Class Mammalia Order Rodentia Family Muridae

NEOTOMA

GENERAL COMMENTS Zakrzewski (1993) provided an overall summary of the fossil record of the wood rats and discussed their dental characters. The terminology he used is not the same as that used herein, although in many cases it refers to the same features. There is no generally accepted terminology for the features that are called by different names in his report and in this chapter. Both reports include illustrations that define the terms. Zakrzewski's intent was to cover all known species of Neotoma, fossil and living, whereas only those involved in the fossil faunas of Porcupine Cave are discussed here; all of the Porcupine Cave fossils represent extant species.

useful dental characteristics Neotoma is a high-crowned cricetid rodent with roots on its teeth. The configuration of the occlusal surface changes dramatically with increasing wear, and it appears impossible to characterize species without taking this fact into account. The most reliable characters are those that remain when the tooth is greatly worn. These characters include the following:

1. The rootward extent and depth at a particular point in wear of the anteromedial groove on the M1 and m1 (figure 18.1, no. 3).

2. The presence or absence of pits (figure 18.1, no. 1) at the base of the reentrants (figure 18.1, no. 5) and, to a limited extent, which reentrants have them.

3. The form of the basal margin of the enamel crown (dentine tract) of m1 (figure 18.1, no. 4).

4. The relative anteroposterior dimension of the buccal reentrants of the lower first molar in occlusal view. (For example, in figure 18.1, center, the posterior reentrant is longer than the anterior one.)

5. The configuration of the occlusal surface on teeth showing little wear, including the position of the commissures between lophids and the configuration of the joined anteroconid-metaconid complex of m1 (which varies with wear, with the depth of the anteromedial groove, and with the presence or absence of an anterior cingulum).

FIGURE 18.1 Left, buccal view (anterior to left); center, occlusal view (anterior up); right, lingual view (anterior to right) of the left ml of Neotoma cinerea, showing dental features discussed. 1, Pits; 2, anteroconid-metaconid complex (anteroconid is most anterior; metaconid posterior to 3); 3, anteromedial groove; 4, base of the enamel crown-dentine tract (ascending the anterobuccal face of the anteroconid, left in the buccal view); 5, reentrants; 6, stylid. The tooth shows minor wear. The anterior cingulum projects posteriorly low along the buccal margin of the tooth from the posterobuccal corner of the anteroconid external to the anterobuccal pit. The posterior cingulum is a broad dentine field extending lingually from the hypoconid, which is the posterior buccal salient angle; these are seen in the occlusal view. Elevation of the dentine tract is moderate for the species (buccal view, left, 4) but is distinctly more wavy that in some other species.

FIGURE 18.1 Left, buccal view (anterior to left); center, occlusal view (anterior up); right, lingual view (anterior to right) of the left ml of Neotoma cinerea, showing dental features discussed. 1, Pits; 2, anteroconid-metaconid complex (anteroconid is most anterior; metaconid posterior to 3); 3, anteromedial groove; 4, base of the enamel crown-dentine tract (ascending the anterobuccal face of the anteroconid, left in the buccal view); 5, reentrants; 6, stylid. The tooth shows minor wear. The anterior cingulum projects posteriorly low along the buccal margin of the tooth from the posterobuccal corner of the anteroconid external to the anterobuccal pit. The posterior cingulum is a broad dentine field extending lingually from the hypoconid, which is the posterior buccal salient angle; these are seen in the occlusal view. Elevation of the dentine tract is moderate for the species (buccal view, left, 4) but is distinctly more wavy that in some other species.

Unless the tooth is fully erupted, the first three of these dental characters cannot be observed in modern dried specimens without dissection of some of the alveolar bone. There is, of course, no such problem with isolated fossil teeth or with modern specimens with fully erupted cheek teeth. The configuration of the occlusal surface of all species changes rapidly with wear and simulates that of other species at different stages of wear. As an example, the configuration of the occlusal surface of the ml of Neotoma cinerea with considerable wear can be quite similar to that of N. floridana or N. stephensi with slight wear. An estimation of the approximate degree of wear is thus important, and this is often not possible on museum specimens in which the tooth is not fully erupted.

The m3 of all extant species of Neotoma in the United States consists of anterior and posterior dentine fields that are joined by a necklike commissure. Variation includes three conditions: the neck is lingual, central, or buccal. The same form of m3 is found in different species. Thus isolated m3s, like M2s and M3s, cannot be identified to species. When wear is great enough that the entire base of the enamel crown is no longer intact, identification is impossible, except in the case of Neotoma cinerea, which has enamel pits at the base of all or most reentrants of ml and m2 that remain as islets even in very worn teeth.

Recognition of the dentine tract (character no. 3 in the previous list) is frequently difficult in dried modern specimens and in some fossil specimens. Helpful clues result from the fact that periodontal fibers do not adhere to tooth enamel, and, where they appear in modern dried specimens (below the alveolar rim in dissected specimens), the base of the enamel crown is indicated by their absence. In most fossil specimens

(where the fibers are no longer preserved) differential secondary discoloration of enamel and dentine often indicates the position of the enamel crown base (but care is necessary because fossil enamel is often translucent). In other fossil specimens the tooth must be manipulated under the microscope to achieve incidental reflection such that the limit of enamel deposition on the tooth becomes evident in the surface topography of the tooth.

The incidental reflection technique requires practice to achieve proficiency. When one looks at the buccal side of the tooth where the dentine tract is present (with the tooth oriented so that the microscope light reflects off it), in many cases the enamel appears higher than the dentine (being deposited on the dentine). However, in some cases it appears that the dentine continues to be deposited after enamel deposition has ended, so the dentine appears to be higher. In either case, the base of the enamel shows up as a slightly different elevation than the dentine on the surface revealed by reflected light. In high-crowned wood rats, waviness of the dentine tract is minor in comparison to fully hypsodont rodents, such as voles and lemmings. In some publications, especially European ones, the dentine tract is called the "linea sinuosa" (Rabeder, 1981; Fejfar and Repenning, 1998:text figure 1). In wood rats, it has been called the dentine tract by Lundelius (1979), Harris (1984), and Zakrzewski (1993). As in all rooted hypsodont taxa, the undulations of the base of the enamel crown vary slightly between individuals of the same species, and therefore the taxonomic utility of this character is most reliable where the tract is most developed, that is, where it ascends the buccal face of the anteroconid. In the wood rats, as in the arvicolines, this greatest waviness is on the buccal face of the anteroconid (anteroconid complex in arvicolines) of ml. In those wood rats that have at least incipient dentine tracts, the height of the tracts is very diagnostic. In the species of the present study, an anterobuccal dentine tract is absent only in Neotoma floridana and is most extreme in N. stephensi and N. cinerea. In most wood rats the anterobuccal dentine tract is eventually cut as the crown wears, causing a discontinuity of the enamel band on the occlusal surface of the anteroconid-metaconid complex.

DIFFERENTIATION OF NEOTOMA TAXA All species are semihypsodont. The ml metaconid (most anterior lingual salient angle) is broadly confluent with the anteroconid (most anterior field of the occlusal surface), varyingly separated by a shallow anteromedial reentrant, or groove; these two dentine fields are herein called the "anteroconid-metaconid complex" (figure 18.1, center, no. 2). The protoconid (medial buccal salient angle) is broadly confluent with the entoconid (medial lingual salient angle), and the two varyingly approach a transverse loph; the hypoconid (posterior buccal salient angle) is completely confluent with a posterior cingu-lum (posterior lingual salient angle) and forms a second (more posterior) transverse loph.

Lophs and the anteroconid-metaconid complex (figure 18.1, no. 2) are all connected by open commissures except in teeth with no or very slight wear. The commissure may be centrally located or have buccal displacement from the midline of the tooth (figure 18.1, center), with the feature varying with wear. The connection is more nearly closed in the very similar subgenera Hodomys and Teanopus (application of names follows that of Hall, 1981). The m2 consists of three lophs formed by the metaconid and anterior cingulum, the entoconid and protoconid, and the hypoconid and the posterior cingulum; it lacks the anteroconid of m1.

The m3 has anterior and posterior dentine fields united by a necklike commissure that may be located lingually, medial, or buccally. The subgenus Hodomys has an S-shaped m3, as does the Pliocene genus Paraneotoma. The generalization that the S-shaped m3 of Paraneotoma distinguishes it from the 8-shaped m3 of Neotoma is true and convenient. But the difference is gradational. Many examples of mid-Pliocene specimens of Paraneotoma (e.g., from the Taunton fauna of eastern Washington, circa 3 million years old) exhibit little-worn m3s with a clear S pattern, but after about half the crown is worn away, they assume the 8 pattern of Neotoma.

Such gradation has not been observed in early Pleistocene faunas. The S pattern is absent in the earliest forms of Neotoma except, possibly, in the completely unworn m3. Parahodomys spelaeus Gidley and Gazin from the circa 830,000-year-old Cumberland Cave fauna of Maryland (and a few unstudied records) bears some resemblance to Paraneotoma, but even it has a less clear S pattern on m3 than extant Neotoma (Hodomys) from Mexico.

The upper teeth of Neotoma superficially resemble the lowers except that the lophs are oriented more anterobuccally. On the M1 the anterocone joins the protocone to form the first loph (the "anterior loop"), which in many species is partly separated by a shallow anteromedial groove. This groove varies in depth from essentially lacking in N. stephensi and most subspecies of N. floridana to maximum development in species such as N. cinerea, N. mexicana, and N. micropus; it also varies in vertical extent and is variable in N. floridana.

The paracone and hypocone form the second loph of M1, and the metacone forms the third and shortest loph, the posterior cingulum being weak in the upper teeth.

The difference in orientation of the lophs of the lower teeth (nearly transverse) and the upper teeth (about 45° from transverse) allows the lophs of the opposing teeth to pass each other in propalinal chewing with much the same action as the blades of a pair of scissors. The pattern is the same in M2 and M3. The upper teeth usually have three roots, whereas the lower ones have only two. As pointed out by Zakrzewski (1993) the puzzling Mexican species Neotoma (Hodomys) alleni has a pronounced primitive S-shaped m3 but seems advanced in having an abnormally great number of roots on its teeth.

In many species, the lower teeth and less commonly the upper teeth possess a sloping crest that may extend posteriorly from the ends of the lophs (the ancestral position of the conids, as is evident in Neotomodon). With greater development, these sloping crests form a low wall across the bottom of the reentrants behind the cusp. In less hypsodont murids this crest, as developed at the posterolateral corner of the anteroconid, has been called the anterior cingulum (e.g., Lindsay, 1972:figure 40). This crest leaves the lowermost part of the reentrant walled off from the side of the tooth, producing a "pit" at the base of the reentrant (figure 18.1, no. 1). Pits occur in the bottom of many reentrants, caused by similar crests projecting posteriorly from this and other cusps and cuspids. They occur in many species as individual variations, but in a few species the pits characterize the majority of individuals and thus serve to help recognize species.

Pits form in the buccal and less frequently in the lingual reentrants of the teeth. On extremely worn teeth the pits remain as enamel islets and thus are one of the most persistent characters throughout the wear of the tooth. Pits cannot be observed without dissection in dried modern specimens that do not have fully erupted teeth. In some species, depending upon the relative openness of the reentrant, the development of an external wall at the reentrant base results in a depressed valley rather than a conical pit, as is most conspicuous in N. floridana and N. micropus, which do not have conical pits.

Excessive development of the lateral walls at the base of the reentrants frequently results in the development of a low cusp, a style or stylid. These form most frequently on the wall across the base of the posterobuccal reentrant of the m1 and m2 (figure 18.1, no. 6). Development of a style or stylid is random, although it may be present more frequently in some species.

If the crest on the buccal side of the anteroconid of the m1 extends upward to the occlusal surface, it forms a buccal spur on the surface's posterobuccal corner. This structure (not illustrated) is more typical of some species in which the crest is higher. Its total absence is typical of species in the western faunal region west of the Rocky Mountains (for example, Neotoma fuscipes). The feature varies with wear in many species; the spur, if present, becomes more prominent with wear.

The development of hypsodonty may be thought of as a delay in root formation. The periodontal fibers that hold the tooth in its alveolus do not adhere to tooth enamel, but do attach to either tooth dentine or tooth cementum. Increasing hypsodonty is therefore accompanied by the increasing exposure of dentine and cementum. Neotoma is only semi-hypsodont; thus no cementum is present in the reentrants between the lophs, and the degree of exposure of dentine (where enamel is not deposited) varies with the species. Some exposure of the dentine is present in most wood rat species and produces a waviness in the basal margin of the enamel, that is, "dentine tracts," which are most conspicuous on the buccal side of the anteroconid-metaconid complex of the ml. The degree of this development appears to be a consistent diagnostic character of the species of Neotoma and persists with wear.

The root pattern on the teeth of Neotoma exhibits the usual condition of low-crowned murids. Two roots are on the lower teeth and three are on the uppers; Ml has a lingual third root at its longitudinal midpoint, whereas M2 and M3 have two anterior roots and one posterior root. Supernumerary roots are rare (except in N. [Hodomys] alleni).

Table 18.1 summarizes the dental differences between species found as fossils in Porcupine Cave. Relevant characters are compared in the following diagnostic summary. Referred specimens for the Pit locality are listed in appendix 18.1.

NEOTOMA CINEREA (ORD, 1815)

MODERN DISTRIBUTION Today the bushy-tailed wood rat, Neotoma cinerea, is the only wood rat species known from the Porcupine Cave area. It was also present throughout the history of the deposits in the cave. In addition to occurring at higher elevations (above 1830 m) at the latitude of Porcupine Cave today, N. cinerea is the only species that ranges north of Oregon and into northwestern Canada.

distinguishing characters Although Neotoma cinerea includes some of the largest individuals known, the size range of its teeth varies widely and overlaps that of other species. Its most distinctive feature is the presence of pits in most reentrants.

ANTEROMEDIAL GROOVE OF ml: Usually prominent to about the base of the enamel cap, but may be only a relatively inconspicuous swale (a shallow and broadly open valley) when the tooth is about half worn. With little wear the groove is uniquely deep, making the anteroconid appear greatly enlarged; this trait is shared with N. mexicana, especially in specimens in which the swale extends half way to the enamel base. The condition in N. cinerea differs from that in N. stephensi and from N. micropus, in which the groove lacks great depth, and from N. floridana in extending to the enamel base.

PITS: Always prominent at the bottom of most reentrants. Always present in the two buccal and posterior lingual reentrants of m1, sometimes present in the anterior lingual reentrant. Some pits are always present on other lower teeth, and pits are present in the reentrants of the great majority of upper teeth. N. cinerea is unique in its many consistently present pits; N. mexicana has none on the lingual side of m1. In some individuals of N. cinerea a depressed valley, rather than a pit, is present at the base of the anterobuccal reentrant of the upper M1, as in N. floridana, but the other reentrants of this tooth retain conspicuous pits, unlike N. floridana.

POSTERIOR BUCCAL REENTRANT OF ml : At least as wide and often noticeably wider in an anteroposterior direction than is the anterior buccal reentrant. The latter condition is unique to the species, but the former condition is shared with most other species. N. cinerea differs from N. mexicana in the width of this reentrant.

DENTINE TRACT OF ml: Variation of this tract is great in N. cinerea, possibly more than in any other species, but exposed dentine runs at least halfway up the buccal face of the anteroconid-metaconid complex. This character is shared with N. stephensi and N. mexicana, but it differs from the condition in N. floridana, which has no dentine tract, and in N. micropus, which has only a slight rise up this face.

BUCCAL CREST FORMS A SPUR: When the m1 is about half worn, the crest running posteriorly from the anteroconid across the buccal base of the anterobuccal reentrant, forming the pit, is met by the occlusal surface and begins forming a posterior spur on the occlusal surface. The degree of development of this spur is shared with N. mexicana and exceeded by N. floridana, in which the spur appears with little wear. The character differs in N. micropus and N. stephensi, in which the crest is very low or absent.

ANTEROMEDIAL GROOVE OF M1: Uniformly deep to the enamel base. Some individuals have a small pit at the base of the groove. This feature is shared with N. mexicana and N. mi-cropus but differs from the condition in N. floridana and N. stephensi.

NEOTOMA MEXICANA BAIRD, 1855

MODERN DISTRIBUTION Today Neotoma mexicana occupies intermediate elevations between N. cinerea (high) and N. floridana (low), occurring in the lower foothills of the Colorado Front Range but also on the Great Plains to the extreme southwest corner of Colorado. The species is rather rare in the fossil record of Porcupine Cave.

DISTINGUISHING CHARACTERS On average N. mexicana is smaller than N. cinerea (Hall, 1981). The teeth, although resembling those of N. cinerea more than those of other species, have unique characters, most distinctive of which is the greatly elongate anterobuccal reentrant.

ANTEROMEDIAL GROOVE OF m1: As in N. cinerea, this groove extends to the base of the enamel cap and may be shallow to an inconspicuous swale when about half worn. The table i8.i

Summary of Dental Differences among the Species of Neotoma Represented in the Porcupine Cave Fauna

Species

Groove, ml

Buccal Pits, Lower

Dentine Tract

Groove, Ml

PosterobuccaJ Reentrant Buccal Pits, Upper Occlusal Length, ml

Anterobuccal Renentrant Occlusal Length, ml

N. cinerea

N. mexicana

N. floridana

N. micropus

N. stephensi

To enamel base; may shallow to a swale in lower half; upper half very deep.

To enamel base; may shallow to a swale in lower half; upper half deep.

About four-fifths to one-half to enamel base, uniformly deep.

One-half to three-quarters to enamel base.

To enamel base, fairly deep with no shallowing and no basal pit.

Present; posterior style frequent.

Posterior reentrant only. No style.

Lacking, but posterobuccal reentrant with valley. Styles very

Lacking, but with valleys.

Pits in most reentrants, similar to N. cinerea. Styles very rare.

High relief up buccal face.

High relief.

Flat with no relief up buccal face.

Moderate relief, one-quarter up buccal face.

Possibly the greatest relief of all, not variable as in cinerea.

To enamel base, uniformly deep.

To enamel base, uniformly deep.

Very shallow and well short of reaching enamel base. Not visible on occlusal surface with modest wear.

Deep and bottoms at enamel base in small pit.

Essentially lacking with slight wear, as in N. floridana.

Present, also on lingual side.

Present, only on buccal side. Tendency to form valleys.

Valleys only.

Valleys only.

Present, no lingual pits.

Frequently elongate.

Not elongate.

Not elongate.

Very elongate.

Not elongate.

Not elongate.

Not elongate.

Not elongate.

Not elongate.

Not elongate.

development of the swale is shared with N. cinerea, but the species differs from N. stephensi and N. floridana in having the shallow swale extend to the enamel base, and from N. micro-pus in having a shallow swale but no pit at the bottom.

PITS: Fewer than in N. cinerea. Pits are limited to the posterior buccal reentrant of ml, and no lingual pits are present in the lower or upper teeth. The pits have a tendency to appear in the form of elongate valleys, and this differs from the condition in N. cinerea.

POSTERIOR BUCCAL REENTRANT OF ml: Resembles other species; reentrant is not great in anteroposterior width. However, the anterobuccal reentrant is greatly elongated antero-posteriorly, and this condition is unique to the species.

DENTINE TRACT: The tract is as wavy as in N. cinerea, and exposed dentine runs halfway or more up the buccal face of the anteroconid-metaconid complex. This trait is shared with N. stephensi and N. cinerea; it differs from the condition in N. floridana, which lacks dentine tracts, and from that in N. micropus, in which the dentine track rises only slightly up this face.

BUCCAL CREST NOT PRESENT: When ml is over three-fourths worn, a weak crest running posteriorly from the ante-roconid across the buccal base of the anterobuccal reentrant may (or, more usually, may not) be present, but it is never prominent enough to form a pit. N. mexicana differs in this character from N. floridana and N. cinerea (which have a crest) but is indistinguishable from N. micropus and N. stephensi.

ANTEROMEDIAL GROOVE OF M1: Uniformly deep to the enamel base. No swale and no small pit at base, as in most N. cinerea; differs from N. micropus (which has a small pit) and from N. floridana and N. stephensi, in which the pit does not extend to the base.

NEOTOMA FLORIDANA (ORD, 1818)

MODERN DISTRIBUTION Neotoma floridana, the eastern wood rat, today is a resident of the southern parts of the central and eastern United States. Its westernmost records in the modern fauna are at Pueblo and Colorado Springs, Colorado, and it ranges into the northern part of eastern Colorado. In eastern Texas and eastward it is present in warm temperate to subtropical areas, with the exception of the subspecies N. f. magister, which occupies the area of southern Indiana, Ohio, nearly all of Pennsylvania, southern New York, and southeastward to the eastern side of the Appalachian Mountains. Including N. f. magister, N. floridana is the most widely distributed wood rat species in the United States, living even on the delta south of New Orleans and New Iberia, Louisiana.

N. f magister is geographically distinct and somewhat unique in dental characters, requiring that considerable dental variation be ascribed to the species N. floridana. It is perhaps arguable whether N. magister Baird, 1858, should be considered a subspecies of N. floridana (Schwartz and Odum, 1957).

DISTINGUISHING CHARACTERS This species is unique in having no dentine tract. Its specific separation from Neotoma micropus has been questioned (Hall, 1981:748).

ANTEROMEDIAL GROOVE OF ml: Prominent to about four-fifths of the distance to the base of the enamel cap, but uniformly deep, a trait shared with N. f. magister and N. micropus. Differs from other species in not clearly extending to the enamel base.

PITS ARE LACKING: The posterior buccal reentrants of the lower molars and all buccal reentrants of the upper molars have elongate valleys at their bottoms; these may be slightly depressed. The distinction between a pit, which is a conical depression in the bottom of the reentrant, and a valley, which is an elongate depression in the bottom of the reentrant along its axis, is gradational but distinctive. The condition of pits in this species is shared with N. micropus and N. mexicana (in which the distinction of pits from valleys is equivocal) but is distinct from other species.

POSTERIOR BUCCAL REENTRANT OF ml: About equal in anteroposterior length to anterior buccal reentrant, as in most species and with some N. cinerea; differs from N. mexicana, which has the anterior buccal reentrant elongate, and from some individuals of N. cinerea with elongate posterior reentrant.

DENTINE TRACT LACKING: The basal margin of the enamel crown is flat with no irregularity (also true in N. floridana magister), differing conspicuously from the other species discussed. No dentine tract rises up the buccal face of the ante-roconid-metaconid complex, a condition that differs slightly from that in N. micropus and greatly from that in other species discussed.

BUCCAL CREST FORMS A SPUR WITH SLIGHT WEAR: Very early in wear the buccal crest makes a prominent, backward-projecting spur on the occlusal surface of the anteroconid. A spur also forms on the protocone of m2. This trait is shared with subspecies N. f magister and with N. micropus, although the crest is smaller in the latter; it differs from the condition in N. cinerea and N. stephensi, both of which exhibit less prominence of the crest, and from that in N. mexicana, which lacks the crest until the tooth is nearly worn away.

ANTEROMEDIAL GROOVE OF M1: Very short and shallow. It disappears with little wear, ending well short of the base of the enamel cap. The groove is even shorter in N. stephensi. N. floridana differs from other species considered here, and also from N. f magister, in its weak development of the groove.

NEOTOMA MICROPUS BAIRD, 1855

MODERN DISTRIBUTION Commonly known as the southern plains wood rat, Neotoma micropus appears to prefer hotter regions than Neotoma floridana and seems better able to endure drier climates; it widely overlaps the range of Neotoma al-bigula, a real "desert rat," over vast areas from the Arizona-New Mexico state line eastward nearly as far as Austin, Texas. This range includes the southeastern quarter of Colorado. N. micropus abuts against the southern limit of N. floridana along the Arkansas River from Pueblo, Colorado, across Kansas to Arkansas City. From there the mutual boundary runs past Enid and Seiling, Oklahoma, to the Canadian River and then southward to Wichita Falls, Texas, to the Gulf of Mexico near

Victoria, Texas (Hall, 1981). The species range continues along the Gulf Coast to Veracruz and San Luis Potosi, Mexico, suggesting that heat rather than aridity controls the range.

Along this boundary between the geographic ranges of N. floridana and N. micropus, Hall (1981:748) mentioned only one place "less than one mile [2.6 km] in diameter" where the two species ranges overlap. In view of the vast area in which N. micropus shares the range of N. albigula, this almost complete allopatry of N. micropus and M. floridana seems curious.

This sharp separation of their modern ranges contrasts with the record from the Pit locality in Porcupine Cave, in which both N. floridana and N. micropus are present in most of the fossiliferous levels from level 3 to level 10 (see table 18.2; see also tables 10.9, 10.10).

DISTINGUISHING CHARACTERS Only specimens of N. micropus from the type area in Meade County, Kansas, were examined during this study, and these seem from dental characters to be fairly distinct from N. floridana. The most distinguishing character of N. micropus is the pit found at the base of the anteromedial groove of M1.

ANTEROMEDIAL GROOVE OF ml: Moderately deep but bottoms between one-half and two-thirds of the distance to the base of the enamel crown. This character is shared only with N. floridana.

PITS: Lacking; the bottoms of many reentrants are elongate and form a valley that may be depressed below the outer margin of the tooth. Valleys are present in the posterior buccal reentrant of m1 and m2 and in all buccal reentrants of the upper teeth. This condition is shared with N. floridana and N. mexicana, although few valleys are sunken below the outer margin of the teeth in N. mexicana. The trait differs from that in other species here considered.

POSTERIOR BUCCAL REENTRANT OF ml: Slightly narrower anteroposteriorly than the anterior buccal reentrant, but difference is minor, unlike the condition in N. mexicana and some N. cinerea. The character generally resembles that in the other species considered.

DENTINE TRACT: Very slightly developed. Base of enamel crown on m1 slightly wavy and rises slightly across the anterior and buccal face of the anteroconid-metaconid complex to form a rudimentary dentine tract, which is not greatly different from that in N. floridana but is a meaningful difference upon detailed comparison. Tract development is intermediate between that in N. floridana and that in other species considered, and in this respect it appears unique.

BUCCAL CREST FORMS A SMALL SPUR WITH SLIGHT WEAR: Posterobuccal spur on the occlusal surface of the ante-roconid-metaconid complex is present but small, and it does not increase in size down the tooth, but appears early in wear. The crest is somewhat similar to that in N. floridana, including N. f. magister, but is not as prominent. The crest differs from that in N. stephensi, N. cinerea, and N. mexicana.

ANTEROMEDIAL GROOVE OF M1: Bottoms at enamel base in a small pit. This condition of the M1 groove is very unusual and is unique in having a pit. In these respects the species differs greatly from N. floridana.

NEOTOMA STEPHENSI GOLDMAN, 1905

MODERN DISTRIBUTION Neotoma stephensi, the Stephens' wood rat, today does not occur anywhere near Porcupine Cave nor in southeastern Colorado. Instead it is confined almost entirely to northern Arizona and western New Mexico. N. stephensi is uniquely specialized in its diet of juniper leaves, a specialization that allows it to live in areas where it could not otherwise survive.

Vaughan (1982) documented the highly selective feeding of the species, noting that it avoids those junipers—and those branches on a single juniper—that contain a higher concentration of defensive chemicals that inhibit digestion (turpentines). His data suggested that the species lives in a state of constant stress because of this diet, resulting in a small litter size and slow recovery rate from major disturbances. In the spring when other plants are green, individuals will eat other vegetation as well as juniper.

Vaughan suggested also that wood rat feeding may cause the juniper plant itself to react, down to the level of specific branches on a single tree, by increasing its production of defensive chemicals. This hypothesis seemed to be supported by the declining conditions of laboratory animals fed juniper clippings from branches that free animals avoided.

DISTINGUISHING CHARACTERS The presence of a high dentine tract on m1 with virtually no anteromedial groove on M1 distinguishes the species.

ANTEROMEDIAL GROOVE OF ml: Moderately deep, shallowing slightly nearly to the base of the enamel but not forming a shallow swale. The groove differs from that in N. cinerea and N. mexicana in its lack of a swale and from that in N. flori-dana and N. micropus in shallowness.

PITS: Present in the buccal reentrants of m1 and in the posterior buccal reentrant of m2; the same pattern persists in the upper M1 and M2. This condition is shared with N. cinerea except for having no medial pits. The pits differ from those of N. floridana, N. mexicana, and N. micropus, all of which have valleys rather than well-developed pits.

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