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Size and age class estimates of North American Eocene palaeopheid snakes.

ABSTRACT

In North America, two genera (Palaeophis and Pterosphenus) and six species of palaeopheid snakes are known from Gulf Coast and Atlantic coastal Eocene marine or marine-influenced sediments. They were specialized aquatic snakes that today are known almost exclusively by isolated vertebrae. Using vertebrae of living boids (Boidae: Boinae) of known lengths and age classes as models, lengths and age classes of the six North America palaeopheid species were estimated. In some cases body morphologies were suggested. As adults, the largest species were Palaeophis grandis and Pterosphenus schucherti, each at about 5.1 m (17 ft), while the smallest species was Palaeophis casei at about 1.3 m (4.3 ft). For the first time, adult, juvenile, and neonate size classes are identified for Palaeophis virginianus.

Key words: palaeopheid snakes, Eocene, sizes, age classes, North America.

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INTRODUCTION

Palaeopheidae is a specialized family of extinct aquatic snakes that is divided into two subfamilies, the Palaeopheinae Lydekker and the Archaeopheinae Janensch (1). Of these, only the more common Palaeopheinae snakes are known in North America. Two genera of Palaeopheinae snakes currently are recognized, Palaeophis Owen and the more derived genus Pterosphenus Lucas (herein abbreviated Pt.), both of which are known almost exclusively by isolated vertebrae from Gulf Coast and Atlantic coastal Eocene sediments (1, 2, 3, 4). These snakes inhabited paleocoastal marine or marine influenced habitats (1, 2, 3, 4, 5, 6, 7), and their vertebrae reflect modifications of their adaptation to an aquatic life (e.g., some degree of lateral compression; well-developed pterapophyses; reduced prezygapophyses, high neural spines; and low set paradiapophyses for rib attachment; see 3, 4 and references within). While the origin of palaeopheid snakes dates back to the late Cretaceous (Masstrichtian) of Africa (8), Palaeophis and Pterosphenus mainly are known from Old World and New World Eocene rocks (1, 2, 4, 5). Both genera were extinct by the end of the Eocene (1, 4).

Despite the fact that palaeopheid snakes have been known to science for over 160 years (e.g., 9), little is known about them. On the basis of shared vertebral characters, Palaeopheidae usually is placed in the same superfamily as living boas and pythons (Booidea; 4). Hoffstetter (10) considered them to be closest to Boidae. Thus, vertebrae of living boids offer the closest model to a generalized palaeopheid vertebral morphology. Recently some workers have speculated on aspects of their skeletal and functional morphology, population structure, habitat requirements, reproductive modes, and feeding habits (e.g., 2, 3, 4, 11, 12, 13). Interestingly, the vertebrae of some species are quite large and much has been mentioned in the literature about the total lengths these snakes may have obtained (e.g., 3, 6, 11, 14), and on occasion they have been referred to as "giant snakes" (3, 6, 15). Conversely, some taxa had small vertebrae and were comparatively small snakes (e.g., 3, 11). It is this size issue that we address here. Using living boids (sensu stricto boas and pythons of the subfamily Boinae) as models, we estimated body lengths of the six North American species of palaeopheid snakes. Vertebral sizes and morphologies seen in living boids of known age classes (see Methods and Materials section) also allowed us to differentiate palaeopheid age classes and in some cases, speculate on body morphologies.

METHODS AND MATERIALS

In an effort to speculate on the body lengths palaeopheid snakes may have obtained, other workers have directly compared their vertebrae to similar sized vertebrae of living snakes (especially boids) of known total lengths (e.g., 3, 11, 14). This suggests that a size relationship exists between total body length (TL) and vertebral centrum length (CL) of a snake (also see Christman, 16). In order to test this, we measured centrum lengths of 20 vertebrae each from skeletons of 16 adult Recent boas and pythons (Boidae: Boinae) of known total lengths (Table I). Only trunk vertebrae of Boinae taxa were measured as they are most like palaeopheid vertebrae in their general morphology, especially in centrum structure. Anterior cervical and precloacal (vertebrae lacking ventrolateral process) vertebrae are unlike the more numerous trunk vertebrae in that they are generally shorter with higher neural arches and smaller condyles. We excluded taxa of the subfamilies Erycinae and Tropidopheinae (e.g., Eryx, Charina, Lichanura, and Tropidophis) as trunk vertebrae of these small to medium-sized boids are unlike those of palaeopheid snakes in several features. For example, they differ from those of palaeopheid snakes in having shorter, wider centra and differently shaped neural arches. In an effort to minimize bias and sampling error, all vertebral measurements were made by the same individual using the same set of digital calipers for all measurements. These data were then subjected to regression analysis using GraphPad InStat[TM], vers. 3 software, which showed that total length is a linear function of centrum length ([r.sup.2]=0.8934). The regression of CL values (see Table 1) on TL produced a robust linear regression model ([r.sup.2]=0.9645; Fig. 1), which may (at least in part) be accounted for by (1) the geometrically simple body plan of snakes, (2) the conservative vertebral number in the species we utilized (see Table 1), and (3) the use of conservative (mean) CL values. Departure from linearity (variance about the line) was likely caused by two main sources of error: (1) the difficulties of accurately measuring the total lengths of large Recent boids prior to skeletonization; and (2) some degree of intravariability of centrum lengths along a vertebral column. Palaeopheid lengths were then estimated by using CL as an estimate of TL (Appendix I). It should be noted that while a boid-to-palaeopheid size comparison on the basis of centrum length is not a perfect match, we believe it offers reasonable estimates of the lengths these extinct snakes obtained, and, as previously noted, no other living group of snakes compare as well in vertebral morphology (especially in centrum structure) to the Palaeopheinae snakes than do living boids.

[FIGURE 1 OMITTED]

As much as possible, the same precautions were taken for measuring fossil palaeopheid vertebrae. However, it was not always possible to determine if a fossil represented a posterior trunk or a precloacal vertebra. Measurements of palaeopheid vertebrae were taken from fossils examined during the course of the study as well as from those published in the literature (see Appendix I). Data were available for six vertebrae of Palaeophis casei, three of P. africanus, two of P. grandis, four of P. littoralis, 32 of P. virginianus, and 37 of Pterosphenus schucherti (Appendix I). Based on this information and vertebral characters of living boids in general, we estimated lengths (shortest and longest) for age classes of each North American palaeopheid taxon (Appendix I). Age classes are here defined broadly as: 1) neonatal, recently hatched or born, probably less than one year old; 2) juvenile, "youthful," probably not sexually mature; and 3) adult, reproductively mature. More specifically, vertebral characteristics that identify the age classes are (see Fig. 2): neonates, vertebrae small with round neural canals that are larger than the condyles, and thin vertebral features, especially the neural arch buttresses (bone area that connects the neural arch proper and the centrum) and the zygosphenes; juveniles, vertebrae larger with larger and more arched neural canals that range from about the size of the condyles to slightly smaller, and thicker neural arch buttresses and zygosphenes; and adults, vertebrae largest with arched neural canals that are much smaller than the condyles, and thick, robust neural arch buttresses and zygosphenes.

[FIGURE 2 OMITTED]

SYSTEMATIC ACCOUNTS

Superfamily Booidea Gray, 1825

Family Palaeopheidae Lydekker, 1888

Subfamily Palaeopheinae Lydekker, 1888

Genus Palaeophis Owen, 1841

Palaeophis casei Holman, 1982

Palaeophis casei presently is known only from two Early Eocene sites in North America, the type locality in Lauderdale County, Mississippi (11, 2) and the Fisher/Sullivan site of Virginia (13). It was a small palaeopheid snake that probably inhabited nearshore estuarine habitat (4), and at present, only vertebrae of adult snakes are known. Holman et al. (3) estimated its size at about 508 mm (1.7 ft). We estimated a larger adult size for the species, ranging from 0.99-1.3 m (3.3-4.3 ft).

Palaeophis africanus Andrews, 1924

This species was previously known only from the Middle Eocene of Nigeria (17), but recently it has been discovered in the Late Eocene Hardie Mine local fauna of Georgia (ca. 36-34.2 mybp; Parmley and Holman, 18, Parmley and DeVore, ms in review). It is known only by vertebrae from adult individuals. Rage (1) considered it to be "morphologically intermediate between the primitive and advanced species" of Palaeophis, the idea being that the vertebrae of primitive species are not greatly modified by aquatic habits, while those of advanced species are. In terms of size, P. africanus appears to have been a moderately-sized snake, ranging from 2.6-3.6 m (8.5-11.7 ft).

Palaeophis grandis (Marsh, 1869)

This species is known by two large vertebrae from adult individuals, one (holotype) from the Middle Eocene of New Jersey (15) and one from the Eocene (undesignated) of Maryland (19, Appendix I). While the validity of P. grandis has been questioned (1), Holman (4) offered a revised diagnosis of the taxon. In terms of the size it may have obtained, Marsh (15) suggested a length of "not less than thirty feet" for the species. Holman (4) estimated its length at about 6.4 m (21 ft). We estimated a smaller size for the species of 3.7-5.1 m (12.2-16.8 ft).

Palaeophis littoralis Cope, 1868

This species is known from the Early Eocene of Mississippi (2) and Middle Eocene of New Jersey (20, 21), which suggests its paleodistribution was considerably more extensive than indicated by the fossil record. It was a comparatively small species. Holman et al. (3) estimated its adult length at about 1 m (3.3 ft), while we estimated a larger size of 1.6-2.0 m (5.2-6.4 ft).

Palaeophis virginianus Lynn, 1934

Holman and Case (14) reported two size classes of this species: juveniles at 0.53 m (ca. 1.8 ft); and large adults at 5.2 m (17 ft). A diverse size-series collection of P. virginianus vertebrae allowed us to identify three age classes: neonatal; juvenile; and adult (see Fig. 2). Vertebrae of neonate P. virginianus are small (CL ca. 2.0-4.5 mm), those of juveniles are larger (CL ca. 6.0-10.0 mm), while those of adults are the largest (CL ca. [greater than or equal to] 11.0 mm; see Appendix I). While the vertebrae of neonate and adult P. virginianus are easily differentiated, the gradation between juvenile and adult is not always well defined. Consequently, some of the P. virginianus vertebrae we assigned to the juvenile age class (Appendix I) may actually represent young adult snakes. Nonetheless, our model suggests adults ranged from 2.2 to 4.3 m (7.3-14.2 ft), juveniles from 1.5 to 2.1 m (4.9-6.8 ft), and neonates from 0.97 to 1.2 m (3.2-4.0 ft).

Compared with other North American palaeopheids, the vertebrae of P. virginianus are quite boid-like in several features and are only slightly modified by aquatic life (1). They are weakly laterally compressed, and have short pterapophyses and relatively high-set paradiapophyses. Based on these characters and the overall large size and robustness of its vertebrae, one can envision a large, heavy bodied snake somewhat similar to the Recent green anaconda (Eunectes murinus) in size, physical build, and perhaps in habits. It seems unlikely that a snake of this size and mass would have inhabited open seas, but rather it may have been an opportunistic nearshore lie-in-wait predator similar to the green anaconda of today (22).

Genus Pterosphenus Lucas, 1899

Pterosphenus schucherti Lucas, 1899

Pterosphenus schucherti is the most commonly reported and widely distributed palaeopheid snake in North America. It is known from Middle to Late Eocene paleocoastal deposits in North Carolina (J. Knight, pers. com.), Florida, Georgia, Alabama, Louisiana, Mississippi, Arkansas, Texas, and New Jersey (see 4, 23, 24, 25). Parmley and Case (2) reported this species from the Early Eocene Yazoo Clay of Louisiana, but this was in error as the Yazoo Clay is Late Eocene in age.

In his original description of Pt. schucherti, Lucas (26) estimated a size of ca. 6.1-7.6 m (20-25 ft) for the species. Based on a large vertebra (length through the zygapophyses 26.3 mm) from the Late Eocene Moodys Branch Formation of Mississippi, Holman et al. (3) suggested the species may have reached a length of about 5.5 m (18 ft). Westgate (6) estimated that the species may have exceeded 6.1 m (20 ft) in length. In a diverse collection of Pt. schucherti vertebrae (n=37, Appendix I), we were able to differentiate juvenile and adult individuals. On this basis, four vertebrae recently collected from the Late Eocene Hardie Mine (7) of Wilkinson County, Georgia (CL 6-10 mm: Parmley, pers. obs.), two from the Late Eocene Twiggs Clay of Georgia (CL 6.7 and 9.9 mm; 2), one from the Late Eocene Cow Creek local fauna of St. Francis County, Arkansas (CL 9.2 mm; 25), and one from the Late Eocene of La Salle Parish of Louisiana (CL 6.2 mm; 26) represent juvenile snakes. Judging from these vertebrae (n=8), juvenile Pt. schucherti ranged from 1.5 to 2.1 m (4.9-6.9 ft). Vertebrae of adult (CL ca. [greater than or equal to] 10 mm; Appendix I) Pt. schucherti suggest the species ranged from 2.3 to 5.1 m (7.6-16.4 ft) in length. Westgate (5) reported a Pt. schucherti vertebra from the middle Eocene of Texas with a CL of only 1.8 mm, suggesting it was that of a juvenile snake. While we did not examine the specimen, its size suggests it represents a neonatal snake with a TL of about 0.84 m (2.7 ft).

Compared with other palaeopheid snakes, the vertebrae of Pt. schucherti show the strongest modifications for an aquatic existence. Whole-body reconstructions of this species often depict it with a compressed paddle-like tail as seen in Recent sea snakes of the family Hydrophiidae (e.g., 3, 4, 6). But there is no vertebral evidence to suggest that this type of tail morphology occurred in this or any palaeopheid species. In living sea snakes, which have strongly compressed paddle-like tails, the caudal vertebrae are modified in their morphology (usually by strong lateral compression and often tall, narrow neural spines and/or compressed ventrolateral processes that project ventrally rather than laterally) and articulation (strongly overlapping neural arches to give rigidity to the tail region). Caudal vertebrae of other Recent snakes that also are highly aquatic (e.g., Farancia) exhibit none of these modifications, but rather are like those of many terrestrial snakes. Very little is known about the morphology of palaeopheid caudal (postcloacal) vertebrae (2), but two recently discovered caudal vertebrae of Pt. schucherti from Late Eocene rocks in Georgia, one of which appears to be a distal caudal (pers. obs., Parmley), are not any more laterally compressed or structurally modified than those of the "neck" or precaudal region of the vertebral column (pers. obs., Parmley). Therefore, they do not exhibit characters that clearly indicate a paddle-like tail was present in the living animal. Nonetheless, Pt. schucherti probably was the most aquatic of all the palaeopheid snakes. In general, its vertebrae are highly modified for an aquatic existence, including lateral compression, tall pterapophyses, high neural spines, and low paradiapophyses that supported weakly curved ribs. Of these characters, lateral compression, low paradiapophyses, and weakly curved ribs indicate Pterosphenus had a laterally compressed body that would have been advantageous for swimming. Hutchison (12) pointed out that Pt. schucherti had pneumocele-like marrow cavities in its vertebrae, which also would have been advantageous for an aquatic existence. In fact, he suggested that Pt. schucherti may have been so modified in its morphology for an aquatic life that it may not have been able to crawl on land.

DISCUSSION

Our results suggest that adult North American palaeopheid snakes can be grouped into four general size assemblages as follows (Fig. 3):

i) 5.0-5.1 m (ca. 16-17 ft): This size-class is comprised of the largest North American palaeopheid snakes, Pterosphenus schucherti and Palaeophis grandis.

ii) 3.6-4.4 m (ca. 12-14.2 ft): This size-class is comprised of two moderately large species, Palaeophis africanus and Palaeophis virginianus. Palaeophis africanus may have been the smaller of the two species by about a meter, but only a few vertebrae of this taxon are known.

iii) 1.6-2.0 m (ca. 6-7 ft): This size-class is comprised of one relatively small species, Palaeophis littoralis, which may have co-existed with the diminutive species P. casei in Early Eocene nearshore estuarine habitats of Mississippi.

iv) 1.0-1.3 m (ca. [less than or equal to] 4.3 ft): This size-class is comprised of the diminutive species Palaeophis casei. It was the smallest species of palaeopheid snake, obtaining a size comparable to some living sea snakes and freshwater natricine species.

[FIGURE 3 OMITTED]
Appendix I. Size estimates of paleopheid snakes. Abbreviations are:
centrum length, CL; total length, TL; adult, Ad; juvenile, Juv; and
neonate, Neo. References for specimens: personal observations and 2, 5,
14, 17, 19, 20, 23, 24, 25, 27, 28.

Taxon CL Estimated TL Age
 (mm) Meters--Feet Class

Pterosphenus schucherti 27.70 4.9-16.1 Ad
P. schucherti 11.20 2.3-7.6 Ad
P. schucherti 18.10 3.4-11.2 Ad
P. schucherti 20.00 3.7-12.1 Ad
P. schucherti 15.00 2.9-9.6 Ad
P. schucherti 19.70 3.7-12.0 Ad
P. schucherti 11.90 2.4-8.0 Ad
P. schucherti 19.50 3.6-11.9 Ad
P. schucherti 18.50 3.5-11.4 Ad
P. schucherti 28.8 5.1-16.7 Ad
P. schucherti 28.9 5.1-16.7 Ad
P. schucherti 13.6 2.7-8.8 Ad
P. schucherti 21.4 3.9-12.9 Ad
P. schucherti 19.2 3.6-11.7 Ad
P. schucherti 20.3 3.8-12.3 Ad
P. schucherti 16.4 3.1-10.3 Ad
P. schucherti 18.5 3.4-11.4 Ad
P. schucherti 19.5 3.6-11.9 Ad
P. schucherti 11.9 2.4-8.0 Ad
P. schucherti 26.30 4.7-15.4 Ad
P. schucherti 22.12 4.0-13.2 Ad
P. schucherti 21.04 3.9-12.7 Ad
P. schucherti 18.60 3.5-11.4 Ad
P. schucherti 18.90 3.5-11.6 Ad
P. schucherti 16.83 3.2-10.5 Ad
P. schucherti 14.09 2.8-9.1 Ad
P. schucherti 11.65 2.4-7.8 Ad
P. schucherti 19.7 3.7-12.0 Ad
P. schucherti 24.0 4.3-14.2 Ad
P. schucherti 16.5 3.2-10.3 Ad
P. schucherti 8.8 1.9-6.4 Juv
P. schucherti 7.8 1.8-5.8 Juv
P. schucherti 6.2 1.5-5.0 Juv
P. schucherti 9.88 2.1-6.9 Juv
P. schucherti 9.09 2.0-6.5 Juv
P. schucherti 9.90 2.1-6.9 Juv
P. schucherti 6.70 1.6-5.3 Juv
P. schucherti 9.20 2.0-6.5 Juv
P. schucherti 1.80 0.84-2.7 Neo
Palaeophis africanus 12.72 2.6-8.5 Ad
P. africanus 17.52 3.3-10.9 Ad
P. africanus 19.20 3.6-11.7 Ad
Palaeophis virginianus 18.5 3.5-11.4 Ad
P. virginianus 24.0 4.3-14.2 Ad
P. virginianus 20.0 3.7-12.1 Ad
P. virginianus 14.6 2.9-9.3 Ad
P. virginianus 18.5 3.5-11.4 Ad
P. virginianus 20.8 3.8-12.5 Ad
P. virginianus 22.0 4.0-13.2 Ad
P. virginianus 20.1 3.7-12.2 Ad
P. virginianus 19.0 3.5-11.6 Ad
P. virginianus 15.6 3.0-9.9 Ad
P. virginianus 15.6 3.0-9.9 Ad
P. virginianus 13.1 2.6-8.6 Ad
P. virginianus 10.7 2.2-7.3 Ad
P. virginianus 11.8 2.4-7.9 Ad
P. virginianus 10.5 2.2-7.2 Ad
P. virginianus 14.0 2.8-9.0 Ad
P. virginianus 11.3 2.3-7.6 Ad
P. virginianus 12.4 2.5-8.2 Ad
P. virginianus 10.8 2.3-7.4 Ad
P. virginianus 11.0 2.3-7.5 Ad
P. virginianus 12.0 2.4-8.0 Ad
P. virginianus 12.9 2.6-8.5 Ad
P. virginianus 7.9 1.8-5.9 Juv
P. virginianus 8.2 1.8-6.0 Juv
P. virginianus 6.4 1.6-5.1 Juv
P. virginianus 7.5 1.7-5.7 Juv
P. virginianus 6.0 1.5-4.9 Juv
P. virginianus 9.6 2.1-6.8 Juv
P. virginianus 6.6 1.6-5.2 Juv
P. virginianus 3.60 1.1-3.7 Neo
P. virginianus 4.30 1.2-4.0 Neo
P. virginianus 2.84 1.0-3.3 Neo
Palaeophis littoralis 8.3 1.9-6.1 Ad
P. littoralis 8.8 2.0-6.4 Ad
P. littoralis 8.5 1.9-6.1 Ad
P. littoralis 6.7 1.6-5.3 Ad
Palaeophis casei 3.9 1.2-3.8 Ad
P. casei 4.5 1.3-4.1 Ad
P. casei 2.8 0.99-3.3 Ad
P. casei 3.3 1.1-3.5 Ad
P. casei 3.5 1.1-3.6 Ad
P. casei 4.0 1.2-3.9 Ad
Palaeophis grandis 29.0 5.1-16.8 Ad
P. grandis 20.2 3.7-12.2 Ad

Table I. Actual and estimated total lengths (TL) of 16 Recent boids:
estimated TLs based on centrum lengths (CL; 20 trunk vertebrae measured
per specimen). Catalog numbers refer to the Recent herpetological
skeletal collections of Georgia College & State University, and all
measurements are in millimeters. Abbreviations include: catalog number,
CAT#; total number of vertebrae, #V (see text).

Taxon CAT# #V Actual TL CL Estimated %Error

Python molorus 1169 324 2450 12.0 2440 0.67
P. molorus 4458 328 3100 16.2 3100 0.00
P. molorus 4460 326 2700 13.5 2676 0.92
Boa constrictor 2249 302 2500 13.5 2676 0.92
B. constrictor 4459 310 2800 14.8 2880 0.00
Liasis boeleni 3867 361 2150 9.3 2015 0.09
L. boeleni 4225 363 2075 9.9 2110 0.24
L. boeleni 3962 360 2550 12.6 2534 0.77
Liasis boa 3822 318 1500 6.6 1591 -0.80
L. boa 3921 322 1510 6.8 1622 -0.72
Liasis albertisii 3862 347 1885 8.3 1858 -0.19
Morelia spilota 3965 371 1960 7.8 1780 -0.35
Epicrates striatus 1188 355 1600 7.3 1701 -0.53
E. striatus 1246 367 1740 6.8 1622 -0.72
E. striatus 1293 348 1495 6.2 1528 -0.97
Aspidites melanocephalus 3855 385 1900 7.8 1780 -0.35


ACKNOWLEDGMENTS

This project was funded in part by the Biological and Environmental Sciences Department and Faculty Research Grants awarded to the senior author by the Office of Research Services, Georgia College & State University. We thank C. Bufford and S. Sawitsky for illustrations. We also thank Laura Abraczinskas of Michigan State University for the loan of palaeopheid fossils and Linda Chandler and three anonymous reviewers for making useful comments that greatly improved this manuscript. Jim Knight of the South Carolina State Museum kindly shared unpublished information on palaeopheid snakes.

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Date:Dec 22, 2003
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