The burrows of the miocene beaver palaeocastor, Western Nebraska, U.S.A

Palaeogeography, Palaeoclimatology, Palacoecology, 22(1977 ): 173--193 © Elsevier Scientific Publishing Company, A m s t e r d a m - Printed in The Netherlands

THE BURROWS OF THE MIOCENE BEAVER WESTERN NEBRASKA, U.S.A.

PALAEOCASTOR,

LARRY D. MARTIN and DEBRA K. BENNETT

Museum of Nalural History, Department of Systematics and Ecology, and Department of Geology, University of Kansas, Lawrence, Kansas 66045 (U.S.A.) (Received May 24, 1976; revised version accepted November 30, 1976)

ABSTRACT Martin, L. D. and Bennett, D. K., 1977. The burrows of the Miocene beaver Palaeocastor, western Nebraska, U.S.A. Palaeogeogr., Palaeoclimatol., Palaeoecol., 22: 173--193.

Daimonelix is a name given to terrestrial lebensspuren of the late Oligocene--early Miocene beaver genus Palaeocastor, and is not a plant or fresh-water sponge as was originally believed by Barbour. Palaeocastor belongs to a lineage of castorids always found in upland habitat, never near evidence of ponded water. Daimonelices are found in high concentrations in the Harrison Formation of western Nebraska and eastern Wyoming, which represents a semiarid, upland paleoenvironment of sandy substrate. Assignment of a large sample of contemporaneous Daimonelix to the species P. fossor is based on a series of incisor width measurements and on mean shaft diameters. The remarkable preservation of daimonelices, which permits detailed analysis of events in beaver burrow construction, is shown to be due to rapid silicification of plant roots invading the burrows. Some aspects of Palaeocastor ecology and ethology are also clarified. The spatial distribution of Daimonelix consists of scattered towns of high burrow density. One such colony is shown in a detailed paleogeographic map. Criteria for time equivalency of such burrow aggregations are established. The entoptychine gopher Gregorymys, the carnivore Zodiolestes, and the larger beaver P. magnus are also shown to be occasional inhabitants of Palaeocastor fossor colonies.

INTRODUCTION I n 1 8 9 2 , E r w i n H. B a r b o u r d e s c r i b e d a n e w g r o u p o f g i g a n t i c s p i r a l f o s s i l s f r o m t h e e a r l y M i o c e n e ( 2 2 m i l l i o n y e a r s B.P.) o f w e s t e r n N e b r a s k a ( F i g . 1 ) . Barbour was influenced by the belief that most of the western Tertiary was l a c u s t r i n e in o r i g i n , a n d r e g a r d e d h i s n e w f o s s i l s as f r e s h w a t e r s p o n g e s [ a v i e w s t i l l l i s t e d as a n a l t e r n a t i v e in t h e Treatise on I n v e r t e b r a t e P a l e o n t o l o g y (H/intzsche!, 1975)]. With the abandonment of the lacustrine hypothesis, Barbour modified his position and regarded the spiral structures, which he n a m e d " D a i m o n e l i x " ( d e v i l ' s c o r k s c r e w ) , as a n e w t y p e o f e x t i n c t p l a n t . S o m e s u p p o r t f o r t h i s i n t e r p r e t a t i o n c o u l d b e f o u n d in t h e p r e s e n c e o f a b u n dant silicified plant tissues in specimens of Daimonelix, and he erected an elaborate phylogeny for this new group (Barbour, 1895). However, speci-

174

Fig.1. Ellicott Ranch specimen of Daimonelix in situ. L. D. Martin and D. Nixon for scale. A, partial cast o f the interior o f the entrance (mound ?); B, turnaround; C, spiral, D, base of coil.

mens of a small beaver, Palaeocastor, c o m m o n l y occur in specimens of Daimonelix, and the plant remains are invariably those of roots and root hairs. In 1893, Cope and Fuchs independently suggested that Daimonelix was the burrow of a large rodent. The validity of this interpretation has since been demonstrated by Peterson (1906) and by Schultz (1942). In Barbour's first "Notice of New Gigantic Fossils" in Science of 1892, he proposed the name "Daimonelix" for these "immense corkscrews". He observed t h a t " n o t less than two genera and three species of the family were n o t e d . . , at least two gigantic and one small species were observed." It seems clear that because Barbour used correct italicization and capitalization in the 1892 publication, and inferred other related zoological taxa, that he originally intended the term "Daimonelix" to be used formally. However, although two figures of daimonelices appear in the 1892 paper, Barbour failed to name a type species or designate a type specimen. In his 1895 work he delineated different forms of daimonelices such as "Daimonelix Regular"

175

and "Daimonelix Irregular", but he did not use these terms with correct italicization or with consistently correct capitalization. Perusal of Barbour's 1894 paper on daimonelices indicates that Barbour had become more unsure of their correct genetic interpretation; he may by then also have been unsure of the correct nomenclatural usage. It is possible that he intended at this time to make the use of the term "Daimonelix" informal. His original failure to use binomial nomenclature may or may not invalidate the term "Daimonelix". At present, the authors recognize the equivocal status of the term but continue to use the singular form in a formal way (Daimonelix) and the plural form in an informal way (daimonelices). As the burrows of Palaeocastor, daimonelices are terrestrial lebensspuren (trace fossils). In the terminology of Seilacher (1953), daimonelices are domichnia, or living quarters, of Palaeocastor. Daimonelices are biogenic sedimentary structures of a bioerosional nature, and, like all lebensspuren, are non-transported and bear the indirect evidence of behavior on the part of the builder and inhabitant. The former property of lebensspuren is of special importance here, since terrestrial vertebrate remains are usually found in transported assemblages. Within the continental sedimentary environment, terrestrial lebensspuren are probably the least studied of all lebensspuren; they are not often preserved and usually go unrecognized even when they do occur (Voorhies, 1975b). Most terrestrial vertebrate trace fossils which have been studied are trackways (repichnia) which were made in the rather limited suite of environments which favor their preservation. Burrows in Cenozoic eolian deposits, however, provide useful data on the distribution and behavior of fossil mammals (Toots, 1963). In only a few types of Cenozoic mammal burrows have the occupants been definitely identified. These include the extinct kangaroo rat (Eodipodomys celtiservator Voorhies, 1975a) and the extinct beaver Palaeocastor (Peterson, 1906). The burrow of Palaeocastor, Daimonelix, is perhaps the most spectacular trace fossil known. In this study we have made use of the high concentrations of Daimonelix in eastern Wyoming and western Nebraska. These daimonelices, with other associated trace fossils, give us a unique picture of Harrisonian (early Miocene) paleoecology. Palaeocastor is a member of a late Oligocene and early Miocene radiation of North American beavers (Castoridae) which resulted in a number of forms adapted for burrowing underground. They are never found in pondedwater environments, but are instead found in upland situations, suggesting that they were inhabitants of steppes as are modern prairie dogs (Cynomys). At the present time, three species are t h o u g h t to be associated with Dairnonelix : Palaeocastor fossor, P. magnus, and P. barbouri. Palaeocastor magnus is much larger than P. fossor and P. barbouri is much smaller. The mean incisor widths for these three species are also distinct. It is possible to discriminate between these three beavers on the basis of the size of their incisors, and therefore also on the basis of the width of incisor marks left in the sand (Figs.2 and 3). No one has previously attempted to discriminate between the

176

B

Fig.2. A. Natural cast of incisor and claw marks on a Dairnonelix. B. Artificial cast of an incisor mark made by drawing the upper incisors of Palaeocastor fossor through damp sand, then casting in plaster. C, Top view of Daimonelix spiral in situ. Arrows indicate individual incisor marks. b u r r o w s o f t h e various beavers, n o r has a n y o n e p r e v i o u s l y a t t e m p t e d t o d e t e r m i n e w h e t h e r these species c o m m o n l y o c c u r r e d t o g e t h e r . Palaeocastor fossor is slightly larger t h a n a black-tailed prairie clog ( C y n o m y s ludovicianus). Its s k e l e t o n is a b o u t as m o d i f i e d f o r digging as a prairie d o g ' s , b u t n o t so m o d i f i e d as a p o c k e t g o p h e r ' s (Geomys). T h e t h i r d

177 I

I

I

n:17 S~24 I

I

I

n:5 S=.33 I

I

n:9

I

S:.27

i

i

I

I

i

I

i

•6

.7

.8

.9

1.0

1.1

1.2

Fig.3. Bar graph of the width of incisors and incisor marks in Palaeocastor fossor. For each sample, horizontal line indicates range, vertical line indicates mean. Numbers below horizontal line indicate sample size and variance. Top: widths of the upper incisor pairs; middle widths of artificial casts of upper incisor marks; b o t t o m : mean widths of incisor marks on daimonelices in the Ellicott colony. Scale is cm.

and fourth toes of the feet are somewhat elongated. The claws are rather short and flat for a fossorial mammal. The tail was not long and was cylindrical (Fig.4). The skull is broad and flattened dorsally. The incisors have flattened anterior surfaces, resulting in a broad, straight cutting edge. The upper incisors are strongly proSdont, a feature often associated with the use of the incisors for digging in modern fossorial rodents. The shape of

A

~

F ~"

jl %

i

Fig.4. Restoration of digging modes used by Palaeocastor fossor. A. Digging in the spiral with the incisors. B, C. U~ing the feet to move loose dirt in the living chamber. E. lT~ing the flattened head to push loose dirt in the living chamber which was scraped oft the wall, D.

178

the ventral surface of the snout suggests that the lips could be closed to prevent swallowing dirt while digging. The major locus of this study of Palaeocastor burrows has been on the Lorena Ellicott Ranch in Sioux County, Nebraska, and adjacent parts of Wyoming. Here Miocene sediments are well exposed under semiarid badland conditions. The Miocene sediments of northwestern Nebraska consist of sandstone, siltstone, claystone, and loess; they are rich in volcanic ash, intermittently concretionary, and are cut by paleo-channels at many horizons. Daimonelix occurs most abundantly in the lower Miocene Harrison Formation. The Harrison in northwestern Nebraska is a massive, grey-brown very fine-grained sandstone. Cliff exposures characteristically have rough faces due to the presence of well-indurated grey sand lenses c o m m o n in the unit. Sandy pockets evident in otherwise grassed-over fields are also characteristic Harrison exposures. The unit is cross-bedded most extensively at the base and contains a basal conglomerate. Ash is dispersed as a sedimentary constituent throughout the unit b u t ash beds also occur, and the one at Agate has been dated at 21.3 million years (Evernden et al., 1964). The Harrison is largely fluvial in origin but displays a loessic facies in areas far from channels (D. A. Yatkola, pers. comm., 1974). It is this eolian facies in which we find concentrations of daimonelices, fossil roots, insect burrows and small mammal burrows. Daimonelices are found less abundantly in the underlying Monroe Creek Formation. Massive cliffs of this light orange-brown siltstone are developed in the Pine Ridge province in Sioux Country. The unit contains abundant calcareous concretions, but fewer preserved plant roots and small burrows than are seen in the Harrison. A single Daimonelix is reported from the Lower Miocene Gering Formation, from Redington Gap, Morrill County, Nebraska (L. Tanner, pers. comm., 1970). The Gering is a'sandstone unit generally representing fluvial deposition. The paleoenvironment indicated by sediments of the Harrison Formation is a semiarid to arid regime with low topographic relief, transected by intermittent streams. During the early Miocene the area was probably highly seasonal in climatic parameters such as rainfall and temperature, much as is Sioux County today (Schultz, 1942). BEAVER BURROW DIAGENESIS

The Harrison Formation is characterized by the presence of abundant fossil roots. These are silicified and sometimes show cellular structure (Peterson, 1906). There is a tendency for the roots to be concentrated at and just below paleosol horizons within the formation. Beaver burrow walls apparently formed a preferred r o o t microhabitat. This pattern of root growth preference may be related to microenvironmental conditions in subterranean voids and to physical properties of such voids.

179

Especially important to dry land plants is moisture. In wet soils, water potential in roots as compared to soil is low and plants find little difficulty maintaining turgor pressure under such conditions. However, in soils developed in semiarid climates, the water potential of the soil is usually lower than that of the roots, because of low water availability and because any interstitial water present forms tight electrolytic bonds with soil particles. After a rainfall on such sediment, due to this tight electrolytic bonding, soil water potential falls quickly as both roots and soil particles compete for water droplets. Under these conditions, water cannot be expected to flow toward roots after an initial wetting phase (Ray, 1973). Plants cannot obtain water unless their water potential is lower than that of the surrounding soil. Thus ~urgor pressure must fall and some wilting must occur in the plant before it can once again obtain water from the soil (Ray, 1973). A dry land plant thus has t w o alternatives: either to grow laterally in the direction of the retreating water, bifurcating to pick off every interstitial drop, or to grow downward far enough to intersect either the water table or a level of sufficiently high water potential (Ray, 1973). Both solutions are seen in modern dry land plants (Weaver, 1968). Great metabolic cost is incurred by a plant opting for either of these solutions (Taylor, 1974). In Harrison sediments however, a third possibility presented itself. At least some plant roots could occupy beaver burrow walls. Here humidity was probably comparatively high due to metabolic activity and respiration on the part of the beavers (McNab, 1966). For example, the atmosphere in burrows of Geomys, Heterocephalus (McNab, 1966), and Spalax has been shown to be saturated with moisture independent of burrow temperature (Nevo, 1973). Even with soil moisture as low as 1.0% this condition prevails in burrows of Geomys (Kennerly, 1964). Although these genera plug their burrows during the day to help prevent water losses (Walker et al., 1964), and although there is no direct evidence that Palaeocastor burrows were plugged, we believe that the shape and great depth of Daimonelix and the absence of a second opening reduced air circulation and helped to keep burrow humidity high in the semiarid environment of the Harrison. Efficiency of roots in absorbing atmospheric as opposed to condensed water would n o t necessarily be reduced if humidity were high enough in the burrows (Ray, 1973). Another factor making Palaeocastor burrows a favorable root micro-habitat is the fact that such burrows constituted a physical void within the sediment. Given otherwise acceptable conditions of light, temperature, pH, and nutrient availability, sediment strength has been shown to be a major factor in slowing r o o t growth (Taylor, 1974). Sediment resistance to root growth has been shown to be especially high in sandy soils (Taylor, 1974). Voids such as the burrows of Palaeocastor would have provided a chance for accelerated r o o t growth for the fortunate plant finding the void. Thus, beaver-burrow walls formed a very desirable microenvironment for dry land plants and doubtlessly each new burrow soon acquired an inner root lining, with the predominant direction of root growth along the trend

180 of the burrow. This process of root infilling was certainly limited by beaver activity during times of burrow occupancy. Offending roots might be nipped off with the incisors, as in Geomys and Thomomys burrows (Walker et al., 1964), or worn down in the course of burrow use. Roots lining the interior of the burrow formed a dense mat which separated the Harrison sands from further impression after initial burrow formation, and which fortuitously protected initial burrow markings from obliteration by subsequent activity in the burrows. Reasonably detailed conclusions regarding the m o d e of burrow construction can thus be drawn. Fossilized beaver burrows are found in all stages of root packing, from a minimum condition displaying an interwoven plant shell around the burrow mold, to a maximum condition of solid packing of the entire burrow by plant roots. Evidence presented above indicates that only after occupancy of a burrow ended could plant roots completely pack it. In some cases, varying amounts of Harrison sand are found central to the root layer. Presumably this type of filling also occurred after burrow occupancy ended (Toots, 1963). F e w roots are seen to cut across partly-packed burrows from one wall to the other, and the dominant trend of root growth is always down the axis of the burrow. R o o t preference for beaver burrow wall habitat led to production of more or less solidly infilled pipes within the Harrison sediment. Surface detail in such burrow casts is good and is an indication of the rapidity with which invading roots found available burrow wall space, and of the importance of the b o u n d a r y between Harrison sediment and burrow space to the invading root system. R o o t growth in fossil beaver burrows is thus limited externally by the desirability of the burrow microhabitat for roots, and limited internally by beaver activity. Permeability and porosity of the Harrison sediments are high, permitting free percolation of ground water through the sediment. The Harrison sediments contain substantial quantities of volcanic ash, which is a major terrestrial source of amorphous silica (Siever, 1962; Siever and Scott, 1963). Where ground water flows through an area of available amorphous silica, equilibrium between ground water and silica can be expected perhaps in a period of weeks (Siever, 1962; Rolfe and Brett, 1969). With replenishment o f silica-saturated water by fresh rainwater, this process can be expected to repeat itself. Both dissolved and colloidally suspended silica has a high affinity for organic material, and is readily and firmly adsorbed onto cellulose or lignin (Rolfe and Brett, 1969). Adsorption of silica onto organic molecules is common and the resulting complex molecule is very insoluble in water (Siever, 1962). Adsorption of silica onto cellulose or lignin is the major probable reaction in processes of fossil w o o d diagenesis (Rolfe and Brett, 1969). Silica produced or secreted by the living plant plays little or no role m the plant's subsequent fossilization (Siever, 1962). However, roots, like other plant parts, have a fairly rigid cell structure which can aid a silica

181

solution in permeating the root. In dead but undegraded roots, silica from such a solution is deposited in intercapillary spaces. In a partially degraded root (one with lignified cell walls remaining), it is deposited in the former structural position of the root cellulose (Rolfe and Brett, 1969). Observation of patterns of silica deposition in beaver burrow walls indicates that both these conditions exist and that silica was also frequently deposited in "intercapillary" spaces between roots as well. The invading silica solution apparently did n o t discriminate between external and internal surfaces of the roots in the tightly interwoven masses. Thus a process of burrow diagenesis, involving a volcanic source area rich in amorphous silica and dependent upon the seasonal movement of ground waters in equilibrium with available silica, probably resulted in a very rapid partial lithification of beaver burrow walls, perhaps within a few years of the death of the plants occupying this microhabitat. Grass roots have been shown to decay fully in natural prairie soil within five years of the death of the plant (Weaver, 1968). Fossilization must have occurred before complete or even very extensive decay to account for the preservation of cellular detail illustrated by Peterson (1906). Even partially lithified burrows would have posed a considerable obstacle to a living beaver seeking a new burrow site. In addition to their evident solitary nature (see below), the fact that Harrison beaver burrows are almost never observed to intersect, and that where one comes near another the later burrow often curves around the path of the earlier one without intersecting it, may be explained by the beavers' distaste for tunneling into an alreaciy lithified sediment. A much easier route was always afforded by the unconsolidated Harrison sands. Peterson (1906) tried to account for the good preservation of the burrows through the secretions and movements of the beavers. This same view was held by Schultz (1942). It is now apparent that most of the preservation of burrow structures in the Harrison Formation is due to the infilling of the walls with roots which rapidly silicified. BEAVER BURROW CONSTRUCTION

Daimonelix burrows were built by sediment excavation. It is fortunate that marks left by Palaeocastor from the final shaping of these burrows are c o m m o n l y well preserved. This provides a unique opportunity to determine exactly how Palaeocastor dug its burrows. Only the presence of claw marks had previously been reported on daimonelices (Schultz, 1942) but our studies show that the d o m i n a n t marks on these burrows are incisor marks. Almost all marks found on the upper one-half of the burrows are incisor marks, while claw marks are largely confined to the b o t t o m and sides of the burrows. Palaeocastor possesses relatively small, unspecialized claws. The third and fourth toes are much the longest, wnmI~ explains t h e narrow claw mark sets found on daimonelices, usually in sets of two or four. The

182 upper incisors of Palaeocastor are p r o S d o n t with a flat anterior surface. When scraped through soft, moist sand they leave a broad flat groove with a slight ridge d o w n the center where the incisors do not quite meet. Plaster of Paris casts made of these marks are identical in all major respects with incisor marks found on natural beaver burrow casts -- the daimonelices (Fig.2). Apparently most of the digging was done by using the incisors to scrape off wall dirt which was then shoveled posteriorly and laterally with the front paws working together. Dirt may have been shoveled through the burrow and thence outside by the hind feet as in Cryptomys, or the beaver may have turned around and used its broad flat head to push it out of the burrow as does Thomomys (Fig.4). The major part of spiral construction was effected by a continuous series of either right- or left-handed strokes with the incisors (Fig.5A and B; Fig.6). Whether the spiral is sinistral or dextral was determined by the direction of the original series of incisor strokes. The sample of spirals is in fact almost exactly divided between sinistral and dextral specimens, suggesting a 50% probability for sinistral or dextral and no marked " h a n d e d n e s s " in the beavers. Examination of the terminus of the living chamber provides an idea of the actual sequence of digging motions within the living chamber. Rightand left-handed strokes with the incisors were alternated in the construction of this portion of the burrow, resulting in a chevron pattern on the ceiling and walls (Fig.7). Digging was apparently accomplished while the animal was stationary. The head was pivoted and the neck was moved in short half-circles. Shapes of natural incisor mark casts show that the incisors were inclined at an angle to wall surface during construction of the spiral, but they were held essentially parallel to the surface for most strokes in the living chamber. The floor of the living chamber and spiral passage usually have a central area devoid of marks; here they were probably obliterated by the animals' daily activities before preservation could occur. Complete daimonelices are rare as portions of the burrow are usually eroded away in natural exposures. The labor involved in excavating a silicified Daimonelix from the surrounding rock is enormous, although this is usually the only way in which a complete burrow may be examined. When detailed examination of a burrow seemed necessary, we prepared latex molds which were then cast in plaster of Paris and studied in the laboratory. Examination of over five hundred complete and partial Palaeocastor burrows (mostly in the field), permits the development of a fairly complete picture of the burrows and of their mode of construction. Study of the plaster cast of the top of one 168-cm section of living chamber showed approximately 400 individual incisor marks. The mean incisor mark width on this burrow is 0.953 cm with a standard deviation (s) of 0.288 cm and variance (S 2 ) of 0.083 cm. This is well within the range for P. fossor incisor marks produced in sand in the laboratory (Fig.3). The mean

183

B A MOUND

TURNAROUND

1.60 M

24 CM i

i

BASE O

\

I J

LIVING CHAMBER

Fig.5. Terminology used for parts of Daimonelix and types of burrows. A. Daimonelix with only slightly inclined living chamber. B. Daimonelix with highly inclined living chamber. C, D. Different types of Daimonelix living chambers illustrated by Peterson (1906, fig.7 ).

length o f incisor marks is 5.332 cm, with a s t a n d a r d deviation o f 2 . 3 1 9 cm. Claw marks are largely c o n f i n e d t o the sides of the b u r r o w (the f l o o r was n o t m o l d e d ) . T h e d i s t r i b u t i o n o f claw and incisor marks o n the t o p o f this b u r r o w is s h o w n in Fig.7. A s e c o n d b u r r o w with a well-preserved f l o o r shows 23 incisor marks on t h e f l o o r surface o f an 80-cm section, a n d 171 claw marks o n t h e sides and floor. Clearly claw marks are largely r e s t r i c t e d t o the floors and sides o f t h e b u r r o w s , while the u p p e r surface bears the greatest p r o p o r t i o n o f incisor marks.

184

Fig.6. Portion

of a Harrison

Daimonelk

Fig.7. Sketch of the top of a Daimonelix better preserved claw and incisor marks,

in situ. A, sink; B, living

living chamber

showing

chamber;

C, base of spiral.

the distribution

of the

Burrows believed to belong to P, fossor contained from six to twelve coils with an average of about nine coils. They ranged in length from about 210 cm to 275 cm, and seemed to be rather consistent in depth at any one locality. In most cases the daimonelices arrange themselves in layers with their upper ends terminating in concentrations of fossil roots which indicate the positions of paleosols. Usually there is no evidence of the mound of earth which must have been associated with each burrow, although we occasionally find fragmentary evidence of a funnel-shaped structure leading into the spiral, which probably represents a portion of the inner wall of such a mound. The mound, of course, is much more subject to erosion than is the remainder of the burrow and would not contain the large numbers of silicified roots responsible for the preservation of the burrow itself. Just below

185

the m o u n d there is often a bulbous structure (Fig.5A and B; Fig.6) which might have provided a place for the beaver to turn around or to sit upright during an alarm. Prairie dog burrows are reported to show similar structures (King, 1955). Within one spiral the coils and shaft are very uniform in diameter. The coils usually show an angle of inclination of from 25 to 30 ° . Cross-sections of the shafts are nearly circular. The shaft diameter widens only after leaving the lowermost coil (base of spiral} (Fig.5A and B; Fig.6), where it enters an upwardly inclined living chamber. Barbour, when he regarded Daimonelix as a plant, called this structure the "rhizome". All of the sever~ hundred 15.

A

13.

C

11.

9' 9¸

>=

z

8-

7" 6 5

7¸

"' ,,o,

5

,,=

3

4

3, 2 I, 0

I

7 13 19 L.C. DIPS. DEGREES

D

B

6 7 8 9 10 11 12 13 14 L.C. DIAMETER, cm

16 17~Ig

19 2"0 2'I 2~'--L

1211 10 9

8"

8

7 6>-

5"

zID

4-

ouJ ,,=

3" 2-

o

>-

o

6

z

w

oLIJ

¢y. U.

F INCISOR MARK WIDTH,mm (Average Per Burrow~

4

3 2 1

°s.

'9 10 11 12 13 16 17 18 1'92"021 22 SHAFT DIAMETER,CM

Fig.8. H i s t o g r a m s s h o w i n g v a r i o u s aspects o f b u r r o w s in t h e E l l i c o t t Palaeocastor c o l o n y . A. Angles o f i n c l i n a t i o n m e a s u r e d o n Daimonelix living c h a m b e r s . B. Incisor m a r k w i d t h s o f o n e Gregorymys b u r r o w (4 m m ) a n d Palaeocastor fossor b u r r o w s . C. Living c h a m b e r d i a m e t e r s o f d a i m o n e l i c e s . D. S h a f t d i a m e t e r s o f b u r r o w s : 1 1 - - 1 4 cm, P. fossor; 21 cm, P. magnus; 5--7 cm, Gregorymys (nonspiral).

186 living chambers examined were inclined upwards away from the spiral. The inclination of living chambers in the Ellicott beaver town is variable, ranging from 2 ° to 37 ° (Fig.8A). There is no relationship between living chamber dip and living chamber diameter (Fig.9C). The low variance in living chamber diameter and in shaft diameter precludes burrow construction by different species of beavers. This indicates that individuals of Palaeocastor fossor were responding to subterranean microenvironmental conditions in construction of their burrows and/or built different living chambers for different purposes. Those living chambers with very steep inclinations (over 30 ° ) seemed to be simple structures without expansions or side chambers (Fig.5B). Many living chambers with lower inclinations have short side chambers and may be greatly expanded in the horizontal plane. Some of these living chambers are quite long and Peterson (1906) reports examples up to 4.5 m in length (Fig. 5C and D). Many low-angle living chambers have short vertical "sinks" dug into their floors (Fig.5A; Fig.6). These might have served for water drainage, b u t are more likely for the deposition of feces as has been described by King (1955) for Cynomys. Neither feces nor nesting materials have been recognized in any specimen of Daimonelix. All of the specimens of Palaeocastor collected by the University of Kansas from burrows were found in the low-angle living chambers. On the basis of the discovery of remains of y o u n g beavers within them, we can also infer that y o u n g were raised in this type of living chamber. In only a few examples of the hundreds of burrows examined could more than one opening to the surface have been present, and we infer that emergency or auxiliary escape openings were either extremely rare or entirely absent. This does n o t mean that the beavers were free from predation. Under normal circumstances, one expects to find the entire skeleton of an animal which has died in its burrow. This is not usually the case with Palaeocastor. Only a few complete skeletons have been found. In most cases, the only parts present are the skull, lower jaws, and sometimes the feet. These are parts which are often left by predators and the consistent presence of this set of remains suggests that the beavers had either been killed or that their carcasses had been scavenged by some carnivore. Lack of escape openings in the burrow suggests that the beavers must have been willing to defend their burrows. In the spiral where the diameter of the shaft is only slightly greater than that of the beaver, they would have been very formidable opponents. In a few cases, bones from mammals other than beavers have been found in daimonelices. These may have washed in as was suggested by Peterson (1906), or in some cases the beavers m a y have brought them in. Pocket gophers are known to keep bones in their nests to gnaw on (Smith, 1948). A Zodiolestes skeleton found in a Palaeocastor burrow (Riggs, 1945) indicates that beaver burrows were entered by this carnivore. This pattern of burrow occupancy is also seen in prairie dog towns with black-footed ferrets (King, 1955).

187 BURROW DISTRIBUTION

Our data suggest that daimonelices were arranged in " t o w n s " similar to those formed by the modern Cynomys, although the existence of a similarly complex social structure cannot be demonstrated. Daimonelices may be absent over a wide area but enormously abundant in other areas in the same upland facies and at the same stratigraphic level. The original surface openings of the fossil burrows in each town seem to terminate in fossil soil horizons which can be traced over a wide geographic area. Thus, it is possible to estimate the number of burrows in a town, to determine the approximate boundaries of a town, and to show the distribution of neighboring towns at a given time in the past. Such clear distributional data also make it possible to determine other members of the upland ecosystem of which Palaeocastor towns formed a part. The density of burrows in towns is remarkable, with individual burrows c o m m o n l y less than three meters apart. Taking into account the presence of a large mound, it seems doubtful that enough grazing space would remain available per individual if all burrows were occupied at the same time. How long a single town or even a single burrow might have been occupied is uncertain. Perhaps a high percentage of the burrows were unoccupied at any one time. The high density of the burrows may explain their spiral structure, as this permits the digging of a very deep vertical burrow which occupies little horizontal space. It was our intention to map some of these towns, but it proved necessary first to establish some criteria for time equivalency of burrows. It is difficult to find an ancient surface exposed showing large numbers of the original surface openings of the burrows. Neither the mounds nor the upper parts of the burrow are generally preserved, as they were not heavily packed with roots as were the coils. In 1975, we were fortunate to find a large surface of spiral bases, fragmentary coils, and living chambers on the Lorena Ellicott Ranch in Sioux County, Nebraska. A contour map was made with a plane table and alidade, using 186 burrows as c o n t o u r points (Fig.10). The total sample of Palaeocastor burrows observed indicates that neighbouring burrows tend to be of similar depth. Thus burrows shown on this map can probably be regarded as belonging to one paleosurface and as being approximately time equivalent. Erosion would have eliminated any short burrows and they therefore do n o t appear on our map, but the high densities of Daimonelix mapped suggest that this is not a serious consideration. Measurements and other observations were taken on the mapped shafts, bases of spirals, and living chambers. The directional data amassed from the Ellicott beaver town suggest that living chambers of these burrows are randomly oriented (Fig.9A, 11), with regard to both trend and plunge (Toots, 1965). The identification of the modal burrow class as belonging to Palaeocastor fossor is substantiated by correspondence of the width of incisor marks on the burrows with widths of incisor marks produced in sand with P. fossor incisors in the laboratory, and with widths of P. fossor

188

B

30 28,

261

O=L.C.

O=SHAFT

24, 22 20, ~

18'

ua Il,u

161

cq~o

141 "

12', 10, 8"

4: 2: 0

" ~'4

'~

" 8"1b'12

I N C I S O R S I Z E , mm 30 28 26 24

u.

0 u -

211 18

.1

iILl

161

1

14:

•

•

•

•

•

•

12. • 2 " ~ " 6 " I~ ' l b ~ ' 1 2

I ; 4 16 ' 1 8 " 2 ' 0 2"2 ' ~;4" 26 " 28 " 3"0 " 32 " 3;4 " 3"6

~-

Fig.9. Graphs showing various aspects o f the burrows in the Ellicott colony. A. Rose diagram o f the o r i e n t a t i o n of living chambers. B. Plot o f incisor marks and shaft diameters. O p e n circles living chambers, closed circles shafts. Gregorymys b u r r o w with 6-cm coil d i a m e t e r ; all t h e rest are P. fossor. C. Living c h a m b e r inclinations and diameters.

incisors (Fig.3). Diameters of both living chambers and bases of spirals are variable (Fig.8C, D), b u t the diameter of the shaft fragments proved almost as diagnostic as width of incisor marks (Fig.8D). Spiral shafts reflect closely the diameter of the animal digging them (Toots, 1963) and are thus consistent in size, as is shown by a variance (S 2 ) of 0.5816 for 25 coils with a mean diameter of 12.28 cm.

189

,oo~ o

o o

o o

o •

eo e

o

o

o

\

o

o

~ o

qlo ~

~

f

IOPOGRJPfllC

MAP OF BEAVER BURROW DISTRIBUTION

coNmo~

I.TEeVAI

,~,~m,b,.=h.~i

p , ° . ° 1.b,O ° ' ° , , . O . ~

O,b,~,,.l~ ~ " o , ° . Io b . ~OO

pLANE TABLe

.~N

® COIL* . . S"AVT DIA.ETe" 0, 2 ' ~ O CO'L • SASE ~ SP'~AL 0, U W " e C " ~ S E ~

147M

Fig.10. Map of part of the Ellicott colony with contours on the burrows.

Besides the modal group of burrow sizes, there are seven non-spiral burrows of very small diameter in the Ellicott town (Figs.8B, D, 9B); these display a mean of 5.81 cm with a standard deviation (s) of 0.121 and variance (S 2) of 0.015. One of these burrows bears small incisor marks. We regard these as burrows of the extinct entoptychine gopher Gregormys. Based on observations of large burrows elsewhere in the Harrison, a single burrow from the Ellicott t o w n with a very large (21 cm) coil diameter is believed to belong

190

OOL OL

\12

Le

L

eL

'''L

L; L

7 R

2/

? L

~

l/ I_

.J

J /

R

~RO

L R

SPATIAL RELATIONSHIPS OF BEAVER BURROWS

L

L

I STRIKE OF LIV NG CHAMBER LSTRIKE OF LIVING CHAMBER WITH COIL ,

2DiP VALUE IN OEGREE$ 4 Of: LIVING CHAMBER

|.47 M

$ FT. NJ[ IroN

DIP VALUE IN 0EGREES OF LIVING 4 CHAMBER WITH COIL OCOIL ONLY O OR 0 OIRECTION OF COdLiNG R t

Fig.11. Mhp of part of the Ellicott colony showing only those burrows from which orientation data could reliably be taken.

to P. magnus (Fig.8D). We c a n n o t be certmn that it is contemporaneous with the P. fossor burrows, but the great h o m o g e n e i t y of coil diameter in the Ellicott beaver t o w n is at least suggestive that the two beaver taxa did n o t usually burrow in exactly the same areas at the same time.

191 CONCLUSIONS

Daimonelix is the dwelling burrow of at least two species of fossorial beaver, Palaeocastor fossor and P. magnus. The incisors were used to excavate the burrow and the loose dirt was shoveled out in part by the feet. The burrows were invaded by roots" soon after excavation and these roots were rapidly silicified. It is the silicification of the roots in the walls of the burrows that accounts for preservation of remarkable detail on most Daimonelix. The burrows of Palaeocastor fossor and P. magnus can be distinguished on the basis of burrow coil diameter and on the basis of the width of the incisor marks preserved on burrow walls. It appears that the two larger Palaeocastor species did n o t share the same " t o w n s " . Within a single P. fossor colony, we have been able to determine the following: (1) the burrows do not intersect; (2) the burrows have only one external opening; (3) the spirals are about 50% dextral and 50% sinistral; (4) the living chambers are inclined upwards away from the spiral; (5) the living chambers are inclined from 2 ° to 37 ° and there is no relationship between living chamber inclination and living chamber diameter; (6) living chambers are oriented randomly; and (7) fossil beaver remains, including those of young beavers, have thus far been found only in living chambers with low angles of inclination. Palaeocastor colonies provide insight into the paleoenvironment of the Harrison Formation in western Nebraska and eastern Wyoming. They must have been developed on upland surfaces which were to a large extent open grasslands. Dense concentrations of Palaeocastor burrows of the type found in the Harrison Formation would n o t be likely to occur in forested areas or in moist areas with continuously saturated soils. In fact, the very deep daimonelices can be considered evidence that the water table must have been deeper than 3 m (Toots, 1963). Palaeocastor fossor did not tunnel after roots like the modern pocket gopher, Geomys, which exists to a large extent on tubers and roots which it finds just underground. The shape of the daimonelices in fact minimized the a m o u n t of infrasurface root contact. It seems likely that Palaeocastor fossor must have derived most of its food supply through grazing at the surface, as does the modern prairie dog Cynomys. The fact that beaver burrow wall space constituted a favorable root microhabitat may, however, have meant that Palaeocastor fossor took advantage of a food supply which followed its tunneling activities. ACKNOWLEDGEMENTS

The authors wish to thank C. B. Schultz, L. G. Tanner, and Daniel Yatkola for helpful c o m m e n t s concerning the distribution of daimonelices and the paleoenvironment of the Harrison Formation. Gilbert Parker assisted during

192

the field season of 1974 and Orville Bonner has helped throughout the study. We are grateful to M. R. Voorhies and C. "W. Teichert for critically reading the manuscript and offering many valuable suggestions. We are especially grateful to the Clarence Dout, Lorena Ellicott and Breuger families, who permitted us to c o n d u c t research on their land and helped us in many ways. This research was supported by a University of Kansas General Research Grant.

REFERENCES Barbour, E. H., 1892. Notice of new gigantic fossils. Science, N.S., 19: 99--100. Barbour, E. H., 1895. Progress made in the study of Daemonelix. Proc. Nebr. Acad. Sci., Publ. 5: 1894--1895. Cope, E. D., 1893. A supposed new order of gigantic fossil from Nebraska. Am. Nat., 27: 559--569. Evernden, J. F. et al., 1964. Potassium-Argon dates and the Cenozoic mammalian chronology of North America. Am. J. Sci., 262: 145--198. Fuchs, T., 1893. Ueber die Natur von Daimonelix Barbour. Ann. K. K. Naturhist. Hofmus., Wien, 7: 91--94. Hh'ntzschel, W., 1975. Trace fossils and problematica. In: C. Teichert (Editor), Part W, Suppl. 1, Miscellanea, Treatise on Invertebrate Paleontology. University of Kansas Press, Lawrence, Kansas, pp. 177--245. Kennerly Jr., T. E., 1964. Microenvironmental conditions of the pocket gopher burrow. Texas J. Sci., 16: 395--411. King, J. A., 1955. Social behavior, social organization, and population dynamics in a black tailed prairie dog town in the Black Hills of South Dakota. Contrib. Lab. Vert. Biol. Univ. Mich., No. 67: 1--123. McNab, B. K., 1966. The metabolism of fossorial rodents: a study in convergence. Ecology, 47: 712--733. Nero, E., 1973. Selection vs. neutrality: A test in natural populations. Nature, 244: 575--585. Peterson, O. A., 1906. Description of new rodents. Mem. Carnegie. Mus., vol. II (W. J. Holland, Editor). Ray, P. M., 1973. The Living Plant. Holt, Rinehard and Winston, Chicago, Ill. Riggs, E. S., 1945. Some Early Miocene carnivores. Field Mus. Nat. Hist. Geol. Ser., 9(3): 69--114. Rolfe, W. D. I. and Brett, D. W., 1969. Fossilization Processes. In: G. Eglington and M. T. J. Murphy (Editors), Organic Geochemistry. Springer-Verlag, New York. Schultz, C. B., 1942. A review o f the Daimonelix Problem. Univ. Nebr. Stud. Sci. Technol., No. 2. Seilacher, A., 1953. Studien zur Palichnologie. I. Uber die Methoden der Palichnologie. Neues Jahrb. Geol. Pal~iontol., Abh., 9 6 : 4 2 1 - - 4 5 2 . Siever, R., 1962. Silica solubility, 0--200°C, and the diagenesis of siliceous sediments. J. Geol., 70(2): 129--150. Siever, R. and Scott, R. A., 1963. Organic geochemistry of silica. In: A. Breger (Editor), Organic Geochemistry, 16. Pergamon Press, Oxford. Smith, C. F., 1948. A burrow of the pocket gopher (Geomys bursarius) in eastern Kansas. Trans. Kans. Acad. Sci., 51(3): 313--318. Taylor, H. M., 1974. Root behavior as affected by soil structure and strength. In: E. W. Carson (Editors), The Plant Root and its Environment. Univ. Press of Virginia, CharIottes4ille, N. C.

193 Toots, H., 1963. Helical burrows as fossil movement patterns. Wy. Contrib. Geol., 2(2): 129--134. Toots, H., 1965. Random orientation of fossils and its significance. Wy. Contrib. Geol., 4(2): 59--61. Voorhies, M. R., 1975a. A new genus and species of fossil kangaroo rat and its burrow. J. Mamm., 56(1): 160--176. Voorhies, M. R., 1975b. Vertebrate burrows. In: R. Frey (Editor), The Study of Trace Fossils. Springer-Verlag, Berlin, pp. 325--350. Walker, E. P., et al., 1964. Mammals of the World, II. Johns Hopkins Press, Baltimore, Md. Weaver, J. E., 1968. Prairie Plants and Their Environment: A Fifty Year Study in the Midwest. Univ. of Nebr. Press, Lincoln. Nebr. Wilmarth, M. G., 1936. Lexicon of geologic names of the United States (Including Alaska), Part I. U.S. Geol. Surv. Bull., 896.