noted between P. miocenica, with its blunt-ended postzygapophyseal wings, and, P. delawarensis with its pointed ends (Holman 1998). In contrast, Rage (1984) illustrates a trunk vertebra of P. miocenica from the Myers Farm Site that does exhibit more pointed ends to the postzygapophyseal wings. We infer that this character of blunt ends versus pointed ends to the postzygapophyseal wings is variable (as stated in Holman 2000) and in likelihood should not be used to distinguish between the two species. The postzygapophyseal wings on Pterygoboa trunk vertebrae are variable in length as compared to the centrum length, which we infer depends on the position within the vertebral column. Those that are more mid-trunk length (MLF UF 268751, P. delawarensis holotype, Myers Site P. miocenica) are typically shorter. The postzygapophyseal wings on more caudally situated and caudal vertebrae exhibit wider, more robust, longer, and more blunt-pointed ends. Three (including the caudal vertebra) of the eight vertebrae from the MLF, two from TF, and two from WS exhibit pterapophyses. We infer that the presence of these represent trunk vertebrae that are more caudally positioned and not mid-trunk or cervical in position. These pterapophyses have not been mentioned in previous descriptions of Pterygoboa (Holman 1976a, 1998, 2000; Rage 1984; although figured by Holman 1998:fig. 1A) and clearly demonstrate that Pterygoboa is indeed a member of the Erycinae (sensu Szyndlar and Rage 2003). The morphology of the type, P. miocenica, is stated to be clearly understood (Holman 2000), with variations fairly well delineated based on many specimens from the type site. However, only a single specimen is known for P. delawarensis. In our opinion, its morphological characters seem to blend into the amount of variation observed on P. miocenica rendering the specific status of P. delawarensis questionable. Consequently we feel that Pterygoboa specimens described here from Florida could either represent a large geographic and/or temporal range extension for P. miocenica, or, a separate species of Pterygoboa with similar vertebral morphology. Future recovery of cranial elements might help resolve this issue. Pterygoboa was found associated with other early Hemingfordian-age extinct erycine boids (Ogmophis miocampactus Holman, Calamagras weigeli Holman, Charina prebottae Brattstrom) and extinct colubrines at the Black Bear Quarry II type locality (Brattstrom 1958; Holman 1972, 1976a). At the Pollack Farm locality of equivalent age, Pterygoboa was recovered with 436 Southeastern Naturalist Vol. 12, No. 2 the erycine Calamagras, colubrines (Ameiseophis robinsoni Holman, Pollackophis depressus Holman), a viperid (Viperidae), along with the anguid lizard (Ophisaurus) and a possible Crocodylus (crocodilian) (Holman 1976c, 1998). The snake fauna from the MLF is just now being analyzed and nothing is reconstructed regarding the early Hemingfordian environmental setting. Of near-equivalent age to MLF is the well-known TF locality, an extremely rich and diverse vertebrate assemblage. Snakes from TF include the erycine (Calamagras floridanus Auffenberg), boine (Pseudoepicrates stanolseni Vanzolini), and colubrines (Paraoxybelis floridanus Auffenberg, Pseudocemophora antiqua Auffenberg) (Auffenberg 1963, Vanzolini 1952). With the report here, Pterygoboa is now known from TF. By the middle Barstovian NALMA at Myers Farm locality, Pterygoboa was found with fewer other erycine snakes (Geringophis depressus Holman) but with multiple species of colubrids, elapids (Elapidae), and possibly a viperid (Holman 1976b, see list in Holman 1977). Clearly much remains to be studied from the MLF, TF, and WS localities as well as other well-known fossil sites where the minute squamates have not received sufficient analytical attention. Acknowledgments We thank Andreas Kerner for bringing the Miller local faunal fossil remains to our attention and their donation to the Florida Museum of Natural History. Andreas Kerner, Sandra L. Swift, and Kevin Chovanec are greatly appreciated for sorting bones from sediments. Richard Hulbert is thanked for the loan of the fossils from the Florida Museum of Natural History and discussions about the Thomas Farm locality. We greatly appreciate Sandra L. Swift for making the illustrations. Partial funding support for this project was received from the Department of Geosciences and the Don Sundquist Center of Excellence in Paleontology, East Tennessee State University. Zbigniew Szyndlar and Dennis Parmley facilitated immensely in helping us to fully understand the complexities of erycine osteology and for edits to our manuscript. Literature Cited Albright, L.B., M.O.Woodburne, T.J. Fremd, C.C. Swisher, B.J. MacFadden, and G.R. Scott. 2008. Revised chronostratigraphy and biostratigraphy of the John Dave Formation (Turtle Cove and Kimberly Members), Oregon, with implications for updated calibration of the Arikareean North American Land Mammal Age. Journal of Geology 116:211–237. 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