SCOYENIA BURROWS FROM ORDOVICIAN PALAEOSOLS OF THE JUNIATA FORMATION IN PENNSYLVANIA

by GREGORY J. RETALLACK

ABSTRACT. Scoyenia beerboweri is a new ichnospecies of burrow from the late Ordovician (Ashgill) Juniata Formation in central Pennsylvania, USA. The burrows are abundant in red calcareous palaeosols, and were created by animals living at the time of soil formation, because they are ®lled with red sediment like that of the palaeosol matrix, and both cut across, and are cut by, nodules of pedogenic carbonate. The isotopically light carbon and oxygen of carbonate in the palaeosols indicate a terrestrial ecosystem of well-drained ¯oodplains in a tropical seasonally-dry semi-arid palaeoclimate. Back®ll layering within the burrows is evidence of a bilaterally symmetrical animal. Size distribution of the burrows reveals discontinuous growth, as found in arthropods. Ferruginized faecal pellets in the burrows indicate that they ingested sediment. For these reasons the burrows of Scoyenia beerboweri are most likely to be the work of millipedes. The nature of vegetation supporting them is unknown, although a single problematic plant-like fossil cast was found, and liverwort spores are widespread in rocks of this age. Vegetative biomass was limited judging from the degree of chemical weathering, extent of burial gleization and isotopic composition of carbon in the palaeosols. These distinctive respiration-dominated liverwort-millipede polsterlands lived at a time of global green- house climate, following Precambrian±Cambrian lichen-algal microbial earths and supplanted by Silurian brakelands of early vascular land plants.

KEY WORDS: Scoyenia, millipede, burrows, Ordovician, Pennsylvania.

C ALCAREOUS red beds of the late Ordovician Juniata Formation of Pennsylvania include many palaeosols containing fossil burrows (Retallack and Feakes 1987; Retallack 1992a, b, 1993). The trace fossils are common and well preserved, but have raised a variety of questions. Are the rocks marine or non- marine? Did the burrows form before, after or during soil formation? What kind of creatures formed the burrows? These questions will be further explored here, but the main purpose of this account is to formally describe these trace fossils. The ®rst discovery of evidence for burrowing organisms on land during the late Ordovician was a surprise (Retallack and Feakes 1987), because the body fossil record of terrestrial animals was then known back only to the Silurian (Almond 1985). It did however con®rm evidence for life on land as ancient as late Ordovician from spores of liverwort-like plants (Gray 1985). Now the idea is less surprising, following discovery of myriapod trackways from the mid-Ordovician (Llandeilo±Caradoc) Borrowdale Volcanics of the Lake District, England (Johnson et al. 1994). The myriapod trackways and euthycarcinoid body fossil in the ¯uvial-eolian Tumblagooda Sandstone of Western Australia (White 1990; Trewin 1993; McNamara and Trewin 1993; Trewin and McNamara 1995), probably also are late Ordovician (Iasky et al. 1998). The fossil record of early land plant spores has also improved, with liverwort-like spores now known back to the mid-Ordovician (Llanvirn: Gray 1993; Strother et al. 1996), and perhaps even mid-Cambrian (Strother et al. 1998). There also are possible late Ordovician (Ashgill) trilete spores (Nùhr-Hansen and Koppelhus 1988; Richardson 1988). Plausible, but problematic, Ordovician land plant megafossils have been reported (Argast 1992; Snigirevskaya et al. 1992). Ordovician palaeosols are also revealing details of ecosystems on land, including high rates of soil respiration (Yapp and Poths 1992, 1996; Yapp 1996; Mora et al. 1996; Retallack 1997a). Mounting evidence from fossils and palaeosols now presents an increasingly detailed view of Ordovician ecosystems on land.

[Palaeontology, Vol. 44, Part 2, 2001, pp. 209±235] q The Palaeontological Association 210 PALAEONTOLOGY, VOLUME 44

MATERIAL AND METHODS Trace fossils described and illustrated in this paper are available to the public in the Condon Collection, Oregon Museum of Natural History, Eugene (W. N. Orr, Curator). Uncollected burrows were also measured in the ®eld with vernier calipers. The trace fossils and their palaeosols were studied in petrographic thin sections and with x-ray diffractometer traces. The palaeosols also were analyzed chemically for major-elements using Atomic Adsorption by Christine McBirney of the University of Oregon; bulk density was determined using the clod method (Feakes and Retallack 1988). Carbon isotopic composition of palaeosol organic matter and carbonates was determined by M. Rice of Global Geochemistry Corp. (Retallack 1993).

GEOLOGICAL SETTING All of the studied fossil burrows were collected from the late Ordovician Juniata Formation in road cuts along a divided section of US highway 322 east of Potters Mills (Text-®gs 1±2). Strata dip at 21 degrees east at the base of a measured section (Text-®g. 2) on the western side of the highway, 100 m east of the junction with crossroads to Faust Flat and Seven Mountains Camp. Some 200 m east of the road junction and 78 m higher stratigraphically are type pro®les of the Potters Mills and Faust Flat clay palaeosols (Text- ®g. 3), from which most of the burrows were collected. Fossil burrows were found throughout the exposed section of red beds, and some were collected in loose blocks of uncertain stratigraphic level. The measured section continues south to a point above the ®rst excavated bench in the road cuts, where the dip of the beds declines to horizontal. High in the road cut here, the red late Ordovician (Ashgill) Juniata Formation is overlain by white sandstones of the early Silurian (Llandovery) basal Tuscarora Formation (Strother and Traverse 1979; Cotter 1982). The Juniata Formation represents the upper part of a thick clastic wedge which thins to the west into grey and green marine rocks (Drake et al. 1989). Red beds of the Juniata Formation have been interpreted as alluvial outwash of the high Taconic Mountains, a mixed sedimentary, metamorphic and plutonic fold- mountain range that once existed to the east (Yeakel 1962; Meckel 1970). Palaeogeographic reconstruc- tions place the Potters Mills locality, which is stratigraphically high in the Juniata Formation, on the outwash plain at least 200 km east of marine rocks to the west and about 120 km west of the mountain front (Dennison 1976). A variety of sedimentary structures within the purple-red sandstones of this outcrop con®rm their ¯uvial origin (Cotter 1978, 1982; Retallack 1993). Also important for the palaeoenvironment of the trace fossils are clayey red interbeds, which are in most cases interpreted as palaeosols (Text-®gs 2±4). Only a few ®ne-grained beds do not show some degree of ancient soil formation. Notable in this regard is a unit of grey shale with soft sediment deformation (ball- and-pillow structure) at eye level near the southern end of outcrop (108 m in section of Text-®g. 2). No trace fossils were found in this facies, which is similar to lacustrine deposits. In contrast, palaeosols are riddled with the trace fossils described here. Palaeosols in this exposure can be recognized in the ®eld from their clayey texture, sharply truncated tops with alteration down into bedded rocks, hackly appearance, and common small yellow (Munsell reddish yellow or 7´2YR7/6) carbonate nodules (Retallack 1985; Feakes and Retallack 1988). The hackly appearance is from soil clods (peds) de®ned by former cracks in the soil (now closed) and by clay coatings (now slickensided irregular surfaces, or argillans of Brewer 1976). Another feature of the palaeosols is red colour (Munsell dark reddish brown or 2´5YR3/4 to weak red or 2´5YR4/2), which is more intense with greater development of the palaeosols as indicated by degree of bioturbation of bedding and by prominence of carbonate nodules. In thin section, the palaeosols compared with their parent alluvium are much more clayey and ferruginized with submicroscopic haematite, and have fewer easily weathered grains such as mica and rock fragments. There is no trace left of laminated fabric, but in its stead is the random aggregation of clay minerals characteristic of soils (microstructure called skelmosepic by Brewer 1976). The carbonate nodules, though recrystallized, are replacive, as is usual for pedogenic carbonate (Wieder and Yaalon 1982). The carbonate is mixed calcite and dolomite, and both minerals have the light carbon and oxygen isotope composition of palaeosol carbonate, as opposed to isotopically heavy marine or lacustrine carbonate (Text-®g. 4; Retallack 1993; Koch 1998). The Potters Mills clay palaeosol shows a regular pattern of variation in chemical composition downward RETALLACK: ORDOVICIAN BURROWS 211

TEXT-FIG. 1. Road map (lower right) and topographic map (left) for locality of late Ordovician Scoyenia beerboweri ichnosp. nov. in the Juniata Formation near Potters Mills, Centre County, Pennsylvania. Contour interval is 20 feet (6´1 m). toward its parent alluvium, re¯ecting ferruginization due to oxidation, clay formation by hydrolysis and chemical leaching (Feakes and Retallack 1988). This isotopic, petrographic and geochemical evidence for open system weathering negates theories of closed-system oxidation and ferruginization during burial (Thompson 1970a), although much other burial alteration can be documented (Retallack 1985).

SYSTEMATIC PALAEONTOLOGY

Ichnogenus SCOYENIA White, 1929

Type species. Scoyenia gracilis White, 1929.

Type locality. Grand Canyon National Park, Arizona: lower part of Hermit Shale, Early Permian.

Diagnosis. `Irregularly walled, longitudinally striated burrows of slightly varying diameter, ®lled with conspicuously meniscated sediment' (Frey et al. 1984, p. 517).

Comparisons. Taenidium and Beaconites are similar back-®lled burrows to Scoyenia, but differ in lacking the striated clayey wall lining of Scoyenia (HaÈntzschel 1975). The diagnostic clayey wall lining of 212 PALAEONTOLOGY, VOLUME 44

TEXT-FIG. 2. A measured section of Late Ordovician palaeosols in the Juniata Formation near Potters Mills, Centre County, Pennsylvania. Palaeoenvironmental interpretations of facies and palaeosol pedotypes (Potters Mills and Faust Flat) are shown, as well as estimates of the degree of development of the palaeosols, per cent area of exposed carbonate nodules, and Munsell hue (following scales of Retallack 1997c).

Scoyenia can be revealed by thin sections, but is most apparent from the way Scoyenia tends to split out in bas-relief. Taenidium and Beaconites in contrast, usually split through the middle to reveal fully their meniscate back®ll. Even those portions of Taenidium and Beaconites breaking out in bas-relief show menisci rather than wall lining (Text-®g. 5). In the terminology of Bown and Kraus (1983), Taenidium and Beaconites are adhesive burrows, whereas Scoyenia is discrete. Other adhesive burrows, less similar to RETALLACK: ORDOVICIAN BURROWS 213

TEXT-FIG. 3. Interpretive sketch of view toward the west of the type Potters Mills clay and Faust Flat silty clay palaeosols in road cut exposures during 1982 (since considerably overgrown). The top of the Potters Mills clay is at the base of the upper black band on the staff, which is graduated in feet. Photographs of this view have been published elsewhere (Retallack 1985, 1993).

TEXT-FIG. 4. Chemical weathering indices, phosphorus content and stable isotopic composition of the Potters Mills clay palaeosol (above) and Faust Flat silty clay palaeosol (below) near Potters Mills, Pennsylvania (data from Feakes and Retallack 1988; Retallack 1993). 214 PALAEONTOLOGY, VOLUME 44

TEXT-FIG. 5. Different kinds of back®ll structures in trace fossils and suggested terminology. In each case the burrowing animal is assumed to have moved toward the rear of the block (adapted from Bradshaw 1981; Frey et al. 1984; Retallack and Feakes 1987; D'Alessandro and Bromley 1987; Bromley 1996).

Scoyenia, include Imbrichnus with chevronate back®ll structure (Hallam 1970; Marintsch and Finks 1982) and Imponoglyphus with conical nested back®lls (HaÈntzschel 1975). Ancorichnus is very distinct in having a central zone of U-shaped back®lls ¯anked by a wall zone of reverse back®ll (Bromley 1996). Phoebichnus, Jamiesonichnites and Cladichnus also have meniscate back®lls, but have radiating or branching burrows unlike Scoyenia (D'Alessandro and Bromley 1987). The ichnotaxonomy of Taenidium and Beaconites has been controversial and remains unsettled. Taenidium was revised by D'Alessandro and Bromley (1987), who rejected some names because of inadequate typi®cation (Keckia, Muensteria) and synonymized other comparable genera into three broadly de®ned genera (Nereites, Scolicia and Psammichnites), which all differ from Scoyenia, Taenidium and Beaconites in having a connecting thread or groove through the back®lls. Beaconites was revised by Keighley and Pickerill (1994), who maintained that it is distinguished by a thick, lined wall. This conclusion has been widely disputed (Goldring and Pollard 1995; Keighley et al. 1996; Keighley and Pickerill 1997), and is not supported by the illustrated type material of Vialov (1962) or by my inspection of specimens and the type locality of Beaconites antarcticus (Bradshaw 1981; Retallack 1997b). There is no clear clayey wall lining like that of Scoyenia or zoned mantle like that of Ancorichnus in trace fossils referred to Beaconites by Vialov (1962), Gevers et al. (1971), Ridgway (1974), Pollard (1976), Bradshaw (1981), Allen and Williams (1981), Pollard et al. (1982), BruÈck (1987), or Smith (1993). Local irregularities of Beaconites confused for wall-linings or mantles are probably deeply invaginated menisci or asymmetric menisci. Some confusion may also come from the restrictive de®nition of wall by Keighley and Pickerill (1994, p. 305), who wrote `A wall and a lining (a type of wall) are herein restricted to features actively constructed by the burrower, and are considered distinct from peripheral features produced by simple excavation or during locomotion.' I do not ®nd this restricted de®nition useful, because the wall of a mine tunnel is still a wall whether lined with bricks or left as bare rock. The term wall should be quali®ed as lined or unlined to avoid confusion (Text-®g. 5). Keighley and Pickerill (1994) made a signi®cant contribution to the terminology of wall linings, meniscus composition, pelletoidal structure, zoning and mantles (Text-®g. 5), and I have found the following additional terms useful. External ornament of the clayey wall lining may be `striated' if it has ®ne scratches (Text-®g. 5A), and `ropey' if it has ®ne scratches organized into wider scratches (Text-®g. 5B), like the threads and strands of a rope. `Arcuate' menisci are U-shaped (Text-®g. 5B), `subrectangular' menisci are distinctly ¯at-bottomed, whereas `sinusoidal' menisci are broadly W-shaped, with a sine wave cross section between the walls of the burrow (Text-®g. 5A). Sinusoidal menisci have been called `bilobed meniscate' (Smith 1993), but that term implies an unrealistically cuspate rather than sinuous central ¯exure. `Simple' meniscate burrows have menisci parallel within one another (Text-®g. 5A±C), whereas RETALLACK: ORDOVICIAN BURROWS 215 `shuf¯ed' menisci are asymmetric, and ¯aring in cross-sectional width alternately from one side of the burrow to another (Text-®g. 5D). Shuf¯ed menisci have been called `megamenisci' by Bromley and Asgaard (1979) and Frey et al. (1984), but this term is unfortunate for possible confusion with what is here called `well spaced' menisci, where the individual menisci are almost as long down the burrow as the burrow is wide (Text-®g. 5A, C). This can be contrasted with `dense' menisci in which individual menisci are so thin that a length of the burrow comparable to burrow width includes three or more menisci (Text-®g. 5B). `Normally graded' menisci vary in grain size from silt, sand or micrite to the rear of the animal's movement to clay, organic or fecal material toward the direction of the animal's movement in laying down back®lls (Text-®g. 5A), whereas `reverse graded' menisci are ®ner grained towards the base of the meniscate layer (Text-®g. 5C). This usage parallels that for normal and reverse graded bedding, and as in that case is easier to pick from the truncated surface of the ®ne-grained layer than its graded contact below. Thus normally graded menisci have the sharp contact of the ®ne-grained material more or less concave (Text-®g. 5A), whereas reverse graded menisci have the sharp contact of the ®ne-grained material more or less convex (Tex-®g. 5C). These new terms and concepts effectively distinguish the type species of Scoyenia (S. gracilis), Taenidium (T. serpentinum), Beaconites (B. antarcticus)andAncorichnus (A. ancorichnus:Text-®g.5).

Discussion. A notable feature of the type material of Scoyenia gracilis, as well as the fossils described here, is variability in ®ll, in width of the burrows along their length, and in orientation within palaeosols. They commonly have prominently meniscate back®lls, but segments of the same burrow also are ®lled with undifferentiated clayey matrix or faecal pellets (Frey et al. 1984). One approach to this variability would be to assign segments of the burrows with undifferentiated ®ll to Palaeophycus and segments with faecal pellets to Planolites, while noting that in this case they are compound ichnotaxa (Pemberton and Frey 1982; Goldring and Pollard 1995). Another approach is to assign poorly preserved segments of a burrow to the ichnogenus appropriate for its better preserved segments (Keighley and Pickerill 1996). This approach was followed here because the most distinctive features of Scoyenia in the ®eld are their ornamented clayey wall linings and meniscate back®lls. Because of common cover by the wall lining and indistinctness of faecal pellets, the faecal and undifferentiated ®ll is only convincingly revealed by petrographic thin sections. Furthermore, undifferentiated ®ll appears largely due to local collapse of poorly preserved burrows. Such taxonomic lumping is more commonly applied to trace fossils of palaeosols than in marine rocks, because of the complex morphology and multiple use of burrows in soils by dung beetles, bees, termites and mammals (Retallack 1984, 1990; Genise and Bown 1994; Hasiotis and Dubiel 1995; Hasiotis and Demko 1996). There has also been a tendency to consolidate laterally variable marine trace fossils into single ichnotaxa, because the trace maker can be better understood from a variety of its activities than from only one activity (D'Alessandro and Bromley 1987; Bromley 1996; Keighley and Pickerill 1996). Scoyenia is best known as an indicator of non-marine ichnofacies (Seilacher 1967). This palaeoenvir- onment is con®rmed by the abundance of Scoyenia in palaeosols, as is clear from the material described here, as well as from my observations of the type species (Scoyenia gracilis) in palaeosols along the Kaibab Trail, in the Grand Canyon, Arizona (White 1929). As far as I am aware, Scoyenia has not yet been recorded from marine rocks (Bromley and Asgaard 1979; Frey et al. 1984), although many ichnogenera are found in both marine and non-marine rocks (Retallack 1976; Buatois et al. 1998). However Scoyenia is neither widespread nor representative of non-marine ichnofacies, considering the diversity of non-marine ichnofossils discovered since Seilacher's (1967) pioneering classi®cation of ichnofacies. Within palaeo- sols, many distinct assemblages can be recognized, represented for example by Coprinisphaera and Termitichnus as well as Scoyenia, and additional ichnofacies characterize eolian, lacustrine, deltaic and coal measure sequences (Buatois et al. 1998).

Scoyenia beerboweri ichnosp. nov. Text-®gs 6±7 1985 Skolithus sp. Retallack, p. 118, pl. 1, ®gs 4±5. 1985 Planolites sp. Retallack, p. 118, p. 1, ®g. 6. 1987 fossil burrows, Retallack and Feakes, p. 61, ®gs 1±3. 216 PALAEONTOLOGY, VOLUME 44

TEXT-FIG. 6. Fossil burrows of Scoyenia beerboweri ichnosp. nov. from Ordovician palaeosols near Potters Mills. A, subhorizontal burrows from a loose block. B, H, vertical burrows with wrinkled clayey lining from upper Faust Flat clay palaeosol. C, E, subhorizontal burrows with lower medial groove from near the surface of the Potters Mills clay palaeosol. D, G, subhorizontal burrows turning vertical from loose blocks. H, burrow encrusted with pedogenic carbonate low in Potters Mills clay palaeosol. Arrows indicate burrow margins. Scale bars represent 10 mm. Condon Collection numbers are F35149 (A), F36201 (D, G), F36203 (E), F36204 (C), F36205 (F), F36206 (I), F36208 (B, holotype), and F36209 (H).

Holotype. Oblique back®lled burrow, F36208, Condon Collection, Oregon Natural History Museum (Text-®g. 6B).

Diagnosis. Scoyenia with ®nely striated clayey lining, sometimes including a basal median groove; burrows subhorizontal within upper 10 cm of palaeosol, below that vertical and penetrating an additional 40 cm down into the parent material; back®lls are sinusoidal, subrectangular and arcuate, simple to slightly RETALLACK: ORDOVICIAN BURROWS 217

TEXT-FIG. 7. Burrows of Scoyenia beerboweri ichnosp. nov. in petrographic thin section, showing back®lls (A±E), faecal pellets (F±G), clay-lined walls (C±G), burrows truncated by calcareous nodules (A, C) and burrows truncating pedogenic carbonate (H). Scale bars represent 1 mm. Specimens are from the lower Potters Mills clay palaeosol (A±E, G±H) and upper Faust Flat clay palaeosol (F). Condon Collection numbers are F36210 (H), F36211 (A, D), F36212 (E), F36213 (B), F36215 (C, G), and F36216 (F). shuf¯ed, moderately well spaced to densely meniscate, and normally-graded in grain size, with truncated surface of the clayey concave face.

Derivation. The species is named in honour of Richard Beerbower, who introduced me to these palaeosols and to studies of early terrestrial ecosystems.

Additional specimens. Other specimens from near Potters Mills used to characterize this species are Condon collection numbers F35149, F36201±F36216. Burrows comparable to Scoyenia beerboweri were also seen at several other road cuts through the Juniata Formation in Pennsylvania: 1 km east of Reedsville, 1 km west of Matternville, and 1 km east of Loysburg. Comparable burrows have also been found in the late Silurian (Ludlow) Bloomsburg Formation near Palmerton, Pennsylvania (Retallack 1985).

Type locality. Road cut 4 km east of Potters Mills, Centre County, Pennsylvania: upper Juniata Formation; late Ordovician (mid-Ashgill). 218 PALAEONTOLOGY, VOLUME 44

TEXT-FIG. 8. Three dimensional distribution of burrows of Scoyenia beerboweri ichnosp. nov., in the type Potters Mills clay palaeosol (A, F36202A±J) and the type Faust Flat clay palaeosol (B, F36207A±E), near Potters Mills Pennsylvania. Scale bars represent 10 mm.

Description. Scoyenia beerboweri burrows are locally conspicuous in clayey horizons interpreted as palaeosols (Text- ®gs 2±4; Retallack 1985, 1993), imparting a strongly bioturbated appearance to their upper parts (ichnofabric index 3 of Droser and Bottjer 1986) and a less bioturbated pipe-rock appearance to the lower part of the palaeosol (ichnofabric index 2). Serial sectioning of large blocks revealed that the subhorizontal galleries bend down to terminate within the vertical galleries (Text-®g. 8). The burrows vary in width from 1 to 21 mm (Text-®g. 9). Internally they show a variety of structures, even within a single specimen. One burrow for example (Text-®g. 7A, G) showed sinusoidal to subrectangular and simple meniscate back®ll structures, as well as massive and pelletoidal ®ll within only a 2 cm length. In others the meniscate ®ll is disrupted as an escape structure (Text-®g. 7D±E). Other burrows show faecal pellets which are ellipsoidal to conoid in shape (Text-®g. 7F±G). The burrows also vary in their surfaces, ranging from ®nely striated clay (Text-®g. 6C) to rough with encrustation of pedogenic carbonate (Text-®g. 6I), again in different segments of the same burrow. This variability can be seen in relatively short lengths of the burrows and re¯ects local differences in preservation and in behavioural repertoire of a single trace maker.

Comparison. The only other validly named species of Scoyenia is S. gracilis which has ropey scratches modelled by the clayey lining to the burrow (Frey et al. 1984), unlike the ®nely striated walls of S. beerboweri. I have not seen any hint of a groove or median thread in any specimens of Scoyenia gracilis from type area of the Hermit Shale in the Grand Canyon, although these features are found in several specimens of S. beerboweri (Text-®g. 6B, F±G). There also are differences in back®lls of Hermit Shale Scoyenia gracilis, which are arcuate, simple and dense, whereas back®lls of S. beerboweri are both dense and moderately well spaced, arcuate, subrectangular, and sinusoidal, simple and slightly shuf¯ed (Text-®g. 7A±D). In all these features, German and French Permian Scoyenia gracilis is like the Arizonan type material and also differs from S. beerboweri (Schwab 1966; Martens 1975). However, fossil burrows referred to Scoyenia `gracilis' from the Triassic Fleming Fjord Formation of Greenland (Bromley and Asgaard 1979) have thick, shuf¯ed meniscate back®lls, and less elongate, less highly oriented, wall scratches than either S. gracilis or S. beerboweri, and may represent a distinct species. Similar to these Greenland fossils are burrows assigned to Scoyenia `gracilis' from the Triassic±Jurassic Newark Supergroup, USA (Olson 1977; Metz 1995), which I have also examined. Scoyenia burrows from the Late Triassic Chinle Group of Arizona are reported to have the distinction of paired scratches on the clay- lined wall (Hasiotis and Dubiel 1993). Two additional distinct forms of Scoyenia are burrows with very widely spaced menisci from the Carboniferous±Permian San Giorgio Formation of Sardinia (Bechstaedt 1983) and burrows with very wide, shuf¯ed meniscate back®ll from the Eocene Duchesne River Formation of Utah (D'Alessandro et al. 1987). Other occurrences inadequately illustrated and described RETALLACK: ORDOVICIAN BURROWS 219

TEXT-FIG. 9. Width distribution of Scoyenia beerboweri ichnosp. nov. from Ordovician type Potters Mills clay (A) and Faust Flat silty clay (B) palaeosols near Potters Mills, Pennsylvania. Subhorizontal burrows near the palaeosol surface are connected with near-vertical burrows deeper within the palaeosols (black). This compilation includes measure- ments of Retallack (1993), additional to those of Retallack and Feakes (1987). for comparison include traces referred to Scoyenia from the Cretaceous, LoÂs Santos Formation of Columbia (Vargas et al. 1985), and the Triassic Sloan Canyon and Sheep Pen formations of New Mexico (Conrad et al. 1987).

Discussion. In a preliminary publication (Retallack 1985), burrows of Scoyenia beerboweri were provisionally referred to Skolithos and Planolites, but subsequent serial sectioning of the burrow systems demonstrated that the horizontal (Planolites-like) and vertical (Skolithos-like) components form a single burrow system (Retallack and Feakes 1987), as also found in the type material of Scoyenia gracilis (Frey et al. 1984). There is also the meniscate ®ll of Scoyenia beerboweri, unlike the structureless clayey ®ll of Planolites (Pemberton and Frey 1982; Alpert 1975), or the passive sandy ®ll of Skolithos (Alpert 1974).

BIOLOGICAL INTERPRETATION OF THE BURROWS The ichnogenus Scoyenia has been attributed to arthropods (Frey et al. 1984), insects, polychaetes (D'Alessandro et al. 1987), and decapod crustaceans (Olsen 1977). An important issue for narrowing options for the trace maker of Scoyenia beerboweri burrows is their relationship to enclosing palaeosols. Did they form before, during or after the palaeosols? Were they marine or non-marine? These issues are addressed before proceeding with biologically signi®cant features of the burrows and comparisons with living and fossil animals.

Environmental context Petrographic and geochemical studies of the palaeosols provide the most convincing evidence that Scoyenia beerboweri burrows were excavated at the same time as the palaeosols developed. The density of the burrows, particularly the subhorizontal ones, varies with other indices of soil formation, such as the abundance of clay skins and carbonate nodules, iron and aluminium enrichment and intensity of red colour 220 PALAEONTOLOGY, VOLUME 44 (Feakes and Retallack 1988). In short, the best developed palaeosols have the most burrows. Only the thick red burrowed units also contain carbonate nodules, which are known from their replacive petrographic appearance and light carbon isotopic values to have been pedogenic and to have formed under a regime of high soil respiration (Text-®g. 4; Retallack 1985, 1993; Yapp and Poths 1994). Carbonate nodules are found isolated in the soil matrix as well as ensheathing some of the burrows (Text-®g. 6I). In the type Potters Mills clay palaeosol 81 out of 157 burrows (61 per cent) were ensheathed with carbonate. Some nodules are cut by the burrow walls (Text-®g. 7H) and other nodules cut across ®lled burrow walls (Text- ®g. 7C). This is the kind of distribution of carbonate expected if burrowing and nodule precipitation occurred during a period of soil formation. Fossil burrows in the outcrop near Potters Mills (Text-®g. 1) have been interpreted as marine (Cotter 1982), and one could make an argument that the burrows represent an aquatic environment predating soil formation. However, Scoyenia beerboweri burrows are well preserved with sharp outlines. One could argue that burrows would not be destroyed in an Ordovician soil lacking abundant animals responsible for the bioturbated fabric of many modern soils. However, those burrows ®lled with non-meniscate ®ll, and formerly open, would have collapsed, or their clayey walls ¯aked off during a prolonged period of soil formation on a burrowed marine or lacustrine shale. Considering the degree of development of pedogenic carbonate in these palaeosols compared with that in modern soils of known age, the palaeosols were exposed for periods of 4000±5000 years in a dry palaeoclimate (Retallack 1985, 1993). From these observations it is unlikely that the burrows predate soil formation. Yet another idea is that the soils formed subaerially without burrows and then were inundated by lake or lagoonal waters with an aquatic burrowing fauna (Driese and Foreman 1991, 1992a, b). However, the ®lling material of the burrows is uniformly red, highly oxidized material similar in chemical and mineralogical composition to the surrounding palaeosol matrix. Such deep and complicated burrow systems in relatively impermeable shale would not have remained so oxidized during inundation. Pyritization or reduction of the burrow ®lls, and partial ®ll with grey estuarine or lacustrine shale would be expected following inundation of palaeosols. There are small, diffuse, reduction spots and layers near the surface of some of the palaeosols, but none ®lling the burrows, nor any rock or fossil trace of an omission sequence. Ostracods and lingulid brachiopods are especially conspicuous in their absence, because they are locally abundant in intertidal deposits of Ordovician and Silurian age in this region (Hoskins 1961; Driese and Foreman 1991; Patzkowsky 1995; Swain 1996). At least one of the many burrowed surfaces should have preserved marine fossils or drab sediments if the sea or lakes had inundated this area with the observed frequency of the burrowed horizons (Text-®g. 2). The red burrows in palaeosols near Potters Mills, Pennsylvania, are very different texturally and chemically from pyritized and drab burrows with associated marine fossils into the uppermost red, late Ordovician, Juniata Formation at Beans Gap, Tennessee (Driese and Foreman 1991, 1992a, b). Another line of argument against a marine origin of Scoyenia beerboweri is the very different trace fossil assemblages in associated lacustrine and marine rocks. Marginal marine deposits of the late Ordovician (Ashgill) Bald Eagle and Juniata Formations with brachiopods and marine clams (Orthor- hynchula-Ambonychia community of Bretsky 1969; Thompson 1970b) yield trace fossils of Arenicolites, Arthrophycus, Lockeia, Planolites, Rusophycus, Sinusites, and Skolithos (Freile and Baldwin 1988; Driese and Foreman 1991). The overlying early Silurian Tuscarora Formation is marine near Potters Mills, with a trace fossil assemblage dominated by Arthrophycus and Monocraterion, and including Arenicolites, Cruziana, Phycodes, Planolites, Rusophycus and Skolithos (Cotter 1982; Friele et al. 1988; Dorsch et al. 1994). No trace fossils were seen in possible lacustrine facies of the Juniata Formation (108 m in Text-®g. 2). Throughout their long fossil record, Scoyenia-bearing palaeosols have represented a very different ichnofacies of soil animals, unlike trace fossil assemblages of marine and lacustrine rocks (Buatois et al. 1998).

Biologically signi®cant features of the burrows Direct fossil remains of the burrowing organisms are unlikely to be preserved in such oxidized calcareous palaeosols of well drained locations and dry palaeoclimates (Retallack 1998), yet the living animals were RETALLACK: ORDOVICIAN BURROWS 221 capable of withstanding dry soil conditions. Former mean annual rainfall of palaeosols can be estimated from depth to their calcic (Bk) horizon using a transfer function derived from modern soils (Retallack 1994a), and correcting for burial compaction with a standard equation (Retallack 1997c). The Potters Mills clay palaeosol with a Bk horizon now at a depth of 19 cm (Text-®g. 4), but originally more like 23±34 cm, gives mean annual rainfall estimates of 280±342 6 141 mm. There is no indication from chemical analyses for sodium enrichment that would be expected during salinization found in soils of dry climate (less than 300 mm mean annual rainfall: Retallack 1985, 1993). Thus a semiarid climate of some 300±500 mm per annum is indicated by the Potters Mills clay palaeosol. Such water stress may have been exacerbated by high temperature and evapotranspiration, considering the low late Ordovician palaeolatitude of these palaeosols (Ziegler et al. 1979). Monsoonal seasonality in precipitation and a palaeotemperature of 238C was estimated from oxygen isotopic analysis of a late Ordovician palaeosol in Wisconsin (Yapp 1993). Animals in the burrows excreted solid faeces that dried out, as indicated by faecal pellets with distinct ferruginized margins. The faeces include much clay and silt, which was probably ingested by the animal, for the purpose of burrowing through stiff soil, processing for organic food, or both (Text-®g. 7F±G). The fossil faeces are broadly ellipsoidal to conoid in shape, some 300±500 mm in minor diameter and 600± 900 mm in major diameter. The diameter of the digestive tract and anus of the organism was thus some 20± 25 per cent of its body diameter. A variety of small organisms leave faecal pellets in burrows of larger organisms (Retallack 1990), but these proportions indicate that the faecal pellets are from an organism as big as the burrows. The burrowing organism also had bilaterally symmetrical limbs with ®ne points or bristles to create the ®ne striation of the burrow wall (Text-®g. 6A±C). The burrow back®ll structures of alternating siltstone and claystone show subrectangular to sinusoidal outlines in both longitudinal and transverse section (Text-®g. 7A±B), indicative of a bilaterally symmetrical organism, rather than a worm-like creature. There is a median groove on the underside of some of the back®lled burrows preserved in bas-relief (Text-®g. 6B±C, F), which corresponds to the central undulation of sinusoidal back®lls seen in thin section. Burrow diameters range from 1 to 21 mm, but are polymodal (Text-®g. 9), as if this was an animal like an arthropod that grew in distinct increments or moult stages. Comparable data have long been known from fossil marine arthropods such as trilobites (Kopaska-Merkel 1988; Sheldon 1988; Chatterton and Speyer 1997), and also are known from other fossil non-marine burrows and trackways of arthropods (Stanley and Fagerstrom 1974; Bradshaw 1981; Bracken and Picard 1984; Trewin and McNamara 1994). Data shown here include additional measurements to those reported in 1987 (Retallack 1993) and were pooled from both subhorizontal and vertical burrows (white and black respectively in Text-®g. 9), which were found to be interconnected (Text-®g. 8). All measurements were taken with vernier calipers accurate to 100 mm. Modes in these expanded data are signi®cant using the Komolgorov-Smirnov test (Kopaska- Merkel 1988; Retallack 1993). Both new data-sets of width of Scoyenia beerboweri from the Potters Mills and Faust Flat palaeosols fail to show a signi®cant ®t to a normal distribution (correlation coef®cient, r ˆ 0´76). These general considerations narrow options for the trace-maker to soil-ingesting arthropods tolerant of dry climates.

Comparisons with modern organisms Among burrows and faecal pellets of living soil arthropods, Scoyenia beerboweri is most like those of millipedes (Diplopoda: Romell 1935; Toye 1967; Paulusse and Jeanson 1977; Bullock et al. 1984). Shuf¯ed and sinusoidal menisci of Scoyenia beerboweri were probably created by packing behind the animal of soil carried back by the legs. Broadly comparable back®lls have been found on the burrow walls of snake millipedes (Paulusse and Jeanson 1977) and in the burrows of mole crickets (Orthoptera, Gryllotalpidae), beetles (Coleoptera, Cynidae) and crane-¯y larvae (Diptera, Tipulidae: Willis and Roth 1962; Ahlbrandt et al. 1978; Ratcliffe and Fagerstrom 1980; Pemberton et al. 1992). Irregularities in back®ll here termed shuf¯ed meniscate (Text-®g. 5) can be attributed to digging orientations slightly oblique to the long axis of the burrow, for the purpose of gaining purchase. Short animals like beetles (Coleoptera) can straddle their burrows obliquely (Willis and Roth 1962), and snake millipedes (Julida) adopt a helical posture as a brace for head ramming (Manton 1954). 222 PALAEONTOLOGY, VOLUME 44

Whereas some back®lls of Scoyenia beerboweri are thin and poorly sorted (Text-®g. 7D), other menisci show extreme grain size sorting (Text-®g. 7A±B). The thick ®ne layers were probably packed deliberately, perhaps with faecal material. After a layer of soil was carried back by the legs, a faecal cap would then be compacted by backward ramming. These may have been locally hard parts of the soil, in which living millipedes are known to resort to earthworm-like ingestion of soil to supplement ramming and digging (Eisenbeis and Wichard 1987). Living millipedes also fashion faecal pellets into a variety of structures such as wall linings, moulting and egg chambers (Romell 1935; Toye 1967). Some of the wall linings appear to have been actively smeared by the trace maker (Text-®g. 6B±C), but in others their internal lamination is more like a clay skin washed down within a soil (Text-®g. 7D). Faecal-sediment alternation is not limited to millipedes, and is seen, for example, in meniscate burrows of the Eocene marine trace fossil Taenidium satanassi (D'Alessandro and Bromley 1987). The marine fossil burrows have clearly preserved faecal pellets and the sharply truncated edge of the ®ne-grained faecal portion is convex (Text-®g. 5). Unlike the maker of Scoyenia beerboweri, the marine animal was tamping sediment rather than faeces. The opposite behavior of tamping faeces onto sediment layers inferred for S. beerboweri may have been an effort to shore up walls of dry, silty soil, subject to collapse. The bottom side walls of one specimen (Text-®g. 7B) show evidence of such small-scale slumping. The sinusoidal meniscate back®lls of Scoyenia beerboweri (Text-®g. 7A±B) are from an animal with legs near the edges of the body, but not projecting well beyond it as in centipedes, pauropods, symphylans, isopods, many insects and polydesmid millipedes (Text-®g. 10). Nor would the centrally- raised parts of the back®ll of S. beerboweri have remained if the body was cylindrical with centrally attached limbs as in snake millipedes (Julida) such as Cylindroiulus (Text-®g. 10). The animal excavating S. beerboweri more likely had a semicircular or lenticular cross section as in pill millipedes (Glomeridae) and polyzoniid millipedes (Polyzoniida). The distinct ¯oor and roof ®ll of some of the burrows (Text-®g. 7B) is compatible with a boring technique of burrowing, as shown by the polyzoniid Polyzonium germanicum, rather than wedging of polydesmids or bulldozing of julids (Eisenbeis and Wichard 1987; Hopkin and Read 1992). Like those in Scoyenia beerboweri, the faeces of millipedes are strongly pelleted because they are excreted with peritrophic membranes from a hindgut that strongly resorbs moisture (Shear and Kukalova- Peck 1990; Hopkin and Read 1992). Millipedes commonly ingest soil with their leaf mould and in burrowing (Lewis 1971; Bailey and de Mendonca 1987; Eisenbeis and Wichard 1987; Hopkin and Read 1992). Some parts of the burrows of Scoyenia beerboweri are passively ®lled with material similar to palaeosol matrix (Text-®g. 7G±H). Presumably these were in part open holes into which the animals retreated during the heat of the day or over the dry season. Scoyenia beerboweri were probably multipurpose feeding, dwelling and breeding burrows like those of terrestrial burrows of millipedes, ants, bees and termites (Hasiotis and Dubiel 1995; Hasiotis and Demko 1996). Water stress is an important physical constraint for millipedes, but transpiration rates of millipedes are among the lowest known among soil arthropods, much lower than in centipedes, insects, collembolans and isopods (Eisenbeis and Wichard 1987). Millipedes can be common in desert regions, where they reduce water loss with exceptionally low-permeability cuticles, waxy epicuticles, large size and dormancy in the dry season (Crawford et al. 1987). The histogram of Scoyenia beerboweri width (Text-®g. 9) is inadequate for precise reconstruction of growth stages, as has been found in the study of trilobites where additional measures such as length and segment number are needed (Sheldon 1988; Kopaska-Merkel 1988; Chatterton and Speyer 1997). Nevertheless, there is nothing unusual about these measurements of S. beerboweri compared with similar data from living millipedes. For example, the 11±12 modes observed in width of S. beerboweri (Text-®g. 9) are comparable in number to 11 growth stages recognized in living Julus scandinavius, which has successive width increments of 0´1±0´3 mm from a width at stage III of 0´05 6 0´03 mm to stage XI at 2´47 6 0´24 mm wide (Blower 1970). This small species passes through four moults per year, and dies after three years. Like S. beerboweri, other species of millipedes are an order of magnitude larger, especially in tropical dry regions (Lewis 1971; Crawford et al. 1987). Ophistreptus guineensis with a diameter up to 20 mm, is close to the maximal diameter of S. beerboweri, and was considered by Manton (1954) to be near the upper size limit for a millipede capable of burrowing. The large desert millipede Archispirostreptus syriacus takes up to nine RETALLACK: ORDOVICIAN BURROWS 223

TEXT-FIG. 10. Cross-sections of body segments of millipedes, a centipede, isopod, arthropleurid and trilobite, for consideration of their likely burrowing pro®le (A, B adapted from Manton 1979; C±E, G±H from Eisenbeis and Wichard 1987; F from Hopkin and Read 1992). years to mature because of a short annual growth period and poor quality and quantity of its food (Crawford et al. 1987). Narceus americanus has an annual moult during the dry season (O'Neill 1969). Life spans of 11 years are known in Glomeris marginata (Carrel 1990). Annual moulting and longevity of about a decade for animals making Scoyenia beerboweri are compatible with its size and the nature of its palaeosols (Feakes and Retallack 1988). Other kinds of creatures can be ruled out for Scoyenia beerboweri on the basis of their different modern behaviour and water relations. Many insect burrows are more complex than S. beerboweri, with branches and chambers (Ratcliffe and Fagerstrom 1980; Buatois et al. 1998). Isopods are mostly surface dwellers, but some desert species form elaborate burrows with branches and chambers to depths of a metre (Shachak 1980; Warburg 1993). Centipedes do not burrow, but use existing cracks within the soil, as do the centipede-like Symphyla and Pauropoda (KuÈhnelt 1976; Eisenbeis and Wichard 1987) and many millipedes (Hopkin and Read 1992). Tardigrades are commonly aquatic, but many live in moss cushions and waterlogged soils (Kinchin 1994). Neither tardigrades nor onychophorans form burrows, relying instead on pre-existing cracks in the soil (Kinchin 1994; Hay and Kruty 1997). Onychophorans and worms do not show such distinct size classes, nor do they tolerate soil dry enough to include pedogenic calcite and dolomite (Little 1990; Hay and Kruty 1997). Carnivorous spiders would not have left faecal pellets including a lot of silt and clay, and spiders do not make such lengthy burrows (Ratcliffe and Fagerstrom 1980; Eisenbeis and Wichard 1987).

Comparisons with extinct animals The early Palaeozoic fossil record of myriapods remains sparse. The oldest known fossil myriapod is Cambropodus from Cambrian marine rocks of Utah (Robison 1990). As reconstructed by Delle Cave and 224 PALAEONTOLOGY, VOLUME 44 Simonetta (1991), Cambropodus is most like a living scutigerimorph centipede, which are denizens of crevices in buildings, mammal burrows, and leaf litter (Lewis 1981). Regardless of whether it was marine or terrestrial, this Cambrian animal is too slender-limbed to have been a burrower. Another puzzle is an early Silurian uniramian from marine shales at Waukesha, Wisconsin (Mikulic et al. 1985a, b). Heterotomy (short-long alternation of tergites) and overall body form of this fossil are similar to lithobiomorph centipedes, which are now creatures of leaf mold and soil crevices, and do not burrow (Lewis 1981). Extinct devoniobiomorph centipedes appear by late Silurian and Devonian, by which time they are at last in non-marine settings (Jeram et al. 1990; Shear et al. 1996; Shear 1998). There are no fully marine centipedes today, although large populations live on the sea-shore (Lewis 1981) and some have survived as long as 178 days submerged in sea-water (Suomalainen 1939) and 64 days in fresh water (Schubart 1929). If Cambropodus and the Waukesha uniramian were indeed marine centipedes, then centipedes predate millipedes, as is also indicated by phylogenetic analyses (Wills et al. 1998; Wheeler 1998). While indicating a possible pre-Ordovician ancestry of myriapods, all these centipede-like creatures had limbs projecting too far from the sides of their bodies to have excavated Scoyenia beerboweri. Extinct Carboniferous arthropleurids and dorso-ventrally ¯at, Devonian millipedes (such as Archidesmus) appear to have been functionally similar to living polydesmid millipedes of the wedge ecomorph (Almond 1985; Shear 1998), which crawl within litter and use existing cracks in soil, but do not burrow (Manton 1979; Hopkin and Read 1992). Even more extremely maladapted for burrowing are extinct millipedes bristling with long, branched spines (Amynilypses and Acantherpestes: Hannibal 1997; Shear 1998). Kampecarids are another extinct millipede ecomorph with a bulbous posterior, and were probably aquatic (Almond 1985; Edgecombe 1998). The idea of extinct aquatic millipedes should not be surprising because several species of normally-terrestrial millipedes survive under water for as long as 75 days (Adis 1986). Carboniferous Isojulus and Nyranius (Kraus 1974) are comparable to living julid and spirobolid millipedes, both bulldozer ecomorphs (Hopkin and Read 1992). Such creatures are plausible excavators of some of the fossil burrows identi®ed as Beaconites antarcticus (Bradshaw 1981; Keighley and Pickerill 1994). The borer ecomorph thought more likely for Scoyenia beerboweri on the basis of its sinusoidal back®lls (Text-®g. 5) is exempli®ed by Pleurojulus, a Carboniferous colobognath (Hofman 1969; Dzik 1981). A late Silurian (Pridoli) millipede from the Old Red Sandstone at Stonehaven, Scotland (Almond 1985), also has ¯exible tergites and prominent sternites as in borer ecomorphs (of Eisenbeis and Wichard 1987; Hopkin and Read 1992). This is closest among known fossils to the kind of creature envisaged for Scoyenia beerboweri burrows. Fossils other than millipedes include no likely excavators of Scoyenia beerboweri. An onychophoran- tardigrade plexus of Cambrian fossils including animals much larger and probably with harder exoskeletons than their living descendants (Delle Cave and Simonetta 1979; RamskoÈld and Chen 1998) is now thought to be within the basal arthropod evolutionary radiation (Ballard et al. 1992; Friedrich and Tautz 1995; Wills et al. 1998). These Cambrian fossils are all presumed marine and many had feathery appendages unsuitable for burrowing. However, the early Cambrian aglaspid Kodymirus, the mid- Ordovician xiphosur Chasmataspis, and the late Ordovician euthycarcinoid uniramian Kalbarria were found in non-marine facies (Gray 1988; McNamara and Trewin 1993; Iasky et al. 1998). Kalbarria is associated with trackways indicating that it ventured onto land with associated eurypterids (Trewin and McNamara 1995). Euthycarcinoids, aglaspids, early xiphosurs and eurypterids have similar ¯attened and streamlined body shapes, and were presumably primarily aquatic. Even though fossil tracks indicate that they were occasionally amphibious, they are unlikely to have burrowed. A fossil super®cially identical to an earthworm has been found in Ordovician marine rocks (Morris et al. 1982), but trace fossil evidence of earthworms on land is still unreported earlier than Triassic (Retallack 1976). Trigonotarbid arachnids appeared in the Silurian (Jeram et al. 1990), and true spiders and wingless insects in the Devonian (Labandeira et al. 1988; Labandeira 1997).

PLANT-LIKE FOSSIL There is little evidence of the vegetation of these Ordovician palaeosols expected as fodder for their abundant burrowers, nor would it be expected to have fossilized given the highly oxidized chemical RETALLACK: ORDOVICIAN BURROWS 225

TEXT-FIG. 11. Problematical calcite cast of a plant-like fossil from late Ordovician lacustrine facies in the Juniata Formation near Potters Mills, Pennsylvania (Condon collection number F35150). Scale bar represents 10 mm. composition of the palaeosols (Retallack 1998). However, a single specimen of thin, yellow, calcite-®lled tubes was found in red, bedded, siltstone, loose at the surface 100 m to the south of the type pro®les of palaeosols that were the focus of this investigation (Text-®g. 11). The clear relict bedding in this specimen is most like a thin unit interpreted as lacustrine in the cliff above this site (108 m in Text-®g. 2). The fossil is a calcite-®lled external cast of a body fossil. The main axis of the fossil, 2 mm in diameter, runs vertically through this relict bedding, and has verticils of slender (1 mm), dichotomously forked axes, at an interval of 15 mm (Text-®g. 11). This fossil is not named here, but reported as a search image for future collectors to ®nd more representative specimens. Its af®nities are a puzzle. The specimen is unlike mosses, cooksonioids, zosterophylls, or the problematic Akdalophyton, in lacking visible enations or consistently dichotomous branching (Snigirevskaya et al. 1992; Kenrick and Crane 1997). It also is more regular in its branching pattern than rhizomes, rhizines and other root-like traces found in Silurian and Devonian palaeosols (Retallack 1992b; Driese et al. 1993, 1997; Elick et al. 1998; Retallack et al. 1998). Nor is it a thalloid fossil comparable to bryophytes, extinct nematophytes, or other problematic fossils (Strother 1988; Argast 1992; Edwards et al. 1995a). There is some resemblance to Pinnatiramosus, an early Silurian (Llandovery) vascular plant from China (Cai et al. 1995, 1996), but the fossil from Pennsylvania has fewer laterals and these branch dichotomously. The verticillate laterals of the fossil from near Potters Mills (Text-®g. 11) are most like a charophyte alga, and such an interpretation would be compatible with its likely lacustrine facies, because living Chara and Nitella are common pond weeds. Charophyte fossils are known as far back as latest Silurian (Taylor et al. 1992; Feist and Feist 1997) and have been widely touted as ancestors of embryophytic land plants (Stebbins and Hill 1980; Graham 1993). However, the most likely ancestral forms are thalloid, like living Colochaete (Kenrick and Crane 1997), and unlike this ®lamentous Ordovician fossil. 226 PALAEONTOLOGY, VOLUME 44

TEXT-FIG. 12. A reconstruction of the Potters Mills clay palaeosol and its biota during the Late Ordovician. Scoyenia beerboweri ichnosp. nov. is the only trace fossil known from these palaeosols. Tetrahedaletes grayae is widespread in rocks of similar age (Strother 1991). The millipede is modelled after the Stonehaven specimen of Almond (1985) and the liverworts after fossils of Edwards et al. (1995a). This reconstruction depicts minimal likely biomass, in contrast to maximal likely biomass depicted by Feakes and Retallack (1988).

ORDOVICIAN LIVERWORT-MILLIPEDE POLSTERLANDS Although pre-Devonian landscapes commonly continue to be regarded as sterile as the surface of Mars (McGhee 1996), the evidence presented here reveals a different picture (Text-®g. 12). Scoyenia beerboweri is the most obvious of remains of a formerly widespread late Ordovician ecosystem of alluvial landscapes in semi-arid, seasonally dry tropical regions. These distinctive burrows can be attributed to millipedes, because of a variety of details of their ®ll and size distribution (Retallack and Feakes 1987). Furthermore, isotopic indication of high soil respiration indicates large populations of animals and heterotrophic microbes in late Ordovician palaeosols (Yapp and Poths 1994). Such large and complex animals were surely supported by a variety of other organisms not preserved. Traces of plants and of humi®cation in these palaeosols are meagre. Yet plants or plant-like microbes must have been present to fractionate isotopes of carbon in organic matter and carbonate of the palaeosols (Text-®g. 4: Retallack 1993; Koch 1998). Liverworts were a likely part of this community, judging from the wide distribution of their spores in Ordovician rocks (Gray 1993; Strother et al. 1996). These liverwort-millipede polsterlands of the Late Ordovician represent a distinct phase in the evolution of early land communities (Eoem- bryophytic of Gray 1993), replacing microbial earths of the Cambrian±Precambrian (Retallack 1992b, 1994b; Retallack and Mindszenty 1994) and later supplanted by tracheophyte-trigonotarbid brakelands of the Silurian±Devonian (Retallack 1992a, b; Eotracheophytic of Gray 1993). As in overgrazed semi-arid RETALLACK: ORDOVICIAN BURROWS 227 tropical ecosystems today (Ahn 1970; Jones and Wild 1975), animals and respiring microbes of Ordovician polsterlands dominated their plant resources to keep the soil clean of organic matter. Ordovician communities were distinct in their high oxygen consumption relative to low carbon sequestration and meagre primary production (Retallack 1996). The precise temporal range of Ordovician liverwort-millipede polsterlands remains poorly constrained. The liverwort component of these communities dates back to mid-Ordovician (Llanvirn), or perhaps even mid-Cambrian, judging from their spore record (Gray 1993; Strother et al. 1996, 1998). The most ancient records of the myriapod component are trackways from late Ordovician (Llandeilo±Caradoc) Borrowdale Volcanics of the Lake District, England (Johnson et al. 1994). Millipede burrows are found through much of the Juniata Formation of Pennsylvania (Text-®g. 2; Retallack 1985), corresponding to early±middle Ashgill (Thompson 1970a), but remain unproven for the latest Ordovician (Hirnant) because of local unconformities (Dennison 1976; Dorsch et al. 1994). The hegemony of liverworts and millipedes was challenged by the appearance of mosses or other stomatophytes as rare fossils in the late Ordovician (Caradoc±Ashgill: Nùhr-Hansen and Koppelhus 1988; Snigirevskaya et al. 1992; Strother et al. 1996; Kenrick and Crane 1997), a rise to dominance of vascular plants in the early Silurian (Llandovery: Gray 1993; Cai et al. 1995, 1996), and the appearance of carnivorous trigonotarbids and centipedes by late Silurian (Ludlow: Jeram et al. 1990). Nevertheless, millipedes and liverworts persisted into Devonian communities (Edwards et al. 1995a; Shear et al. 1996), and remain with us today. It has long been assumed that herbivory did not occur in the earliest terrestrial communities, because the earliest known land animals included millipedes and carnivorous centipedes and arachnids (Beerbower 1985; Shear and Kukalova-Peck 1990; Labandeira 1997). Living millipedes are largely detritivores (KuÈhnelt 1976; Eisenbeis and Wichard 1987; Hopkin and Read 1985). Leaves softened by bacteria and fungi are the most common food of millipedes, and even in deserts they clean up detritus around the bases of plants (Crawford et al. 1987). Detritus-based early terrestrial ecosystems are indicated by Silurian ascomycete fungi in faecal pellets of presumed detritivores (Sherwood-Pike and Gray 1985). The most ancient direct evidence of fresh land-plant feeding comes from spore-dominated coprolites of late Silurian age (Edwards et al. 1995b) and wounds in fossil plants of early Devonian age (Banks and Colthart 1983; Scott et al. 1992). Suitably-preserved palaeosols, plants and coprolites are not yet known from Ordovician rocks to test the early primacy of detritivory or herbivory, and the fossils described here offer no de®nitive evidence. The oxidized, clayey, faecal pellets of Scoyenia beerboweri could have been made to facilitate burrowing, rather than feeding on organic soil. The high soil respiration and low humi®cation indicated by geochemical studies of Ordovician palaeosols would be surprising, but not impossible, if due only to decomposition and detritivory (Feakes and Retallack 1988; Retallack 1993; Yapp and Poths 1994). Similarly thorough exploitation of low plant biomass may be indicated by the association of the likely myriapod burrow Beaconites antarcticus with small lycopsids in an organic-lean early Devonian palaeosol in Antarctica (Retallack 1997b). Now that there is widespread trace fossil evidence for Ordovician myriapods on land (Retallack and Feakes, 1987; Johnson et al. 1994; Trewin and McNamara 1995), the most persuasive arguments for early terrestrial herbivory come from observations that living millipedes have a diverse diet, including fresh plants (KuÈhnelt 1976; Hopkin and Read 1992). Carnivory and scavenging have also been reported for living millipedes (Srivastava and Srivastava 1967; Hofman and Payne 1969), but ¯esh eating does not appear sustainable by millipede digestive tracts (SchluÈter 1980). Many millipedes consume their own exuviae after moulting (Hopkin and Read 1992). Some millipedes have a specialized diet of algae from the bark of trees (Eisenbeis and Wichard 1987). Others are serious crop pests of ¯owers, seedlings, roots and tubers (Baker 1974). Other millipedes feed on live liverworts, mosses and young shoots of vascular plants. In Spain for example, Ommatoiulus cingulatus contained predominantly mosses (44 per cent) and liverworts (18 per cent), in clear preference to other herbaceous plants and litter of its grassland habitat (Bailey and de Mendonca 1990). A vision of early polsterlands as simple plant-herbivore ecosystems was popularized some time ago by Attenborough (1979). There is no reason, considering their modern diet, why millipedes in such early ecosystems would not have eaten soil algae and plant thalli, both fresh and rotting. The Ordovician reign of millipedes as generalized detritivores and herbivores of communities low in biomass on land would have been curtailed by Silurian evolution of carnivores such as trigonotarbids and 228 PALAEONTOLOGY, VOLUME 44 centipedes (Jeram et al. 1990), by the Silurian rise of tracheophytes with lignin which is indigestible to most land animals (Robinson 1990; Retallack 1997a), and by Devonian±Carboniferous evolution of herbivorous insects (Labandeira 1997). Millipedes remain surprisingly abundant in gardens and the wild, despite dramatic evolutionary innovations in vascular land plants, insects and mammals (Hopkin and Read 1992). As outlined above, they thrive on non-vascular plants, in deserts and in crops where competition by insects is limited by low nutritional value, aridity or insecticides. Millipedes have lower metabolic requirements than insects (Reichle 1968), so are better able to exploit detritus and non-vascular plants than insects with their preference for fresh, nutritious plant foods. Perhaps a part of the millipede's success in consuming non-vascular plants and detritus was being there ®rst, as an example of what Price (1988) has labelled `ecology recapitulates phylogeny'. A similar case could be made for mosses and liverworts, still thriving in a world dominated by vascular plants more effective at casting shade and commandeering nutrients from the soil. Liverwort-millipede polsterlands exempli®ed by Scoyenia beerboweri can be viewed as an early phase in the long sequence of events that built modern terrestrial ecosystems.

Acknowledgements. Lynn Feakes was instrumental in helping to get this project off the ground in its early stages, with initial funding from the society of Sigma Xi. This contribution was enabled by a 1998 Faculty Summer Research Award from the University of Oregon. Two anonymous reviewers made a world of difference. It is also a pleasure to acknowledge helpful discussions with R. Beerbower, J. L. Wright, V. P. Wright, S. G. Driese, C. I. Mora, and H. D. Holland.

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G. J. RETALLACK Department of Geological Sciences University of Oregon Typescript received 7 January 1998 Eugene Revised typescript received 28 June 2000 Oregon 97403-1272, USA