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"Lycostrobus" chinleana, an equisetalean cone from the Upper Triassic of the southwestern United States and its phylogenetic implications¹

²EOST—Géologie, Université Louis Pasteur, F-67084 Strasbourg Cedex, France and Laboratoire de Paléobotanique, Institut des Sciences de l'Evolution, UMR 5554 CNRS, Montpellier, France; and ³Department of Earth and Planetary Sciences, Northrop Hall, University of New Mexico, Albuquerque,New Mexico 87120

Received for publication September 15, 1998. Accepted for publication March 9, 1999.

	ABSTRACT

TOP ABSTRACT INTRODUCTION PREVIOUS INVESTIGATIONS MATERIALS AND METHODS SYSTEMATIC DESCRIPTIONS DISCUSSION LITERATURE CITED

 
Detailed study of the cone Lycostrobus chinleana Daugherty shows that the fossil was incorrectly attributed to the Lycopodiales by the author and to the quillworts by Retallack and that it actually should be assigned to the Equisetales. The cone, which occurs in the Upper Triassic Chinle Formation at several localities in the southwestern United States, is 2.5 cm wide and nearly 6 cm long and consists of a stout axis bearing whorls of peltate sporangiophores. Each sporangiophore is composed of a slender stalk and a hexagonal disk, which typically bears a single, generally long, lanceolate, forward-directed leaf-like umbo tip on the outer surface and several recurrent sporangia on the inner surface. Small round to oval trilete spores occur in the sporangia. Since the leaf-like umbo tip is similar to the sterile leaves found in certain calamite cones and the recurrent sporangia are equisetalean-like, it appears that the cone may represent a intermediate stage between Calamites and modern Equisetum. According to this hypothesis, the nonbracteate Equisetum cone could have developed from a bracteate calamite cone, through reduction and fusion of the bracts and the sporangiophores, rather than by the loss of whorls of bracts of the Calamites cone as suggested earlier by others. As a result of this study the cone is assigned to the new Equisetalean genus Equicalastrobus and redescribed under the name E. chinleana (Daugherty) Grauvogel-Stamm and Ash, n. comb.

Key Words: Chinle Formation • Equicalastrobus • Equisetales • Lycostrobus • southwestern United States • Upper Triassic

	INTRODUCTION

TOP ABSTRACT INTRODUCTION PREVIOUS INVESTIGATIONS MATERIALS AND METHODS SYSTEMATIC DESCRIPTIONS DISCUSSION LITERATURE CITED

 
Although the work of Good (1975) appeared to confirm earlier suggestions (e.g., Andrews, 1961 ) that Equisetum and Calamites were closely related, it has remained unclear whether both taxa arose from a common Devonian ancestor or whether Equisetum descended directly from the Carboniferous calamites. Also, it has remained unclear what anatomical changes took place in its cone during the evolution of Equisetum. Those relationships and data on some of the changes that occurred during that process now seem to be clarified by the features of a large equisetalean cone that has been found in the Upper Triassic Chinle Formation in the southwestern United States. Each of the sporangiophores of this cone consists of a radially symmetrical Equisetum-like head typically bearing a long, lanceolate, bilaterally symmetrical leaf-like umbo tip on the outer surface (Figs. 1, 2). These features suggest that the equisetalean cone could have indeed developed from a calamitalean cone through reduction and fusion of the bracts and the sporangiophores.

View larger version (29K): [in this window] [in a new window]   Figs. 1, 2. Reconstructions of the sporophyll of Equicalastrobus chinleana. The trichomes on the margins and outer surface of the disk are represented by the stippling and short lines in the figures. 1. Outer surface in the plan views based on the specimens in Figs. 22–27. Note the raised hexagonal umbo in the middle of the outer surface of the disk, the slightly inclined flange of the disk, and the long leaf-like tip that originates from the upper edge of the umbo. 2. Longitudinal section based principally on the specimens in Figs. 19, 20, 22, 23. Note the numerous cylindrical sporangia attached to the inner surface of the disk and directed perpendicularly to it and the cone axis. Scale bar = 1 mm

	PREVIOUS INVESTIGATIONS

TOP ABSTRACT INTRODUCTION PREVIOUS INVESTIGATIONS MATERIALS AND METHODS SYSTEMATIC DESCRIPTIONS DISCUSSION LITERATURE CITED

 
Since it was originally described and assigned to the Lycopodiales by Daugherty (1941) , Lycostrobus chinleana has evoked only a little interest and discussion. The fossil was evaluated by Chaloner and Boureau (1967) who concluded that its assignment to Lycostrobus should be considered hazardous. In his detailed investigation of the Upper Triassic lycopod stem Chinlea, Miller (1968) suggested that L. chinleana could have been borne on the same plant. Bock (1969) reported that L. chinleana had a coniferous aspect and compared it to the cones of Primaraucaria Bock (1954) . In 1979 Ash suggested that L. chinleana together with the genotype supported the contention by Helby and Martin (1965) that arborescent lycopsids survived into the Mesozoic. Taylor (1981) and Taylor and Taylor (1993) stated without explanation that it was "thought to be associated with the Pleuromeiales." Lycostrobus chinleana was not mentioned by name when Stewart (1983) and Stewart and Rothwell (1993 , p. 208) discussed an Upper Triassic cone described by Daugherty (1941) , but it is clear that they were referring to the fossil because Daugherty did not consider any other cone in his report. Although Stewart and Rothwell misinterpreted some of its features, they were essentially correct when they indicated that the fossil belonged to the Equisetales and represented a potential intermediate stage in the evolutionary development of the equisetalean cone. Later Retallack (1997) stated that the fossil (called both L. chinleana and L. chinleanus in his report) had keeled sporophylls each of which bore a single large sporangium and that it was the cone of an arborescent quillwort.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION PREVIOUS INVESTIGATIONS MATERIALS AND METHODS SYSTEMATIC DESCRIPTIONS DISCUSSION LITERATURE CITED

 
This study is based on the holotype of L. chinleana and 15 additional specimens collected from three localities in the Colorado Plateau region of the Southwestern United States (Fig. 3). Their locations are as follows: (1) 2 km east of the headquarters of Capitol Reef National Park, Utah in SE 1/4, Sec. 15, T. 29 S., R. 6 E. Same as U.S. Geological Survey (USGS) Fossil Plant Locality 10154 (see Ash, 1977 ). Shinarump Member of the Chinle Formation; (2) 60 m east of the Tepees in Petrified Forest National Park, Arizona in NE 1/4, Sec 22, T. 18 N, R. 24 E. Same as University of California Museum of Paleontology (UCMP) Locality P3901-4 (see Daugherty, 1941 ). Monitor Butte Member of the Chinle Formation; (3) 3 km south of the post office at Fort Wingate, New Mexico. Same as USGS Fossil Plant Locality 10060 (see Ash, 1970 ). Monitor Butte Member of the Chinle Formation. Palynological studies indicate that all three localities are Late Carnian in age (Litwin, Traverse, and Ash, 1991 ).

These fossils include two fragmentary cones (the holotype, UCMP 1563, and another specimen, UCMP 148991) from locality 2 in Arizona (Figs. 4–10, 21–23, 33–36), two complete and 11 more or less fragmentary cones together with several isolated sporangiophores (UCMP 148993–149000, 153161) from locality 1 in Utah (Figs. 11–20, 24–27, 37–40), and a single incomplete specimen (UCMP 148992) from locality 3 in New Mexico. Although all of the fossils described here are preserved in mudstone, their condition varies from locality to locality. Those from locality 1 are unexpanded and are represented by thick, brownish carbonaceous residues broken into many small cubic fragments together with an underlying impression. Only a few of these cones are represented by both part and counterpart. Most of the fossils are surrounded by large quantities of comminuted plant debris (Figs. 11–20). Fine details, such as cell walls and trichomes, are not clearly visible on any of these specimens. The holotype and the second specimen from locality 2 are somewhat expanded and are represented principally by red-stained impressions, which bear small, thin fragments of black coaly material. Both of these two specimens are incomplete, but the part and the counterpart of each are present. The holotype appears to consist of about the lower two-thirds of the entire cone excluding the peduncle (Figs. 4, 5). The other specimen (UCMP 148991) from locality 2 is much less complete and represents just the middle part of the cone (Figs. 9, 10, 21). Imprints of cell walls are visible on both the red-stained impressions and on the surfaces of the fragments of carbonaceous material in both specimens. Euhedral gypsum crystals also are present on the red-stained impressions of the holotype and sometimes obscure observations, depending on where they occur (Fig. 36, large arrowhead). Several of the disks on these two specimens show a smaller raised hexagonal imprint or umbo on the outer surface where the leaf-like tip is attached. The umbo appears concave in the specimens showing the imprint of the sporangiophores (Figs. 5, 7, 10, 33–36). Impressions of small spine-like trichomes occur on the marginal flange of the disk around the umbo and along the margin of the disk itself in these specimens (Figs. 6, 8). Although they are very obvious on the specimens from locality 2 the trichomes are poorly preserved on the specimens from locality 1 and just give a granular aspect to the marginal flange (Fig. 18). The specimen from locality 3 is represented by an impression of a small part of a cone (several sporangiophores in face view) and bears only a few flakes of carbon.

View larger version (171K): [in this window] [in a new window]   Figs. 4–8. Equicalastrobus chinleana. 4, 5. Part and counterpart of the lower two-thirds of an expanded cone. The sporangiophore opposite the arrow near the top of Fig. 5 is shown at a higher magnification in Fig. 36 and those opposite the arrowhead are shown at a higher magnification in Figs. 7, 8, 33–35 . Holotype, UCMP 1563. 6. Imprint of the outer surface of lower part of the sporangiophore disk shown at left in Fig. 8. Note the tiny spine-like trichomes along the border. The tiny bosses and black dots on the surface correspond, respectively, to trichome imprints and to cross section of trichomes imbedded in the rock matrix. Photographed in polarized light. Holotype, UCMP 1563. 7. Detail of the group of sporangiophores near the base of the cone in Fig. 5 (opposite arrowhead). Note that only sporangiophores a–c are nearly complete in this specimen and show the entire disk and its attached leaf-like umbo tip. Note also that the leaf-like tips designated by horizontal arrows are slightly folded longitudinally along the midrib. Sporangiophores a–c are shown at a higher magnification in Figs. 33–35. Holotype UCMP 1563. 8. Imprint of the outer surface of the two lowest sporangiophore disks in Fig. 7. The sporangiophore disk to the right is also shown at a lower magnification in Fig. 34. Note that the umbo is rather smooth except for the imprints of epidermal cells and is separated by a low ridge (arrowheads) from the marginal flange which is covered with tiny trichomes. Low ridges also extend from the lower corners of the marginal flange to the lower corners of the umbo. Holotype UCMP 1563. Scale bars in Figs. 4, 5, 7 = 1 cm; bar in Figs. 6, 8 = 1 mm

View larger version (183K): [in this window] [in a new window]   Figs. 9–18. Equicalastrobus chinleana. 9, 10. Part and counterpart of the central region of an expanded cone. The arrow near the top of Fig. 10 indicates the imprint of the outer surface of a sporangiophore in which the umbo contains a low oval structure, that may be related to withering or, more likely, it may represent the trace of the peduncle attachment on the inner side of the sporangiophore. The lower part of Fig. 9 is shown at a higher magnification in Figs. 21–23 after a buried sporangiophore was dégaged. UCMP 148991. 11–13. Imprints of the outer surface of three complete unexpanded cones with attached peduncles that show several nodes and internodes. Narrow longitudinal striations are visible at places in the internodes in Figs. 11, 13. Note that these imprints show mostly sporangiophore disks, and the leaf-like umbo tips are buried in the underlying sediment. The lower part of the cone and peduncle in Fig. 13 is shown at a higher magnification in Figs. 14, 15. Photographed in polarized light. From left to right UCMP 148995, UCMP 148992, UCMP 148994. 14. Detail of the lower half of the unexpanded cone in Fig. 13 and peduncle in which several nodes (small arrows) and short internodes are clearly visible. The uppermost node at arrow A forms the annulus or collar of the cone (see reconstruction in Fig. 51 ). Note the two, slightly arched whorls (between the arrowheads) of sporangiophore disks (marked by white circles with black centers) just above the peduncle. Photographed in polarized light. UCMP 148994. 15. Enlargement of the peduncle of the cone in Figs. 13, 14, showing the nodes and the short internodes covered with numerous longitudinal striations. UCMP 148994. 16. Apex of a cone showing the leaf-like umbo tips on the uppermost sporangiophores. UCMP 148999. 17. Basal portion of an unexpanded cone showing the imprint of the outer surface of the sporangiophore disks. The peduncle shows several nodes and internodes covered with narrow longitudinal striations. Note that the marginal flanges of the disks look like pads around the umbo. UCMP 148993. 18. Enlargement of the outer surface of the adjoining sporangiophore disks in the two consecutive whorls of the cone in Fig. 14. The disks indicated by arrows a, b, and c are interpreted in Figs. 48–50. Note the large central umbo delimited by a deep groove (small arrows) and surrounded by the slightly inclined marginal flange of the disk. Note also the leaf-like tip extending upwards from the umbo of the sporangiophore disk indicated by arrow c. UCMP 148994. Scale bars in Figs. 9–14, 16–17 = 1 cm; bars in Figs. 15, 18 = 5 mm

View larger version (179K): [in this window] [in a new window]   Figs. 33–40. Equicalastrobus chinleana. Sporangiophore in plan view. Figs. 33–36. Enlargements of sporangiophore of the holotype (UCMP 1563) showing the imprint of the outer surface of the disks and the attached leaf-like tip. Note the oval structure (at arrowheads) in the middle of the more angular umbo (at small arrows). The difference in surface structure between the umbo and the marginal flange of the disk is clearly visible. A low ridge (small arrows) marks the boundary between the two areas. 33. Sporangiophore opposite arrow a in Fig. 7 and reconstructed in Fig. 43. In this specimen the thin layer of sediment containing the imprint of the upper marginal flange of the disk is nearly unbroken and covers the base of the underlying leaf-like umbo tip. 34. Sporangiophore opposite arrow b in Fig. 7 and reconstructed in Fig. 44. In contrast to the specimen in Fig. 33 the layer of sediment containing the imprint of the upper marginal part of the disk has partly fallen away and exposes most of the base of the leaf-like umbo tip. Note the small oval structure (arrowheads) near the lower margin of the umbo. 35. Sporangiophore opposite arrow c in Fig. 7 and reconstructed in Fig. 45. The layer of sediment containing the imprint of the upper marginal flange of the disk has fallen away completely in this specimen, and the slightly constricted base (large arrow) of the leaf-like tip that connects with the upper margin of the umbo is exposed. At places, a low ridge (small arrows) marks the boundary between the umbo and the marginal flange of the disk. Note the oval structure in the umbo (arrowheads). 36. Impression of a fragmentary sporangiophore opposite the small arrow and near the top in Fig. 5 and reconstructed in Fig. 46. At left, the marginal flange of the disk is covered with spine-like trichomes and is clearly delimited from the rather smooth umbo by a well-marked groove (small arrows). Note also the longitudinally oriented cell imprints, which extend from the umbo into the leaf-like tip without interruption. The place where the lateral border of the leaf-like tip joins the umbo is indicated by the large arrow. The margin of an incomplete oval structure in the umbo is indicated by the small arrowheads. Impressions of a ring of gypsum crystals are present near the base of the leaf-like umbo tip in this specimen (large arrowhead at right). 37. An aggregation of several disassociated sporangiophores, three of which have rather short leaf-like umbo tips (opposite arrows a, b, d). The other sporangiophores, are devoid of any evidence of a leafy outgrowth. Note the long isolated univeined leaf-like tip at the base (arrow e), which has become detached from a disk. UCMP 148997. Figs. 38–40. Enlargement of three isolated sporangiophores in Fig. 37, at arrows b, c, and d, respectively. Note that the surface of the umbo of the sporangiophores in Figs. 38–40 are devoid of the oval structure typically present in most of the organs of this species (e.g., Figs. 33–36). 38, 39. Sporangiophores with short folded leaf-like tips attached to the upper edge of the umbo (broken in Fig. 39, at arrow). 40. Sporangiophore opposite arrow c in Fig. 37, showing only a slightly protruding umbo devoid of a leafy outgrowth. Scale bars = 1 mm in Figs. 33–36, 38–40; bar = 1 cm in Fig. 37. The specimens in Figs. 33–36 are parts of the holotype, which came from locality 2

All of the specimens used in this study are deposited in the collections stored at the University of California Museum of Paleontology, Berkeley, CA (UCMP).

	SYSTEMATIC DESCRIPTIONS

TOP ABSTRACT INTRODUCTION PREVIOUS INVESTIGATIONS MATERIALS AND METHODS SYSTEMATIC DESCRIPTIONS DISCUSSION LITERATURE CITED

 
Division
Sphenophyta.

Class
Equisetopsida.

Order
Equisetales.

Family
Equisetaceae Equicalastrobus gen. nov., Grauvogel-Stamm and Ash.

Genotype
Equicalastrobus chinleana (Daugherty) Grauvogel-Stamm and Ash, n. comb. from the Upper Triassic Chinle Formation of the southwestern United States.

Generic diagnosis
Cone, cylindrical, pedunculate, consisting of a stout axis bearing peltate sporangiophores in whorls; sporangiophores consisting of a slender stalk and a hexagonal disk; middle of outer surface of disk bearing a single lanceolate, generally long, forward-directed leaf-like umbo tip; inner surface of disk bearing several recurrent sporangia containing small round to oval trilete spores.

Etymology
The generic name denotes the intermediate morphology of the cone between those of Equisetum and Calamites.

Comparisons
This cone is easily distinguished from the cone Lycostrobus Nathorst (1908) and other lycophyte cones because it has whorls of peltate sporangiophores, which bear recurrent oblong sporangia on the inner side of the sporangiophore disk. In contrast, in most lycophyte cones, including Lycostrobus, the sporangiophores are arranged in a helix and a sporangium is borne in the axil or on the upper surface of the pedicel of each sporangiophore.

Equicalastrobus chinleana (Daugherty) Grauvogel-Stamm and Ash, n. comb.

1941 Lycostrobus chinleana Daugherty, p. 63–64, pl. 12, figs. 1–2.

1967 Lycostrobus chinleana Daugherty. Chaloner and Boureau, p. 431. Discussion only.

1969 Lycostrobus chinleana Daugherty. Bock, p. 58. Discussion only.

1972 Lycostrobus chinleana Daugherty. Ash, p. 25. Discussion only.

1980 Lycostrobus chinleana Daugherty. Ash, p. 163. Listing only.

1981 Lycostrobus chinleana Daugherty. Taylor, p. 168. Discussion only.

1983 "Lycostrobus chinleana Daugherty." Stewart, p. 167. Discussion only; name not given.

1993 Lycostrobus chinleana Daugherty. Taylor and Taylor, p. 297. Discussion only.

1993 "Lycostrobus chinleana Daugherty." Stewart and Rothwell, p. 208. Discussion only; name not given.

1997 Lycostrobus chinleanus [sic] Daugherty. Retallack, p. 515, fig. 1, fig. 11, table 1. Discussion and diagram only.

1997 Lycostrobus chinleana Daugherty. Retallack, fig. 10. Diagram only.

Emended diagnosis
Cone, cylindrical, before expanding 5.5–6.0 cm long and 2.5 cm wide in middle, tapering slightly to rounded apex, strongly contracted basally to a short peduncle 1 cm wide. Peduncle showing several nodes and short internodes. Internodes 2 mm long, showing numerous narrow longitudinal striations. Uppermost node of the peduncle bearing fused leaf-like lobes, forming annulus. Cone axis 1.0 cm wide, bearing whorls of sporangiophores, whorls arising every 7–8 mm in compact cones. Sporangiophores peltate, consisting of narrow stalk and hexagonal disk; stalks 4–6 mm long, 2 mm wide at base where they join cone axis, disks equidimensional (4–6 mm in diameter) to transversely elongated (up to 7 mm wide, 5 mm high). Outer surface of disk bearing a slightly raised umbo in center; umbo hexagonal to diamond-shaped resembling sporangiophore in outline, bearing a single univeined, leaf-like tip directed towards the cone apex. Leaf-like umbo tip up to 2.5 cm in length, 2–5 mm wide near base, tapering evenly to an acutely pointed apex, sides often slightly enfolded. Many small spine-like trichomes present on outer side of disk on marginal flange between base of leaf-like umbo tip and margin and along margin of disk; trichomes 100 µm tall, 25 µm wide at base. Inner surface of peltate disk bearing numerous recurrent sporangia. Sporangia cylindrical, 4 mm long and 0.6 mm in diameter, containing many small trilete spores. Spores delicate, thin-walled, round to oval, 37–48 µm in diameter, rays of trilete mark 9 µm long, ornamentation scabrate to rugulate.

Holotype
UCMP 1563. Part and counterpart of a nearly complete specimen from locality 2 of this report (Figs. 4–8, 33–36).

Localities
Locality 1 in Capitol Reef National Park, Utah; locality 2 in Petrified Forest National Park, Arizona; and locality 3 near Fort Wingate, New Mexico.

Stratigraphic position
Chinle Formation.

Age
Late Carnian stage of the Late Triassic.

Description
A short, stout peduncle 1 cm in diameter is preserved at the base of four of the cones from locality 1 (Figs. 11–15, 17). In three of them the peduncle shows traces of several nodes and short internodes (Figs. 11, 13, 15, 17, 14 at small white arrows). Faint impressions of longitudinal striations are present on the internodes of the peduncle in these cones (Fig. 15). The uppermost node of the peduncle, just beneath the lowermost whorl of sporangiophores, bears what apparently are fused, leaf-like lobes resembling somewhat the sporangiophore disks, but they are not peltate and are devoid of an umbo. The grainy tips of these lobes indicate that they were covered with tiny trichomes similar to those of the marginal flange of the sporangiophore disks (Figs. 14, 51 at arrow A). This particular whorl is probably an annulus or collar similar to that of Equisetum (Page, 1972 ).

Since the spores extracted from some of the unexpanded cones are thin walled, delicate, and nearly featureless, they are probably immature. This is also indicated by the fact that it was difficult to totally separate or isolate any of the spores.

In both specimens from locality 2 the sporangiophores are rather widely separated and more or less disorganized, but they show clearly the leaf-like tip attached to the umbo on the hexagonal head (Figs. 4, 5, 7, 9, 10). In contrast, the sporangiophores of the cones from locality 1 are tightly arranged and mostly show only their hexagonal disk (Figs. 11–14, 17, 18). In one of them two adjacent, slightly arched whorls (between black arrowheads, Fig. 14) of tightly arranged disks (indicated with a black dot enclosed in a white ring) are clearly visible. Leaf-like umbo tips are visible along the margins (as compressed) and the apex of some of these cones (Figs. 11–14, 16). Apparently the expanded cones, such as the holotype, are "ripe," whereas the unexpanded cones from Utah are somewhat immature. The absence of spores in the holotype and their presence, although presumably immature, in the unexpanded cones from locality 1 seem to confirm this hypothesis. The dimensions of the cones given above are based on the unexpanded cones from locality 1 and thus are somewhat at variance with the dimensions reported by Daugherty (1941) , which were based on a single expanded and incomplete cone from locality 2.

The general variation in the appearance of the cone on the rock surface is seemingly directly related to the maturity of the organ when it was buried. In the unexpanded immature cones from locality 1 the disks of the sporangiophores were relatively contiguous when buried and formed a continuous zone of weakness along which the rocks usually cleaved during collecting. However, since the leaf-like umbo tips in these cones were not on the same cleavage plane as the disks, they usually were not exposed when the rock was split. Consequently, in most unexpanded cones the disks typically are clearly visible, whereas the leaf-like umbo tips usually are not exposed because they are still buried in the rock matrix (Figs. 11–14, 17, 18, 47–49). On the other hand, the sporangiophores were not contiguous when the expanded mature cones from Arizona (Figs. 4, 5, 9, 10) were buried, and thus did not form a similar continuous zone of weakness. Consequently, these cones broke along a somewhat irregular plane. Since the space between the disks and attached leaf-like umbo tips in the mature cones contains only a comparatively thin layer of sediment it is apparent that the two structures were tightly compressed when the cones were buried, and both structures usually appear to be on virtually the same cleavage plane after the rocks containing the cones were split (Figs. 5, 7, 33–36, 42–46).

Since the cones are preserved in a variety of orientations and preservational states their general structure and the organization of the sporangiophores could be determined reasonably accurately. For example, several specimens show that the outer side of the head of the sporangiophores bears a rather large central umbo, which is hexagonal to diamond-shaped in outline, comparable in shape and orientation to that of the head. In the specimens in which the outer side of the sporangiophore disks is exposed, the umbo protrudes convexly a short distance above the marginal flange and appears as a clearly delimited area surrounded by an obliquely inclined border (Figs. 21–27). On the transversely elongated sporangiophore head measuring 6.5 x 2.8 mm, the umbo measures 4 x 1.2 mm (Figs. 26, 27) and on another head, which is 7.4 mm wide, the diamond-shaped umbo measures 5 x 2 mm (Fig. 25). In one of the specimens showing the imprint of the sporangiophores, the umbo appears as a clearly delimited depressed area (Figs. 14, 17, 18). Their equidimensional heads, which are 6 mm in diameter, contain an umbo 3 mm in diameter (Fig. 18, at arrow a). The inclined border, which surrounds the umbo, is visible in all the specimens, even in those that show the sporangiophores in side view (Fig. 20, at arrowheads) and in those that exhibit the imprint of the outer side of the disks (Fig. 18). A faint ridge is sometimes visible in the marginal flange of the disk, extending from its hexagonal corner to that of the umbo (Figs. 7, 8, 23, 27, 34).

View larger version (179K): [in this window] [in a new window]   Figs. 19–27. Equicalastrobus chinleana. 19. Longitudinal section of an unexpanded cone showing two superposed sporangiophores in lateral view at the left (opposite arrows). Part of the cone is shown at a higher magnification in Fig. 20. Photographed in polarized light. UCMP 148998. 20. Enlargement of the two sporangiophores marked with arrows in Fig. 19. Note the numerous cylindrical sporangia (SP) attached to the inner side of the sporangiophore disk and directed perpendicularly to it and the cone axis as in Equisetites Sternberg and Equisetum Linné. The inclined marginal flange and curved edge of the disk are indicated by the arrowheads at the base of the figure. Photographed in polarized light. UCMP 148998. 21. Enlargement of a part of the cone in Fig. 9 after the outer surface of a complete sporangiophore was dégaged at the base of the fossil. The dégaged sporangiophore, which was preserved in plan view, is shown at a higher magnification in Figs. 22, 23. UCMP 148991. 22. Detail of the dégaged sporangiophore in Fig. 21. The variation in the surface texture reflects the difference in epidermal features between the marginal flange (covered with tiny trichomes) and the umbo and leaf-like tip (relatively smooth with imprints of epidermal cells). Also note that the leaf-like umbo tip is slightly constricted at the base (inclined arrows in Fig. 22) and that it overlies a large part of the upper marginal flange of the disk (horizontal arrow in Fig. 23). The base is shown at a higher magnification in Fig. 23. 23. Detail of the base of the sporangiophore in Fig. 22 showing several sporangia (small arrows) at the lower right where the inclined flange of the disk is broken off. 24. Fragmentary cone showing two rather well-preserved entire sporangiophores (arrows) in plan view. The sporangiophore above the vertical arrow is shown at a higher magnification in Figs. 26, 27. Note that the two long leaf-like umbo tips visible in the lower left, are slightly folded along a well-defined longitudinal midrib. UCMP 148996. 25. Sporangiophore in plan view in which the slightly protruding umbo is diamond shaped. Note that its leaf-like tip is attached to the upper edge of the umbo and that it overlies a large part of the upper marginal flange of the disk. UCMP 153161. 26. Basal part of the sporangiophore above the vertical arrow in Fig. 24 photographed in low-angle lighting. This photograph shows that the midrib of the leaf-like tip extends to the upper edge of the umbo. 27. The entire sporangiophore in Fig. 26. Note that here the slightly protruding umbo is hexagonal and that the leaf-like tip attached to its upper edge also overlies the upper marginal flange of the disk (at arrow). The horizontal line, which appears to be a crack at the base of the leaf-like umbo tip in the sporangiophore in Figs. 25–27, is merely the coaly material in the cross section of the buried flange of the disk. This line appears after the coaly material of the fossil has fallen away from the surface of the impression. A series of diagrams illustrating the origin of the line is presented in Figs. 28–32 . Scale bars in Figs. 20, 22, 23, 25–27 = 5 mm, bars in Figs. 19, 21, 24 = 1 cm. The specimens in Figs. 19, 20, 24–27 came from locality 1 and those in Figs. 21–23 from locality 2

View larger version (23K): [in this window] [in a new window]   Figs. 28–32. Equicalastrobus chinleana. Diagrams illustrating the origin of the horizontal line that appears as a black line or crack at the base of the leaf-like umbo tip on the sporangiophore in some specimens of the species ( Figs. 25–27 ). In all the sections the organic matter of the sporangiophore is shown as solid black and the rock matrix is tippled. For simplification the sporangia are shown only in Fig. 29. 28. Plan view of the outer surface of the disk and base of a leaf-like umbo tip of the sporophyll in life. 29. Longitudinal section of the sporophyll in life. 30. Longitudinal section of a buried sporophyll that has been reduced to a thin layer of coaly material, which tends to fall away from the impression when the rock is split open. 31. Longitudinal section of a buried sporophyll after the enclosing rock has been split open and all of the coaly material has fallen away except for that of the marginal flange of the disk, which was under the leaf-like umbo tip and remains buried in the rock. Only the cross section of the marginal flange (at arrow) is visible and looks like a black line or crack on the impression as shown in Fig. 32. 32. Impression of a sporophyll entirely devoid of coaly material in plan view, except in the space at the base of the leaf-like umbo tip, which is the exposed cross section of the buried coalified upper marginal flange of the disk. Compare with Figs. 25–27. Scale bar = 1 mm

In most specimens showing the imprint of the outer surface of the disks, the umbo is clearly delimited from the marginal flange of the disk. In specimens from locality 1, the boundary is indicated by a relatively deep groove and the umbo is represented by a slight depression [Figs. 18 (at small arrows), 48–50]. In one, the marginal flange of the disks looks like a pad around the central depressed umbo imprint marked by the groove (Fig. 17). In the holotype, the delimiting groove is less strongly marked [Figs. 8 (at arrowheads), 33–36 (at small arrows)]. As clearly shown in the counterpart, there is moreover considerable contrast in the morphology of the outer surfaces of the umbo and leaf-like tip (smooth) and the marginal flange of the disk (densely covered with spiny trichomes) (Figs. 5, 7, 8, 33–36). No stomata were observed among the longitudinally oriented cells, which are visible on the outer surface of the umbo and leaf-like tip.

The spine-like trichomes, which are present all over the marginal flange on the outer side of the disk up to the edge of the umbo, are very small, 100 µm long and 25 µm wide at their base (Figs. 1, 6, 8). They are preserved in lateral view along the margins of the disk and in plan view all over the surface of the marginal flange of the disks (Figs. 6–8, 33–36). The trichomes can be seen either imbedded in the sediment or as imprints on the surface giving it a granular appearance. In some of the specimens from locality 1 showing the imprint of the outer side of the disks, the marginal flange also has this grainy appearance (Fig. 18). Trichomes are not present on the outer surfaces of the umbo and leaf-like tip. The difference in surface structures also is obvious in the single sporangiophore preserved in face view from Arizona where the outer surface is exposed (Figs. 21–23). The thin layer of coaly material that covers this impression bears these epidermal structures, i.e., trichomes on the marginal flange and smooth cell imprints on the umbo and leaf-like tip. In contrast, the difference in surface structure is not visible in the sporangiophores from locality 1 where the outer side is exposed after the layer of coaly organic matter has fallen away (Figs. 24–32). These specimens show that the outermost part of the coaly layer covering the impressions corresponds to the epidermis itself. It is not certain that the surface exposed in these sporangiophore impressions, which are devoid of the coaly layer, represents the inner surface since their disk does not show any trace of attachment of the sporangia and central stalk (Figs. 25–27).

The outer surface of the sporangiophore disks in both cones from Arizona show a low oval structure in the middle of the umbo [Figs. 7, 33–36 (at arrowheads), 43–46]. It can be observed in the ten sporangiophore disks of the holotype where the central region is preserved (Fig. 5), as well as in the other specimen from Arizona at locality 2 (Fig. 10, at arrow). In the single sporangiophore in face view exposing its outer surface in the other specimen from Arizona, this structure is not apparent (Figs. 21–23). In some sporangiophore disks, this structure seems to be associated with the base of the leaf-like umbo tip, suggesting that it is somehow related to its attachment to the umbo [Figs. 35 (at arrowheads), 45]. In others, it may mark the place where the stalk is attached to the disk [Figs. 33, 34 (at arrowheads), 43, 44]. This structure may be a sign of withering since it only occurs in the ripe cones from Arizona. The slight constriction visible at the base of the leaf-like tips in these specimens (Figs. 22, 23, 35, 45) and the rather irregular shape of the umbo may be further support for this possibility since similar features do not seem to occur in the specimens from locality 1 (Figs. 18, 25–27, 37–40, 48–50) or locality 3.

Some of the isolated sporangiophore disks show rather short (3–5 mm) leaf-like umbo tips [Figs. 37 (at arrows a, b, d), 38, 39], which contrast with the long (20–25 mm) leaf-like tips that usually characterize the sporangiophores. Others are devoid of a leaf-like tip and show only a slightly protruding umbo [Figs. 37 (at arrow c), 40]. However, it is possible that the leaf-like tips may have become detached from some of the isolated disks, as is suggested by the long isolated leaf-like tips present between them (Fig. 37, at arrow e). The difference in length of the leaf-like tips may be related to the position of the sporangiophores in the cones with the shorter ones being situated near the base of the cone and the longer ones being near the apex (Fig. 16) and possibly elsewhere on the organ (Fig. 51).

View larger version (54K): [in this window] [in a new window]   Figs. 51–52. Equicalastrobus chinleana. The trichomes, which occur on the margins and outer side of the disk, are represented by stippling and short lines. 51. Reconstruction of the cone. For simplification only some sporangiophores are represented with the leaf-like umbo tips. The cone annulus is at A. 52. Reconstruction of a sporangiophore. Note that the leaf-like umbo tip is slightly folded along the midrib. Based on the specimens in Figs. 5, 7, 10, 24, 26, 37, 38. Scale bar = 1 cm in Fig. 51; bar = 1 mm in Fig. 52.

One unexpanded cone, which is an exception because it is longitudinally split, shows several sporangiophores in lateral view (Figs. 19, 20 at arrows), clearly demonstrating that the sporangia (SP, Fig. 20) are attached to the inner side of the head and are directed towards the cone axis (Figs. 2, 52) as in the cones of Equisetites Sternberg and Equisetum Linné. A similarly organized sporangiophore has been dégaged at the base of one of the specimens from Arizona (Figs. 9, 21). In this sporangiophore, that is preserved in plan view, the border of the head shows clearly some sporangia originating from its inner side (Figs. 22, 23 at small arrows).

Interestingly, at all localities Equicalastrobus chinleana is associated with equisetalean stems, which range up to 34 cm in diameter. Most of the stems at locality 1 are uncompressed and in growth position and average 18 cm in diameter (Ash, 1975 ). Many compressed stems 1–3 cm in width occur at locality 2. Also, a few hundred metres west of locality 2 and at about the same stratigraphic horizon stem casts that range up to 34 cm in diameter occur in growth position. Compressions of equisetalean stems 2–3 cm in diameter are also associated with Equicalastrobus chinleana at locality 3 (Ash, 1986 ). Possibly these associated stems are part of the parent plant of E. chinleana n. comb.

	DISCUSSION

TOP ABSTRACT INTRODUCTION PREVIOUS INVESTIGATIONS MATERIALS AND METHODS SYSTEMATIC DESCRIPTIONS DISCUSSION LITERATURE CITED

 
We have had difficulty in recognizing with assurance all of the features reported in the holotype of "Lycostrobus" chinleana by Daugherty (1941) and confidently correlating them with those we have observed in the same fossil. The "seedlike bodies" reported near the base of the fossil and called megasporangia are probably the oval structures visible in the middle of the umbos on otherwise poorly exposed peltate disks preserved in plan view. Presumably, the "four angled stalks" are the umbo in the middle of the exposed hexagonal disks of the sporangiophores themselves. We are uncertain, however, what feature he thought was the "peltate tip of the scale."

Reevaluation of the holotype of "L." chinleana and study of the newly discovered specimens demonstrate that their organization does not correspond to that of the lycopods as reported by Daugherty (1941) . Also, the reevaluation shows that the structure of its sporophylls does not at all resemble the reconstruction presented by Retallack (1997, fig. 1), which shows a large ovate sporangium adaxially attached on a basally dilated leaf ending in a narrow distal limb. Furthermore, our findings indicate that the relationships of the cone do not lie with the quillworts as proposed by Retallack (1997). Rather, our findings demonstrate that its relationships lie with the Equisetales as suggested by Stewart (1983) and Stewart and Rothwell (1993) and is not related to either Lycostrobus or any other lycopod. In fact, the morphology and arrangement of the peltate sporangiophores in the material and the recurrent attachment of the sporangia compare closely with similar features in typical cones of Equisetites and Equisetum. However, they differ from them by having broad, generally long, lanceolate leaf-like umbo tips arising from the middle of the outer surface of the sporangiophores. Only in the equisetalean genus Echinostachys Brongniart, particularly E. oblonga, the cone of the Triassic Schizoneura paradoxa Schimper and Mougeot, is there a such an outgrowth although it is shorter and spine-like (Grauvogel-Stamm, 1978 ). But in E. oblonga the sporangia are attached along the stalk of the peltate sporangiophores, whereas in "L." chinleana they are attached to the inner surface of the disk of the sporangiophore itself, as in Equisetites and Equisetum. Therefore, since these cones actually belong to the Equisetales and do not conform to the features of Lycostrobus they must be referred to another genus. Furthermore, since they do not resemble any known fossil equisetalean cones, we assign them to the new genus Equicalastrobus, with the new combination E. chinleana (Daugherty) Grauvogel-Stamm and Ash.

In this regard it is important to note that the cones shown in the famous restoration of Schizoneura paradoxa by Mägdefrau (1942, 1952, 1956, 1968), which has been reprinted in several textbooks (e.g., Stewart and Rothwell, 1993 ) were wrongly attributed to that fossil. Actually, they are the female cones of the herbaceous conifer Aethophyllum stipulare as demonstrated by Grauvogel-Stamm (1978).

When Stewart (1983) and Stewart and Rothwell(1993) reported that "L." chinleana was equisetalean they stated that it consisted of "... several whorls of sporangiophores separated by a whorl of Equisetum-like leaves." Thus, they considered this fossil as evidence for "... a gradual change from the regular bracteate condition found in the calamite cone to the non-bracteate condition of Equisetum." Furthermore, they interpreted it "... as representing a stage in the loss of whorls of bracts from the calamite cone type."

The present reevaluation shows that in fact, as noted above, the cone of E. chinleana consists only of whorls of sporangiophores. This arrangement, together with the morphology of the sporangiophores indicates, contrary to the conclusions of Stewart (1983) and Stewart and Rothwell (1993) , that the change from a calamite cone to an equisetalean cone may have been produced by reduction and fusion of the bracts and the sporangiophores of two consecutive whorls such as those in calamite cones (Figs. 53–57). Although a less parsimonious explanation for the derivation of equisetalean cones than that of Stewart and Rothwell, this process could have resulted in the development of E. chinleana with long leafy sporangiophores devoid of intercalary whorls of sterile bracts. Therefore, the sporangiophores of E. chinleana may represent a transitional stage towards the typical Equisetum-like sporangiophores devoid of a leafy outgrowth. A proclivity for reduction in E. chinleana is indicated by the presence of very short leaf-like tips on some umbos and their total absence on others (Figs. 37–40). The strongly marked boundary between the leaf-like tip and umbo and the marginal part of the sporangiophore disk and, particularly the great differences in their epidermal features, may be evidence for the dorsiventrally compound structure of the sporangiophores. The differences in symmetry between the leaf-like tip (bilaterally symmetrical) and the peltate disk (radially symmetrical) appear to be further evidence supporting this hypothesis. The presence of the same epidermal structure in both the umbo and the leaf-like tip and its extension from one to the other without interruption demonstrate that they are parts of a single organ with the umbo representing the base of the leaf-like tip. Thus, the umbo and leaf-like tip seem to correspond to the bract of a calamite cone that has fused with the peltate sporangiophore of an adjacent whorl.

View larger version (16K): [in this window] [in a new window]   Figs. 53–57. Hypothetical sequence of stages in the evolution of the Equisetum sporangiophore from the sporangiophore and bract of Calamites. Sporangia shown in solid black; leaf-like umbo tip and/or umbo shown stippled. 53. A sporangiophore and the subadjacent bract of Calamostachys sp. (modified from Zimmermann, 1959 ). The dashed lines in the bract indicate that its length is variable (see the long bract in C. al. schimperia; Meyen, 1987, fig. 26c). 54, 55. Equicalastrobus chinleana, illustrating the postulated fusion of a sporangiophore and the bract of a calamite cone. 56. Sporangiophore of Equisetostachys verticillata from the Triassic of France showing a umbo (arrow) on the outer side of the head (modified from Grauvogel-Stamm, 1978). A similar umbo also is present in the cone of Equisetites arenaceus from the Upper Triassic of Germany (Kelber and van Konijnenburg-van Citter, 1998) and in a Jurassic equisetalean cone from Iran (Corsin and Stampfli, 1997). These umbos are quite similar to those sporangiophores of Equicalastrobus chinleana, when they are devoid of a leaf-like tip (Fig. 40 ). 57. Sporangiophore of the living Equisetum (modified from Bell, 1992 ). Scale bar = 1 mm

Sporangiophores resembling somewhat those of E. chinleana have been reported in abnormal Equisetum cones by Tschudy (1939) and Page (1972) . Even if according to Browne (1926 , p. 305) "... it is unsafe to attach much value to them as a key to morphology ...," these abnormal sporangiophores might suggest a resurgence of an ancestral feature. Goebel (1930) and Zimmermann (1930, fig. 111; 1959, fig. 121B) also figured an abnormal cone of Equisetum and interpreted its sporangiophores, which bore a leaf-like outgrowth, as a transitional stage between a sporophyll and a vegetative leaf. However, in none of these studies is it clearly shown exactly where the leafy outgrowth is attached to the sporangiophore disk. Tschudy (1939) only reported that the leafy outgrowth protruded from the top of the sporangiophore and Page (1972, p. 370) stated that the sporangiophores "... show short upward-pointing prolongations from the upper halves of their peltate heads." Although Page (1972 , figs. 4A, D–F) clearly indicated in some of his drawings that the leafy outgrowth protruded from the middle of the sporangiophore head, as in E. chinleana, he did not present any detailed drawings or interpretations of these sporangiophores.

It is worth mentioning that fossil cones of Equisetaceae have been described showing sporangiophore disks with an hexagonal umbo comparable to that of E. chinleana but devoid of any leafy outgrowth. For example, the Triassic cone attributed to Equisetostachys verticillata (Grauvogel-Stamm, 1978 ) has such a feature. However, it was not mentioned when the fossil was originally described because the feature was not recognized until the cone was reexamined during the present investigation. The sporangiophore heads of Equisetites arenaceus from the Upper Triassic of Germany also show an umbo in the middle of its outer side, which has been interpreted as the scar of its central stalk (Kelber and van Konijnenburg-van Cittert, 1998). An equisetalean cone from the Middle Jurassic Shemshak Formation in Iran also has sporangiophore disks that have hexagonal umbos (Corsin and Stampfli, 1977 ). Perhaps the umbos on the sporangiophore disks of these fossil equisetaleans are vestigial structures and remnants of this ancestral structure. Such a hypothesis may be supported by the fact that in the range of variations of the sporangiophores of E. chinleana, some have short leaf-like umbo tips [see Figs. 33 (at arrows a, b, d), 38, 39, 55] whereas others are completely devoid of this leafy outgrowth and show only the umbo [Figs. 37 (at arrow c), 40], as in the Equisetites cones mentioned above.

The hypothesis of the fusion of the fertile and bracteate whorls in calamite cones was suggested earlier by Jeffrey (1899, p. 185) and discussed by Browne (1926 , p. 303), but it has never found general acceptance according to him, partly because in abnormal Equisetum cones, "... the concrescence seems always to occur in lateral direction, i.e., between members of the same whorl ..." and partly because no fossils have been found previous to the present investigation that show recognizable traces of such a fusion. Equicalastrobus chinleana appears to be that fossil.

The hypothesis that there was a fusion between a bract and sporangiophore is not in contradiction with their supposed evolutionary pathways, which according to Page (1972 , p. 72) implies that each of the organs arose from a branching system and that therefore they are homologous. Furthermore, their successive production in the cones of calamites shows that they are serial homologs.

In conclusion, although several authors (e.g., Chaloner and Boureau, 1967 ; Bock, 1969 ) have expressed doubt about the attribution of "L." chinleana to the lycopods, the fossil has never been studied in enough detail to determine its true systematic position until this investigation. The new study shows that it was incorrectly assigned to the Lycopodiales by Daugherty (1941) and to the quillworts by Retallack (1997) and that it actually belongs to the Equisetales. As a result, the cone is redescribed here under the name Equicalastrobus chinleana (Daugherty) Grauvogel-Stamm and Ash, n. comb. Furthermore, the investigation shows that the sporangiophores of the cone have both archaic and modern features, respectively their leafy aspect and their Equisetum-like structure (i.e., the peltate sporangiophores and the recurrent attachment of sporangia), and that E. chinleana appears to represent a transitional stage between Calamites and typical Equisetum. By filling the gap between the two, this fossil indicates that Equisetum could be a direct descendant of the Calamitaceae.

View larger version (37K): [in this window] [in a new window]   Fig. 3. Index map of a part of southwestern United States showing the localities where E. chinleana occurs. The chart in the upper right corner shows the principal members of the Chinle Formation in the Colorado Plateau and the stratigraphic position of each locality

View larger version (37K): [in this window] [in a new window]   Figs. 41–50. Diagrams illustrating the origin of the differences in appearance of the sporophylls of E. chinleana from localities 1 (Figs. 48–50) and 2 ( Figs. 43–46), which resulted from the difference in maturity of the cones when they were buried. Symbols as in Figs. 28–32. 41. Longitudinal section of a buried sporophyll, which has been changed to a layer of coaly material, as in Fig. 30. Note that the tiny trichomes present on the outer surface of the disk left a clear imprint in the surrounding sediment and are represented by stippling and short lines in the impression of the outer surface of the disks in plan view shown in Figs. 43–46, 48–50. 42. Longitudinal section of a sporophyll from locality 2, deduced from the sporangiophores preserved in plan view in Figs. 33–36 , after the enclosing rock has been split and most of the coaly layer has fallen away leaving an impression of the outer surface of the fossil. In this cone, which is mature, the sporangiophores are more or less disassociated, but each is so tightly compressed that the disk and attached leaf-like umbo tips appear to lie on virtually the same plane. Since the space between the leaf-like tip and the disk is very narrow, the sediment infilling is quite thin and fragile and often falls away when the rock is split. Consequently, the disk is usually more or less absent over the site of the leaf-like tip in the sporangiophore as shown in Figs. 43–46. 43–46. Impressions of the outer surface of four disks of the holotype ( Figs. 43–45 based on disks indicated by arrows a, b, and c, respectively, in Fig. 7 ; Fig. 46 based on the disk marked with a small arrow near the top of Fig. 5 ). In Fig. 43 only a small portion of the upper margin of the disk has fallen away to reveal a small part of the underlying leaf-like umbo tip, whereas in Figs. 44, 45, 46 progressively larger amounts have disappeared and larger areas of the underlying leaf-like umbo tip are visible. 47. Longitudinal section of a sporophyll from locality 1, deduced from the specimens in Figs. 13, 14, 17, 18. In these immature cones the sporangiophores are so tightly arranged that the disks are contiguous, resulting in a continuous zone of weakness along which the rock usually splits. Since the sporangiophores were not tightly compressed, the space between the outer surface of the disk and the leaf-like umbo tip is wide and the sediment infilling is relatively thick and strong and rarely falls away when the rock split. As a result, only the outer surfaces of the disks are exposed in most specimens from locality 1 (see Figs. 11–14, 17, 18 ) and the leaf-like umbo tip, which is in an underlying plane, remains buried in the sediment. 48–50. Imprints of the outer surfaces of three disks of the sporophylls from locality 1 illustrated in Figs. 14, 18 (at arrows a, b, c). Where the leaf-like umbo tip is buried beneath the disk, its margins are represented by dashed lines in these figures, and where the disk is missing, its outline is shown by a dotted line. The leaf-like umbo tip is partly exposed only in the sporophyll shown in Fig. 50 (arrow c in Fig. 18 ). Scale bar = 1 mm

	FOOTNOTES

⁴ Author for correspondence.

	LITERATURE CITED

TOP ABSTRACT INTRODUCTION PREVIOUS INVESTIGATIONS MATERIALS AND METHODS SYSTEMATIC DESCRIPTIONS DISCUSSION LITERATURE CITED

 
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This Article Abstract