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smithsonian contributions to paleobiology • number 99

Plant Fossils from the 

Pennsylvanian–Permian 

Transition in Western Pangea, 

Abo Pass, New Mexico 

William A. DiMichele, Spencer G. Lucas, 

Cindy V. Looy, Hans Kerp, 

and Dan S. Chaney
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smithsonian contributions to paleobiology • number 9 9

Plant Fossils from the 

Pennsylvanian–Permian 

Transition in Western Pangea, 

Abo Pass, New Mexico 

William A. DiMichele, Spencer G. Lucas, 

Cindy V. Looy, Hans Kerp, 

and Dan S. Chaney

WASHINGTON, D.C.

2017
Published by SMITHSONIAN INSTITUTION SCHOLARLY PRESS

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Library of Congress Cataloging-in-Publication Data

Names: DiMichele, William A. | Smithsonian Institution Scholarly Press, publisher.

Title: Plant fossils from the Pennsylvanian-Permian Transition in western Pangea, Abo Pass, New Mexico / William A. DiMichele [and 

four others].

Other titles: Smithsonian contributions to paleobiology ; no. 99.

Description: Washington, D.C. : Smithsonian Institution Scholarly Press, 2017. | Series: Smithsonian contributions to paleobiology, ISSN 

0081-0266 ; number 99 | Includes bibliographical references.

Identifiers: LCCN 2016045255

Subjects: LCSH: Plants, Fossil—New Mexico. | Paleobotany—Pennsylvanian. | Paleobotany—New Mexico—Paleozoic. | Paleobiology—

New Mexico. | Geology, Stratigraphic—Permian. | Geology, Stratigraphic—Paleozoic.

Classification: LCC QE989 .P53 2017 | DDC 561/.11520978962—dc23 | SUDOC SI 1.30:99

LC record available at https://lccn.loc.gov/2016045255

ISSN: 1943-6688 (online); 0081-0266 (print)

Publication date (online): 2 March 2017

The paper used in this publication meets the minimum requirements of the American National Standard for Permanence of Paper for 

Printed Library Materials Z39.48–1992. 

ABSTRACT

DiMichele, William A., Spencer G. Lucas, Cindy V. Looy, Hans Kerp, and Dan S. Chaney. Plant Fossils from the Pennsylvanian–Permian Transition in Western Pangea, Abo Pass, New Mexico. Smithsonian Contributions to Paleobiology, number 

99, viii + 40 pages, 21 figures, 1 table, 2017. — Plant fossils are described from five stratigraphic levels spanning the Pennsylvanian–Permian boundary in Abo Canyon, New Mexico. Charles B. Read collected the fossils in 1940 and 1941; Read’s 

field notes cannot be located. A combination of Read’s bed numbering pattern, his notes in the collections, and collection 

taxonomic composition permits them to be placed in an oldest to youngest sequence. The youngest fossils, from the Abo 

Formation of early Permian age, anchor the collection stratigraphically. A collection labeled “Base of Red Magdalena” is most 

likely equivalent to the modern Bursum Formation, thus immediately below the Abo. The three remaining collections are from 

the Upper Pennsylvanian portion of the Atrasado Formation. All collections are dominated or codominated by plants typical 

of environments with seasonal moisture stress and record increasing moisture limitation through time. Conifers, Sphenopteris 

germanica, and mixoneurid odontopterids are common to abundant in pre–Abo Formation collections. These same collections 

also contain wet-substrate taxa, particularly calamitaleans and marattialean tree ferns, with arborescent lycopsids in the oldest 

collection. The middle three collections contain the remains of microconchids/spirorbids, snails, ostracods, and conchostracans 

closely associated with the plant remains, indicating brackish to marine salinities at the burial sites of the organics. The Abo 

Pass collections document Pennsylvanian–Permian floristic changes in the western Pangean equatorial belt, an important point 

of comparison to better studied floras from this same interval in central Pangea (eastern United States and Europe). Most of the 

plants in the western equatorial assemblages are the same as those of similar age from west central to central Pangea (Euramerica), indicating a widespread tropical biogeographic province at this time, but within which there were several distinct biomes.

Cover images, from left to right: Selected details from Figures 12e, 16h, and 9g. 
Contents

LIST OF FIGURES v

LIST OF TABLES vii

INTRODUCTION 1

GEOLOGY 2

METHODS OF ANALYSIS 4

A Note on Conifer Taxonomy 4

ABO FORMATION 4

Age and Depositional Environment 6

Fossil Flora 7

BASE OF RED MAGDALENA 7

Fossil Flora 10

Age and Environmental Implications of the Flora 14

UNIT 8 16

Fossil Flora 16

Stratigraphic and Environmental Interpretations 19

UNIT 10 19

Fossil Flora 20

Stratigraphic and Environmental Interpretations 27

UNIT 35 27

Fossil Flora 29

Stratigraphic and Environmental Interpretations 33

DISCUSSION 35

ACKNOWLEDGMENTS 36

REFERENCES 36

INDEX OF SCIENTIFIC NAMES 39

Figures

1. Location of the study section in Abo Pass, New Mexico 2

2. Geological section in Abo Pass 3

3. Abo Formation flora: Walchia, Sphenophyllum, Autunia 8

4. Abo Formation flora: Supaia 9

5. Base of the Red Magdalena: animals 11

6. Base of the Red Magdalena: walchian conifers, Taeniopteris 12

7. Base of the Red Magdalena: walchian conifers 13

8. Base of the Red Magdalena: mixed plants 15

9. Unit 8: dominant floral elements 17

10. Unit 8: minor floral elements 18

11. Unit 10: comminuted plant debris, animal fossils 21

12. Unit 10: walchian conifers 22

13. Unit 10: walchian conifers, unidentified coniferophyte 23

14. Unit 10: mixed plants 24

15. Unit 10: medullosans, peltasperms 25

16. Unit 10: medullosans, Sphenopteris germanica 26

17. Unit 10: calamitaleans, ferns, miscellaneous elements 28

18. Unit 35: medullosans, conifers 30

19. Unit 35: Sphenopteris germanica, medullosans, fern 31

20. Unit 35: mixed plants 32

21. Unit 35: lycopsids, ferns, seed 34

Table

1. Results of quantitative analysis 5

1 Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA.

2 New Mexico Museum of Natural History, 

1801 Mountain Road NW, Albuquerque, New 

Mexico, 87104, USA. 

3 Department of Integrative Biology, University of California Museum of Paleontology, 

University of California, Berkeley, California 

94720, USA. 

4 Forschungsstelle für Paläobotanik, Geologisch-Paläontologisches Institut, Westfälische 

Wilhelms-Universität Münster, Heisenbergstraße 2, 48149 Münster, Germany. 

* Correspondence: dimichel@si.edu

Manuscript received 28 July 2016; accepted 

25 August 2016.

INTRODUCTION

The Pennsylvanian–Permian transition encompassed a gradual global environmental 

change that, on average, involved the shift from a wetter Pennsylvanian to a drier Permian. This transition was, however, complex in its expression, varying geographically in 

timing, intensity, and details, not happening at all in parts of eastern Pangea, namely, 

present-day China (e.g., compare the patterns in western Pangea [Montañez et al., 2007; 

Tabor and Poulsen, 2008; Tabor et al., 2013], west central Pangea [Blake and Gillespie, 

2011; Cecil, 2013; Montañez and Cecil, 2013], east central Pangea [Roscher and Schneider, 2006; Opluštil and Cleal, 2007; Cleal et al., 2012], and eastern Pangea [Hilton and 

Cleal, 2007]). Superimposed on this transition were glacial-interglacial cycles of the late 

Paleozoic ice age, which encompassed all of the Pennsylvanian and much of the early 

Permian (Cecil, 1990; Fielding et al., 2008; Montañez and Poulsen, 2013; Cecil et al., 

2014). Furthermore, because of the gradual and areally variable nature of the environmental changes, the boundary between the Pennsylvanian and Permian is not marked 

by a major geological hiatus (Cecil, 2013). As a result, the placement of the boundary 

has been difficult and controversial, particularly in terrestrial sections, both in the means 

used to identify it and in the reliability with which it can be identified in different places 

throughout the world.

In New Mexico, this transition is exposed throughout the central part of the state 

from its southern reaches near Alamogordo, through the central portion of the state east 

of Socorro, and into Abo Pass (Figure 1). Although studies of the regional geological 

and paleontological aspects of this transition have been published (e.g., Read and Wood, 

1947; Lucas et al., 2013), a detailed synthesis of the paleobotanical changes in this important region of western Pangea remains incomplete. A general overview of the changes 

was provided by Ash and Tidwell (1982) and remains a good summary of the basic 

patterns.

We report here on plant-fossil collections from five stratigraphic levels in Abo Pass 

(Figure 1) made principally by Charles B. Read in 1940, and supplemented in 1941, in 

his capacity as a paleobotanist and field geologist for the U.S. Geological Survey (USGS). 

The specimens are housed permanently in the collections of the National Museum of 

Natural History in Washington, D.C. Unfortunately, as will be discussed below, the 

disposition of the field notebooks associated with these collections is not known, forcing us to rely on secondary sources to assess the stratigraphic interval covered by the 

Plant Fossils from the Pennsylvanian–

Permian Transition in Western Pangea, 

Abo Pass, New Mexico

William A. DiMichele,1* Spencer G. Lucas,2 Cindy V. Looy,3

Hans Kerp,4

 and Dan S. Chaney1
2 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

collections. The fossil assemblages compare most closely with 

those described from the Pennsylvanian–Permian boundary elsewhere in Euramerica, including the Dunkard of the U.S. Appalachian Basin (Blake and Gillespie, 2011), Rotliegend of Germany 

(e.g., Potonié, 1893; Barthel, 1976, 2009; Kerp and Fichter, 

1985; Barthel and Brauner, 2015), the Autunian of France (e.g., 

Zeiller, 1890, 1892, 1898, 1906; Doubinger, 1956; Galtier and 

Broutin, 1995, 2008; Steyer et al., 2000; Bercovici and Broutin, 

2008), and other European strata that encompass the Pennsylvanian–Permian transition (e.g., Broutin, 1981; van Amerom et al., 

1993; Wagner and Martínez García, 1982; Popa, 1999; Šimůnek 

and Martínek, 2009).

GEOLOGY

Read measured his stratigraphic section in Abo Canyon (Figure 

1) in the “southeastern part of Casa Colorado Grant, and portions

of T. 2 and 3 N., R. 5 and 6 E., between Sais Quarry and Abo State 

Monument, Socorro and Torrance Counties, New Mexico” (Read 

and Wood, 1947). At the time, the Sais Quarry (or crusher) was on 

the western end of the measured section, a site where ballast was 

quarried and crushed for use by the railroad—this quarry ceased 

operations in the 1950s. The Abo State Monument, on the eastern 

end, refers to the Abo ruins. The study area is on the USGS Scholle 

and Abo 71⁄2 minute quadrangles. Read most likely investigated 

exposures along the railroad, along U.S. Highway 60, which parallels the railroad for much of its traverse through Abo Canyon, and in 

side canyons along this route. There are many canyons branching off 

to the north and south of the railroad, and active copper mining 

occurred in that same area from beds in the Abo Formation (Hatchell 

et al., 1982).

We do not have Read’s field notes, and they have not been 

located in either the archives of the U.S. Geological Survey in 

Denver or in the U.S. National Archives in Beltsville, Maryland; 

the latter holds a large collection of microfiche created by the 

FIGURE 1. Location of the study section in Abo Pass, New Mexico. According to Read and Wood (1947), the study was carried out in Townships 2N and 3N, Ranges 5E and 6E. P = Pennsylvanian, Pa = Abo Formation, Pb = Bursum Formation, PC = Precambrian, Pg = Glorieta 

Sandstone, Psa = San Andres Formation, Py = Yeso Group, and Q = Quaternary.
number 9 9 • 3

Sandia

Formation

Madera Limestone,

gray limestone member

Madera 

Limestone,

arkosi

c

 limestone 

member

"Red 

Magdalena?" Abo Formation

Yeso

Formation

Yeso

GroupAbo Formation Sandia Formation Gray Mes

a

Formation

Atrasado

 Formation

Bursum 

Formation

2016 1940s

fossil plant assemblages

?

?

Middle Pennsylvanian Late Pennsylvanian Early Permian

Age

USGS from paper copies of notebooks they hold. Lacking the 

field notes, we also lack the details of the measured section, 

which, of course, is a serious handicap to understanding the 

stratigraphic positions of the fossil plant collections subsequently described. An Abo Canyon section was published in 

1944 by Read et al. and again in 1947 by Read and Wood 

(their fig. 4, columnar section 13; reproduced as part of Figure 

2 of this work) but in each instance in only a general way, 

as part of a larger study, and at a scale that precludes clearly 

identifying likely source beds.

Nonetheless, the plants themselves point to the general 

part of the section from which they were derived, most bearing 

strong similarities to floras from European sites from the Late 

Pennsylvanian to the Pennsylvanian–Permian transition. Thus, 

the majority of the collections are likely from either the Bursum 

Formation and/or the “upper, arkosic member of the Madera 

Limestone” (Atrasado Formation; cf. Lucas et al., 2014). According to the Read and Wood (1947) section, the Bursum Formation is about 35 m thick, and the arkosic member of the 

Madera Limestone is approximately 125 m in thickness in the 

measured section. More siliciclastic beds are noted in the arkosic member of the Madera Limestone than in the Bursum.

From an age standpoint, the youngest strata from which 

Read and colleagues collected plant fossils were of the Abo Formation, which is lower Permian, spanning the middle to upper 

Wolfcampian and lowermost Leonardian, in regional U.S. chronostratigraphic terminology. These stages are the equivalent of 

the Asselian through the lower Kungurian in international terms 

(Lucas et al., 2005, 2015). Although Read numbered the beds in 

the 1940 section, no bed number was assigned to the collections 

from the Abo. The rest of the collection comes from four beds, 

identified as Unit 8, Unit 10, Unit 35, and Base of Red Magdalena. The plant-fossil content suggests that the lower numbers 

are from near the top of the section, possibly beginning in the 

Bursum Formation, and that the Red Magdalena, collected in 

1941, also may be from the Bursum Formation. The Pennsylvanian–Permian boundary lies either within or at the base of the 

Bursum Formation, depending on which marine index fossil is 

chosen to mark the base (see Lucas et al., 2016, for discussion). 

The Madera Limestone is equivalent to the Gray Mesa (lower 

gray member) and Atrasado (upper arkosic member) formations, according to the graphic correlation of Read and Wood 

(1947) (Figure 2). The lower gray member encompasses most of 

the Desmoinesian (upper Middle Pennsylvanian), whereas the 

upper arkosic member spans the upper Desmoinesian, Missourian (lower Upper Pennsylvanian), and Virgilian (Upper Pennsylvanian) U.S. regional stages, approximately equivalent to the 

upper Moscovian, Kasimovian, and Gzhelian, respectively, in 

international terms (Lucas et al., 2013).

Some attributes of the depositional environments in which 

the floras are preserved can be deduced from the lithological 

characteristics of the rock matrices in which they are preserved. 

Basic grain size; sedimentary structures (lamination, various 

kinds and degrees of cross bedding); the disposition of the 

FIGURE 2. Geological section in Abo Pass (see text for details). Plant 

fossils come from the Abo Formation and from several levels in the 

Bursum and Atrasado formations. Geological column is from Read 

and Wood (1947, fig. 4) in Journal of Geology, used with permission 

of The University of Chicago Press.
4 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

plants in the matrix (Are they flat lying? Do they cross bedding 

planes? Are they folded and twisted?); the presence of comminuted plant debris (CPD); its relative abundance and distribution; 

and the distribution and abundance of such sedimentary particles 

as muscovite all assist in determining the degree of energy in the 

system at the time plant parts were deposited. Animal remains associated with the plants also provide insights into relative salinity 

and the amount of water table or shoreline fluctuation. However, 

the lack of data on outcrop-scale interpretations of the sedimentary environments is a significant handicap. As will be seen from 

discussions of each flora, the environmental conditions under 

which they were deposited, to the degree we can determine them, 

vary from little to significantly different, indicating that whereas 

all may have been deposited under seasonal climatic regimes, the 

position of the depositional environment relative to the coastline 

and the specific environment in which deposition took place vary 

among the collections. Hence, comparisons of them cannot be 

considered “isotaphonomic” (Behrensmeyer et al., 2000) at the 

level of the depositional environment.

METHODS OF ANALYSIS

Each collection was examined primarily at hand-lens magnifications of ×3, ×4, and ×7, with microscopic examination 

supplemented where necessary. Photography and subsequent 

enlargement and image enhancement permitted recognition of 

features in especially difficult specimens.

The plant remains in all collections but those from the Abo 

Formation are fragmentary, often of quite small size. As a consequence, identification of many of these remains was difficult, 

and multiple possibilities often exist for their natural affinities. 

The identification possibilities were narrowed by comparing the 

New Mexico material mainly with floras or monographic works 

encompassing Upper Pennsylvanian and Pennsylvanian–Permian 

boundary strata, which are the most likely points of comparison 

based on lithostratigraphic and marine invertebrate biostratigraphic assessments of the age of the strata in the collection area. 

The authors realize these comparisons place biases on the scope 

of the search for similar forms, but they also facilitate identification within the limited time frame permitted for this study.

Quantitative assessments of each flora were carried out 

using a modification of the hand-sample/quadrat method of 

Pfefferkorn et al. (1975), as described by Bashforth and Nelson 

(2015). In this method, top and bottom surfaces of each hand 

sample are treated as separate sampling quadrats. All plant taxa 

occurring on a surface are noted as present and recorded only 

once, regardless of how many individual specimens of that taxon 

may be present. Barren surfaces are noted and compiled. Part 

and counterpart faces, if present, were treated as the same surface and counted only once. This type of analysis addresses the 

problem of what to count in undertaking a quantitative analysis, 

with the underlying presumption that estimation of biomass is 

the primary objective of the quantification. It accounts for the 

fact that many specimens may be highly fragmented into small 

pieces, whereas others, which may account for significantly more 

biomass than all the small material combined, may be represented only by large, intact, single specimens. 

This kind of quantification yields a frequency of occurrence of each category counted (taxa or objects, such as axes) 

compared to the total number of quadrats (hand samples analyzed). Reported frequencies of occurrence are calculated only 

as a percentage of the total fossiliferous quadrats, with part and 

counterpart surfaces counted only once; barren surfaces are not 

considered in the calculation of the total number of quadrats. 

Therefore, the reported percentages given for all categories 

below may add up to more or less than 100% because the occurrence frequencies of each taxon are independent measurements 

(there is no closure, unlike with absolute count measurements). 

Animal remains were noted, and their occurrences were tabulated in a manner similar to that of the plants. However, if only 

animal remains were present, such quadrats were tabulated as barren with regard to further analysis of plant occurrence frequency.

The results of the quantitative analysis are presented in 

Table 1. These are the actual occurrence data for the categories of 

identifiable material (mainly taxonomic groups) in the collection. 

It is important to remember that these are frequency data reflecting the number of quadrats (hand-sample surfaces) on which a 

category was identified, not category counts. Consequently, they 

are nonadditive within any given collection because more than 

one category may occur on any given quadrat surface.

A Note on Conifer Taxonomy

Conifers dominate or occur at high frequencies in all the 

collections analyzed in this study. These remains have been classified as morphotypes, following the procedures and character 

analysis used by Looy and Duijnstee (2013), rather than being 

assigned to genera or species. We believe all these plants to be 

walchian conifers. The reader familiar with them will note morphologies similar to such taxa as Ernestiodendron (Morphotype 

A), Walchia piniformis (Morphotype B), and Walchia schneideri 

(Morphotype D). Morphotype C, represented by few specimens, has more orders of branching and different leaf angles 

than the other forms and appears to be a previously unrecognized form that may not fit into a described species. Detailed 

analysis suggests that the New Mexico material differs in subtle 

but consistent ways from the type concepts for these described, 

aforementioned taxa.

ABO FORMATION

Two collections were made by Charles B. Read from strata 

of the Abo Formation in Abo Canyon, USGS localities 8952, in 

June 1941 (based on USGS collections catalog), and 8977, undated but which we presume to have been made in July of 1940 
number 9 9 • 5

TABLE 1. Results of quantitative analysis (see text for details). Localities are in presumed stratigraphic order from left to right, youngest (Abo Fm = Abo Formation) to oldest (Unit 35). For Abo Formation collections, counting quadrat occurrences was not practicable; 

X = present. Walchia sp., mixoneurid Odontopteris, and marattialean foliage nonspecific are cumulative categories. Where numbers 

of quadrat occurrences are given, subdivision by morphotype or species was not carried out during the quantification. Morphotypes or 

species identified as PND = present but not differentiated during quantitative analysis. For the marattialean foliage nonspecific category 

in Unit 35, several morphotypes were present, but only one was identified, marked by PND. A long dash (—) = absent from the collection; NA = not applicable.

Category or result of analysis

Locality

Abo Fm Red Magdalena Unit 8 Unit 10 Unit 35

Total quadrats NA 81 95 269 101

Total barren surfaces NA 46 45 115 52

Supaia sp. X — — — —

Walchia sp. X — 12 — 18

Walchia Morphotype A — 18 PND 32 —

Walchia Morphotype B — 49 PND 152 PND

Walchia Morphotype C — 2 — — —

Walchia Morphotype D — 3 — 7 PND

Conifer cones bract scales — 1 — 9 6

Cordaites — 1 6 10 —

Sphenopteris germanica — — 25 6 12

Taeniopteris — 2 — — —

Autunia naumanii — — — — 1

Autunia conferta X — — — —

Rhachiphyllum sp. — 4 — 1 —

cf. Peltaspermum sp. — 1 — 2 —

cf. Peltasperm pollen organs — — — 1 —

Mixoneurid Odontopteris — — 16 16 —

Odontopteris (Mixoneura) schlotheimii — 8 PND PND 3

Odontopteris (Mixoneura) subcrenulata — — PND PND 5

Odontopteris brardii — — — — 10

Reticulopteris sp. — — — 15 —

Alethopteris schneideri — — — 3 —

Alethopteris virginiana — — — — 1

Macroneuropteris scheuchzeri — — — — 18

Pseudomariopteris cordato-ovata — — — — 1

Neuropterids — 3 — 15 15

Marattialean foliage nonspecific — — — — 14

Pecopteris densifolia — — — 2 —

Polymorphopteris polymorpha — — — — PND

Lobatopteris geinitzii — — — 2 —

Oligocarpia gutbieri — — — 6 5

Sphenopteris matheti — — — 1 1

Sphenopteris spp. (ferns) — — 2 — 1

Charliea sp. — — — 1 —

(Continued)
6 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

(on the basis of the noted presence of his field partner for that 

year, H. W. Ervin, who is credited on other collections made at 

that time in Abo Canyon). We made two additional collections 

in 2000 and 2001 as U.S. National Museum (USNM) localities 

41672 and 41873. 

Three of the four total Abo sites (USGS and USNM) can be 

confidently located near the Scholle railroad siding on U.S. Highway 60. Collection USGS 8977 is noted only as being made “at 

Scholle” by C. B. Read (note in collection drawer). Collection 

USNM 41672 is from Cañon Saladito on the south side of the 

highway, and USNM 41873 is from Cañon Salado on the north 

side of the highway. We suspect that the location of USGS 8952 

is close to or identical to that of USNM 41873 on the basis of its 

lithology and fossil content, which are almost identical, including 

the presence of the unusual species Sphenophyllum gilmorei, which 

we have not found at any other of dozens of Abo collecting sites. 

Age and Depositional Environment

The Abo Formation in Abo Canyon is of early Permian age. In 

central New Mexico, the Abo Formation, which is time transgressive from south to north in the state, may be as young as Artinskian, 

in international terminology, and encompasses the middle to upper 

part of the Wolfcampian and lower Leonardian (Lucas et al., 2005, 

2015). The Abo collections reported herein are thus the youngest 

of the specimen suite collected by Read. The lithologies of the specimens that make up all four of these collections are similar and typical 

of many fossiliferous exposures in the Abo Formation throughout 

southern and central New Mexico. They consist of siltstone to finegrained sandstone of pinkish red color. Specimens, many of which 

are large and exceed hand-sample size and may be >10 cm thick, are 

mostly massive to irregularly laminated. Collection USNM 41672 

was noted in the field to be a fine-grained sandstone with weak 

TABLE 1. (Continued)

Category or result of analysis

Locality

Abo Fm Red Magdalena Unit 8 Unit 10 Unit 35

Calamitalean stems — 1 8 3 10

Annularia spicata — — 1 2 8

Annularia carinata — — — — 1

Annularia spp. — 3 — — —

Calamostachys spicata — — — 1 —

Sphenophyllum gilmorei X — — — —

Sphenophyllum thonii — — — 2 —

Sphenophyllum oblongifolium — — — — 12

Sphenophyllum spp. — — 2 1 6

Lepidostrobophyllum hastatum — — — — 3

Lepidophylloides sp. — — — — 2

Small lycopsid axes — — — — 3

Striate axes — 6 15 25 71

Woody axes — 2 27 10 —

Seeds — 2 9 51 10

Comminuted plant debris — 5 — 68 26

Roots — — — — 12

Charcoal — — — 1 1

Conchostracans — — 14 — —

Microconchids — 17 — 10 —

Ostracods — 6 15 2 —

Snails — 1 — 8 —

Linguloids — — 1 — —
number 9 9 • 7

indications of trough cross bedding. Plants were abundant 

in float and were found in place at the top of a low bluff. The 

plants appeared to have been confined to the channel facies, 

which was scoured into purple mottled paleosols. Consistent with entombment during active sediment transport, the 

plants in all four collections include many specimens that are 

disposed at different angles in the rock matrix, often at odds with 

one another, suggesting rapid burial and high rates of sediment 

deposition.

Fossil Flora

No quantitative assessment of fossil taxonomic content was 

made for the Abo collections. All are or appear to have been 

from float and thus are not necessarily reliable indicators of the 

proportional composition of the flora. Additionally, we did not 

make, and there are no indications that Read made, any quantitative assessments of floral composition in the field. Experience 

has shown that the best way to quantify Abo floras is on site, 

from large float or well-exposed bedding surfaces, because of, 

first, the difficulty of digging specimens out of the hard siltstonesandstone matrix and, second, the often sparse occurrences of 

fossils at any given location. 

Collections USGS 8952 and USNM 41873 consist of mixed 

floras, and as mentioned above, the floral composition, including the presence of Sphenophyllum gilmorei, and the similarities 

in color, texture, and bedding characteristics of the two deposits 

suggest possible derivation from the same beds.

The flora includes the following taxa:

• walchian conifers, possibly of more than one species 

(Figure 3a)

• the sphenopsid Sphenophyllum gilmorei, an unusual and 

rarely reported species of groundcover sphenopsid (Figure 3b,c), originally described by White (1929) from the 

Supai Formation (lower Permian) of the Grand Canyon, 

Arizona

• a single specimen of what is possibly the peltasperm Autunia conferta (Figure 3d), encountered at USNM 41873 

• the peltasperm Supaia thinnfeldioides (Figure 4d) 

White (1929), in his original description of Supaia, created 

many species, all which may be morphological variants of a single species, which we treat here as S. thinnfeldioides (see DiMichele et al., 2007, for discussion).

Hatchell et al. (1982) also reported and illustrated plants 

from Abo Canyon in the vicinity of Scholle, from Cañon Salado, 

likely in the same general area as USNM 41873 and USGS 8952. 

Their flora consists of Supaia thinnfeldioides and two morphotaxa of walchian conifers; callipterids also are referenced but not 

illustrated. Thus, this flora also is similar in its mixed composition to the USGS and USNM floras from this location; C. B. 

Read is credited with plant identifications.

Collections USGS 8977 and USNM 41672 consist entirely of 

Supaia thinnfeldioides (Figure 4a–c). Such monospecific composition is typical of the Abo Formation throughout its extent (DiMichele et al., 2013a). The dominant Abo elements at most sites are 

walchian conifers, and these appear to have been abundant across 

the Abo landscape. However, Supaia, the second most common 

element of the Abo Formation, is locally abundant and occurs 

most commonly in monospecific accumulations or with only 

rare walchian conifer specimens. Supaia may have been a small, 

streamside plant that tolerated flood disturbance and tended to 

grow in dense stands (DiMichele et al., 2012). Although the Abo 

flora is relatively diverse, all other elements are rare.

BASE OF RED MAGDALENA

A collection identified as originating at the “base of the Red 

Magdalena” is USGS locality 8951. This collection was made 

in 1941, but a note in the collection drawer says “see coll. of 

1940.” The absence of a bed number is surprising, given this 

note, so it is possible that the note refers to a collection since 

lost or that the fossiliferous bed was identified in 1940 but was 

collected intensively only during a return visit in 1941. The combined flora and lithological matrix of this collection are not similar to any of the others described below, however, suggesting a 

distinct fossiliferous bed. 

The term Red Magdalena does not hold any formal stratigraphic significance, although it was used informally in the early 

to mid-twentieth century. We believe that it most likely refers to 

strata of the Bursum Formation. Read and Wood (1947) specifically refer the Pennsylvanian strata throughout central New 

Mexico to the Magdalena Group and note the more terrestrial 

and more oxidized (= red) nature of the younger units. Thus, it 

is possible that the Red Magdalena is from the upper parts of the 

Magdalena Group, hence the Bursum Formation. In this case, 

USGS 8951 is not the stratigraphically lowest collection made by 

Read in Abo Canyon (that most likely being Unit 35). Although 

its position relative to the other beds remains uncertain, the lack 

of a bed number and a probable Bursum affinity lead us to place 

8951 below the Abo Formation at this time.

In the Red Magdalena collection, plant fossils are preserved 

in a buff siltstone with blocky fracture. Although bedding or 

lamination is not prominent, the plant fossils are horizontally 

disposed, suggesting that bedding surfaces are present, and some 

surfaces are covered with small muscovite flakes. Comminuted 

plant debris is rare, and plant fossils vary in size from small fragments to occasional pieces of larger branches. No charcoal was 

noted. Animal fossils are common and consist of spirorbids/

microconchids, ostracods, and small snails (Figure 5a,c). The 

spirorbids heavily encrust coniferous plant remains. Overall, the 

nature of the matrix and the animal remains suggest a freshwater 

to brackish-water coastal environment, perhaps a quiet embayment (e.g., Taylor and Vinn, 2006; Bennett, 2008).
8 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

FIGURE 3. Abo Formation flora. (a) Walchian conifer, USNM specimen 618975, USNM locality 41873. (b) Longitudinal section through a 

cast of the small sphenopsid, Sphenophyllum gilmorei. USNM specimen 618977, USNM locality 41873. (c) A whorl of Sphenophyllum gilmorei leaves showing typical obovate shape with greatest width in the lower 1

/3 of the leaf and tapering apex. USNM specimen 618978, USGS 

locality 8952. (d) Possibly Autunia conferta, with vaulted pinnule lamina, well-developed pinnule midribs, and steeply ascending, relatively 

straight pinnule lateral veins. USNM specimen 618976, USNM locality 41873. Scale bars = 1 cm.
number 9 9 • 9

FIGURE 4. Abo Formation flora: Supaia thinnfeldioides. (a) One-half of a Supaia leaf (leaves were forked) illustrating typical morphology and preservation as a mold. USNM specimen 618980, USGS locality 8977. (b) Close-up of three lower 

left pinnules in image (a), showing strong pinnule midvein and arching, but vaguely fasciculate, open dichotomous, lateral 

venation. (c) Fragmentary specimen illustrating typical frond shape. USNM specimen 618981, USGS locality 8977. (d)

Fragmentary specimen clearly showing the vaulted character of the pinnules, the ends of which have been broken; pinnule 

bases appear to be constricted because of their irregular thickness and vaulting, resulting in partial burial of the decurrent 

leaf bases. USNM specimen 618979, USGS locality 8952. Scale bars = 1 cm.
1 0 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

Fossil Flora

The base of the Red Magdalena flora is small, consisting of 

81 fossiliferous quadrats and 46 blank surfaces. The high frequency of nonfossiliferous surfaces indicates relative sparseness 

of the plant material and its confinement to one or a few layers. 

Walchian conifers are the most abundant plants in the 

assemblage. These include the following morphotypes.

Morphotype A (22.2% frequency of occurrence; see 

Methods of Analysis section) is represented by isolated shoots 

and several plagiotropic penultimate branch systems with ultimate shoots attached (e.g., Figure 6b,c). The ultimate leaves 

are falcate in profile, with keels on both sides of the leaf; they 

bifacially flatten distally (cf. Type I leaves; de Laubenfels, 1953). 

The leaves are narrow triangular to linear with obtuse apices in 

face view. The leaf attachment areas are rhomboidal in shape. 

Leaves are 3–4 mm in length. Leaf widths range from 0.5 to 

1.0 mm midleaf and are slightly wider at the base. Leaf thickness is 1 mm or less at the base and thins out distally. The leaves 

depart at high angles, 80°–130°; these angles are quite variable 

but consistent within specimens. One branch specimen in which 

the leaf attachment is obscured may be an ovuliferous cone 

(Figure 6d).

Morphotype B (60.5%) is represented by isolated shoots 

and several plagiotropic penultimate branch systems with ultimate shoots attached (Figure 7a–d). Penultimate leaves are up to 

5 mm in length. Ultimate leaves are helically arranged, slightly 

curved in profile, and in some wider specimens distally bifacially flattened and have an incurved apex (cf. Type III leaves; 

de Laubenfels, 1953). The leaves are narrow triangular to linear with obtuse apices in face view. Ultimate leaf lengths range 

from 2 to 4 mm, with widths of 0.4–1.2 mm and a thickness of 

0.3–0.5 mm or less. The leaves depart at low angles, 20°–50°, 

which is generally consistent within specimens.

Morphotype C is represented by two plagiotropic antepenultimate branch systems, with penultimate branches departing 

the antepenultimate branch at angles of 20°–60°, with lower angles near the branch apex (mean values for the individual specimens are 45° and 38°) and ultimate branches leaving at angles 

of 30°–60° (means for individual specimens are 43° and 51°). 

One of these branch systems is not fully developed and shows 

sprays of foliage in its penultimate and ultimate shoots (Figure 

6a); in order to differentiate the dimensions of these immature 

leaves from the mature forms, their dimensions are shown within 

brackets below. The other branch system is fully developed. The 

antepenultimate leaves are slightly curved, bifacially flattened, 

narrowly triangular in face view, 4.0–4.5 mm in length, 0.7–1.0 

mm in width, and arise at an angle of 50°–60°. The penultimate 

leaves are slightly curved to falcate, bifacially flattened, narrow 

triangular in face view, 2.8–3.5 [1.3–2.4] mm long, and 0.6–0.7 

[0.4–0.6] mm wide. They depart at a mean angle of 45° in the 

partially immature specimen and at about 60° in the other; these 

angles are larger when an ultimate shoot arises from the leaf axil. 

Ultimate leaves are helically arranged and slightly curved in profile, with a slightly incurved apex (cf. Type III leaves; de Laubenfels, 1953). The leaves are linear with obtuse apices in face view. 

Leaf lengths range from 1.5 to 2.4 [1.0 to 1.5] mm, and leaf 

widths are 0.5–0.6 [0.3–0.6] mm. Leaves are 0.3–0.5 mm or less 

in thickness. The leaves depart at low angles, 30°–55° [45°–70°], 

and are generally consistent within a single specimen. Leaf morphology of this type is comparable to that of Morphotype B; the 

two shoot specimens might represent different orders of branch 

systems and/or might be from different positions on the tree.

Morphotype D (3.7%) is represented by isolated shoots 

(Figure 5a). The helically arranged ultimate leaves are straight in 

profile, with an inwardly curved apex and a keel on the adaxial 

side of the leaf. Leaf shape in face view and leaf width are not 

known. Leaf length is 3–7 mm, and leaf thickness is ~0.7 mm, 

thinning out distally. The leaves depart at angles between 45° 

and 80°.

A single polliniferous cone (Figure 7e) was found that is 

probably associated with one of these conifer morphotypes. 

The remainder of this flora consists of plants that all occur 

at low frequencies.

Calamitaleans. Calamitaleans are the only certain 

pteridophytes in the flora. The remains consist of fragments of 

foliage attributable to Annularia sp. (3.7%) and a single possible calamitalean stem (1.2%), lacking a node but with regular 

ribbing too coarse to represent the veins of cordaitalean foliage 

and too regular to be sclerenchymatous bundles of a tree fern or 

pteridosperm axis (Figure 6e, right side).

Taeniopteris sp. cf. T. abnormis (2.5%). Two 

small fragments of Taeniopteris sp. cf. T. abnormis foliage were 

found in the collection. The illustrated specimen (Figure 6e, left 

side) appears to come from near the apex of a leaf, given the upcurved aspect of its veins, which are rarely branched. The intact 

apex was likely broad and rounded. The other specimen shows a 

relatively wide midvein and perpendicular, sparsely branched lateral veins. The taxonomy of Taeniopteris is complicated because 

so many species have been named from isolated specimens. Not 

common in European Late Pennsylvanian and Permian floras, 

Taeniopteris is quite common in American floras, particularly of 

early Permian age; see discussion in Barthel (2009) or Wagner 

and Martínez García (1982).

Cordaites sp. (1.2%). A single, questionable fragment of a possible cordaitalean leaf was identified (Figure 8a). 

This specimen may, in fact, be a striate axis, assignable to the 

pteridosperms, possibly Odontopteris (Mixoneura) sp. cf. O. 

schlotheimii. It is not certain that the longitudinal striations are 

straight and unbranched enough to be leaf veins, although they 

appear to be so. In addition, the very flat aspect of the specimen suggests a leaf rather than a flattened axis. The specimen is 

encrusted with spirorbids, which suggests a period of flotation 

prior to burial, again consistent with a flat, individual leaf blade 

rather than a stem.
number 9 9 • 1 1

FIGURE 5. Base of the Red Magdalena, animal fossils. USGS locality 8951. (a) Ostracods (small, white, bean shaped) in sediment 

surrounding walchian conifer Morphotype D. USNM specimen 618982. (b) Microconchids/spirorbids attached to taxonomically 

indeterminate axes and to foliage of a walchian conifer. USNM specimen 618983. (c) Mold of a snail (at S, arrow) and ostracods 

in sediment surrounding a branch of walchian conifer Morphotype B. USNM specimen 618984. Scale bars = 1 cm.
1 2 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

FIGURE 6. Base of the Red Magdalena, USGS locality 8951. (a) Walchian conifer Morphotype C antepenultimate branch fragment; the penultimate and ultimate shoots are still developing. USNM specimen 618990. (b) Walchian conifer Morphotype A 

branch fragment with larger leaves, also showing the oblique insertion of leaves. USNM specimen 618991. (c) Walchian conifer 

Morphotype A of “typical” morphology. USNM specimen 618992. (d) Walchian conifer Morphotype A possible ovuliferous cone 

shown in a section with the external cover of leaves/bracts removed. USNM specimen 619252. (e) Taeniopteris sp. cf. T. abnormis

(left side) and possible calamitalean stem (right side). USNM specimen 618993. Scale bars = 1 cm.
number 9 9 • 1 3

FIGURE 7. Base of the Red Magdalena, USGS locality 8951, walchian conifers, comparative morphology. (a) Walchian conifer 

Morphotype B branch fragment. USNM specimen 618985. (b) Walchian conifer Morphotype B branch fragment. USNM specimen 

618986. (c) Walchian conifer Morphotype B branch fragment. USNM specimen 618987. (d) Walchian conifer cf. Morphotype B 

branch fragment. Note that leaves are smaller and more closely adpressed to the axes than in the other illustrated specimens. USNM 

specimen 618988. (e) Walchian polliniferous cone or bud. USNM specimen 618989. Scale bars = 1 cm.
1 4 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

Odontopteris (Mixoneura) sp. cf. O. schlotheimii (10%). Various pinnule fragments were found that 

have relatively sparse venation that departs steeply from the 

midvein area (a midvein is not well developed but may consist 

of a concentration of veins), arches through the lamina, and 

meets the margin at nearly a right angle (Figure 8b–e). Where 

lateral pinnules are clearly developed (Figure 8d), multiple veins 

enter the lamina, and a midvein is absent to only weakly developed. Overall, the pinnules we assign to this species are quite 

variable. However, the combination of specimens with an 

undulatory margin (Figure 8b), elongate triangular shape 

(Figure 8c,e), or triangular shape with a bluntly attenuated apex 

where small individual pinnules are developed (Figure 8d) is 

broadly consistent with Odontopteris (Mixoneura) schlotheimii 

(e.g., Barthel and Amelang, 2011). Note that Mixoneura has 

been considered by some authors to be a distinct genus (see 

Wagner and Castro, 1998); because of its complex nomenclatural history, we use it here parenthetically and as a subgroup of 

Odontopteris.

Rhachiphyllum sp. cf. R. schenkii (4.9%). Rhachiphyllum is a callipterid, a group of peltaspermous seed plants 

segregated from the genus Callipteris (Kerp and Haubold, 1988).

Specimens assigned to this species are superficially quite like some 

of those attributed to Odontopteris (Mixoneura) schlotheimii in 

having sparse secondary venation and a somewhat odontopteroid 

aspect due to the entry of secondary veins into the lamina directly 

from the supporting rachis (Figure 8f). However, these lateral 

veins ascend steeply and meet the margin at an acute angle. They 

also have a well-developed, albeit thin, midvein that is not sunken 

into the lamina surface (as in Autunia). The tearing of the lamina 

in some of the specimens (as seen in the left-hand specimen in 

Figure 8f) also indicates a thin pinnule lamina. In keeping with 

this assignment, a small, fragmentary specimen with similarities 

to Peltaspermum sp., the reproductive organ of various types of 

callipterids, a subgroup of the peltasperms (Kerp and Haubold, 

1988; Poort and Kerp, 1990), was identified in the collection 

(Figure 8g).

The remainder of the plant material consists of fragmentary 

pinnules of a neuropteroid aspect that are not specifically determinable (3.7%); rare seeds (n = 2, 2.5%); striate axes, possibly of 

pteridosperms (7.4%); and woody axes, probably from conifers 

(n = 2, 2.5%).

Age and Environmental Implications of the Flora

If, as we suggest, the Red Magdalena correlates approximately with the Bursum Formation, then there is a published 

flora by Tidwell et al. (1999; with additional illustrations in Ash 

and Tidwell, 1982) with which it can be compared, from the Red 

Tanks Member of the Bursum Formation at Carrizo Arroyo in 

the Lucero uplift of Valencia County. The Red Tanks Member 

is equivalent to the dominantly nonmarine facies of the Bursum 

Formation as the term is used elsewhere in the state, placing it at 

the Pennsylvanian–Permian boundary. The important similarities between the large Carrizo Arroyo assemblage and the small 

collection made in Abo Pass are the following: (1) conifer dominance of a largely seed-plant flora, (2) conifer morphotypes that 

are the same as those identified by Tidwell et al. as Walchia piniformis, Walchia schneideri, and Ernestiodendron filiciforme, (3) 

common mixoneurid odontopterids, including forms attributed 

to Neuropteris pseudoblissii, which may, in fact, be Odontopteris (Mixoneura) schlotheimii, and also some similar in form 

to those reported here with elongate, triangular pinnules/pinnae, (4) common callipterids, including Rhachiphyllum schenkii, 

and (5) few pteridophytes, but including calamitaleans, which 

are nearly ubiquitous in late Paleozoic floras, even from strongly 

seasonally dry settings, seemingly occupying even ephemeral wet 

areas. The more intensively sampled Carrizo Arroyo flora also 

includes a slightly wider array of medullosan pteridosperms, 

more abundant cordaitaleans, and common Sphenopteris germanica (previously identified as either Sphenopteridium manzanitanum or Arnophyton kuesii), plus such species as Charliea 

manzanitanum and Phasmatocycas.

The authors also have collected extensively in the Bursum 

Formation, both in central and southern New Mexico, and have 

found a flora similar to that reported by Ash and Tidwell (1982) 

and Tidwell et al. (1999). A brief mention and illustration of 

the dominant plants in these floras from Socorro County to the 

southwest of Abo Pass are in Lucas et al. (2013). Key findings 

from these studies are that the Bursum floras are dominated primarily by conifers, with locally common to abundant cordaitaleans, Taeniopteris, Sphenopteris germanica, and callipterids, the 

latter most commonly represented by Rhachiphyllum schenkii. 

However, there is considerable site-to-site variation in composition, including local abundance of marattialean tree fern foliage 

(Pecopteris).

From a biostratigraphic point of view, there are virtually 

no taxa of the Abo Pass Red Magdalena flora or of presumably 

similar aged floras in Carrizo Arroyo or Socorro County that 

will permit an unequivocal age assignment, apart from being 

near the Pennsylvanian–Permian transition. The floras bear a resemblance to those of the Rotliegend, as characterized in detail 

by Barthel (2009).

The abundant occurrence of freshwater to brackish-water 

invertebrates indicates these assemblages occupied a coastal or 

estuarine setting. Tidwell et al. (1999) discuss the possible setting 

of the Carrizo Arroyo Bursum flora and address the conflict that 

exists between interpretations that place the flora in the “uplands” and those that attribute its composition to a climate of 

seasonal drought. Here, we stand firmly on the side of a climatic 

interpretation—the plants were most likely growing close to the 

environment in which they were deposited, and that environment was in the lowlands. The near absence of pteridophytes 

(spore-producing ferns, lycopsids, sphenopsids) in the flora and 

strong dominance of xeromorphic seed plants suggest conditions 

of periodic moisture stress, perhaps a dry subhumid climate in 

the terminology of Cecil (2003). 
number 9 9 • 1 5

FIGURE 8. Base of the Red Magdalena, USGS locality 8951. (a) Cordaitalean leaf fragment. Note flat aspect and fine longitudinal striations (veins). Microconchids are attached to the surface. USNM specimen 618994. (b) Odontopteris (Mixoneura) 

schlotheimii pinna terminus; note undulatory margins. USNM specimen 618995. (c) Odontopteris (Mixoneura) schlotheimii; 

several terminal portions of pinnae likely originally attached to a common rachis. USNM specimen 618996. (d) Odontopteris 

(Mixoneura) schlotheimii, pinnate portion of a pinna. Pinnules have rather widely spaced veins. USNM specimen 618997. (e)

Odontopteris (Mixoneura) schlotheimii, pinna segments. Note strong arc of veins as they approach the margins. USNM specimen 618998. (f) Compare with Rhachiphyllum schenkii. Pair of pinnules with thin, but clearly marked midvein and steeply 

ascending, relatively straight lateral veins that meet the margin without changing trajectory. USNM Specimen 618999. (g) Compare with Peltaspermum type, radially symmetrical, reproductive organ, fragmentary preservation. USNM specimen 619001. 

(h) Ostracods in sediment surrounding walchian conifer branch. USNM specimen 619000. Scale bars = 1 cm.
1 6 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

UNIT 8

Read’s Unit 8 collection, USGS locality 8974, consists of fossil plants on pieces of light to medium gray, slightly silty claystone 

that is blocky in character and may be very finely laminated, although this is difficult to see in hand samples. Bedding surfaces 

are broad and flat, and the plant remains give no indication of 

crossing these horizontal surfaces. Plant fossils are preserved as 

organic compressions with some staining by iron minerals. There 

commonly are halos of reduced iron around plant remains. The 

plant fossils are mainly fragmentary and of relatively small size. 

They appear to have been transported into the depositional environment, which, from the character of the rock matrix, was a site 

of quiet-water sedimentation, perhaps a lake. However, no CPD 

was noted among the plant fossil remains or draping bedding 

surfaces. Comminuted plant debris might be expected in a lowenergy depositional setting into which streams were debouching, 

carrying various sizes of plant litter; perhaps the size of the catchment was large, accounting for both the small size of the plant 

fragments and the absence of CPD, which may have been diluted 

by dispersal. No fossil charcoal was identified in the collection.

Included among the plants were several kinds of animal fossils. These include a single fragment of an insect wing. Among 

the aquatic organisms are conchostracans, ostracods, and a single linguloid, inarticulate brachiopod. Common to abundant ostracods and a single linguloid suggest brackish-water conditions, 

although the linguloid may have been transported given the 

principally marine preferences of these organisms (Emig, 1986). 

The conchostracans, which are abundant in the collection, are 

indicative of ephemeral standing-water conditions, mainly fresh 

but occasionally brackish (Frank, 1988; Brendonck, 1996); they 

may reflect fluctuation in the areal extent of the depositional environment, with marginal drying, or possibly seasonal rainfall 

variation, resulting in ephemeral periods of standing water on 

the land surface surrounding the depositional setting.

Fossil Flora

The collection consists of 140 total hand-sample surfaces; 

95 surfaces contained some form of identifiable plant remains, 

and 45 were barren. 

The dominant element of the fossil flora is the fern or seed fern 

Sphenopteris germanica (26.3%; Figure 9a,b). This plant has been 

characterized as Sphenopteridium manzanitanum in floras and floral lists from the western United States, following its description 

under that name by Mamay (1992; see discussion of the taxonomy 

of this plant in DiMichele et al., 2013b). The Sphenopteris germanica in this collection is variable in form, and on many specimens, the pinnule lobes are rounded and held at somewhat lower 

angles than seen in specimens of Missourian age from the Kinney 

Brick Quarry locality in the Manzanita Mountains (Sphenopteridium manzanitanum). Some of the specimens we have identified as 

S. germanica are labeled “Callipteris lyratifolia” or “Odontopteris 

obtusa” in the collection (presumably by Charles B. Read).

Two or three types of conifer occur in the flora (13%), all 

at low to moderate abundances. Most specimens conform to 

Morphotype B (Figure 9f) and have curved, but not adpressed, 

bilaterally flattened leaves. Others, however, have leaves that are 

significantly more gracile, elongate, and positioned to suggest 

differentiation from Morphotype B and may be Morphotype 

A (Figure 9e). Some specimens have shorter leaves disposed in 

a manner similar to those of Morphotype B. Without cuticular 

anatomy it is difficult to identify strictly vegetative material. 

The flora appears to contain two distinct forms of 

mixoneurid odontopterids, Odontopteris (Mixoneura)

schlotheimii and Odontopteris (Mixoneura) subcrenulata, 

which were not recognized as distinct during the quantification. 

Together, they occur at a frequency of 16.8%. Because some of 

the pinnules of these plants are of distinctly triangular shape and 

have neuropterid features, we originally identified many of them 

as Neurodontopteris auriculata.

The main portion of the material in question is attributable 

to Odontopteris (Mixoneura) schlotheimii, which is characterized by large, somewhat irregularly to triangularly shaped pinnules with relatively sparse, fine, open dichotomous venation 

(Barthel and Amelang, 2011). The larger specimens in the Unit 8 

collection have relatively widely spaced lateral veins, undulatory 

pinnule margins, and triangular pinnule shape (Figures 9g,h, 

10n). Some pinnules show differentiation into small, round, 

basal pinnule lobes or even free pinnules at the base, but these 

also have sparse, fine veins. In rare instances, pinnules were encountered (Figure 10n), always of larger size classes, in which 

vein anastomoses were identified, particularly near the pinnule 

apex but occasionally in the lobed basal portions. Because venation in these pinnules and their overall shape are similar to those 

of O. schlotheimii, we presume a common identity with that species, although such vein reticulations may indicate that erection 

of a new species is called for.

A few of the mixoneuroid specimens appear to be most similar to Odontopteris (Mixoneura) subcrenulata (Figure 10b–d). All 

have dense, fine venation, which enters a pinnule base centrally. The 

illustrated specimens show a rank of small, nearly round, broadly 

attached pinnules (Figure 10b), an elongate pinnule with a slightly 

undulatory margin (Figure 10c), and a series of pinnules possibly 

representing a transition from free, small pinnules to an entire, 

larger terminal pinnule (Figure 10d). These remains are also similar to Odontopteris (Mixoneura) lingulata. The two species can be 

difficult to distinguish on the basis of small fragmentary specimens. 

In Europe, Odontopteris (Mixoneura) subcrenulata is a predominantly Stephanian species, whereas O. lingulata is a typical Rotliegend species. Odontopteris lingulata differs from O. subcrenulata in 

having very long, tongue-shaped terminal pinnules, which may be up 

to half the pinna length. Hence, the number of normal pinnules per 

pinna is relatively low, and they are rather widely spaced. Odontopteris subcrenulata has much shorter terminal pinnules, and normal 

pinnules stand more closely together. The venation also is different: 

in O. subcrenulata a bundle of veins enters the pinnule 

approximately in the middle of the pinnule base, whereas in 
number 9 9 • 1 7

FIGURE 9. Unit 8, USGS locality 8974, dominant elements. (a) Sphenopteris germanica. USNM specimen 619003. (b) Sphenopteris 

germanica. USNM specimen 619002. (c) Sphenopteris germanica and a walchian conifer fragment. USNM specimen 619004. 

(d) Walchian conifer fragment of lateral branch main axis. USNM specimen 619005. (e) Walchian conifer. USNM specimen 619007. 

(f) Walchian conifer Morphotype B. USNM specimen 619006. (g) Odontopteris (Mixoneura) schlotheimii. USNM specimen 619008. 

(h) Odontopteris (Mixoneura) schlotheimii. USNM specimen 619009. Scale bars = 1 cm.
1 8 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
number 9 9 • 1 9

O. lingulata veins enter the pinnule over the entire pinnule 

width and are not bundled. These two species are considered 

synonymous by some authors (see Wagner and Castro, 1998, for 

discussion).

The only other seed plant in the flora is represented by foliage of Cordaites (6.3%). All such leaves are fragmentary but 

of low aspect, elongate, and with fine, relatively dense, parallel 

venation (Figure 10e).

There are few pteridophytic taxa among the plant remains. 

Those identified include calamitalean stems (8%; Figure 10j) and 

foliage, Annularia sp. cf. A. spicata. Groundcover is suggested by 

two specimens tentatively assigned to Sphenophyllum sp. (Figure 

10k) and single fragments of two different kinds of sphenopterid 

fern (Figure 10 l,m). With regard to the matter of groundcover, 

Sphenopteris germanica also may have been a small, centrally 

rooted plant with a cormose stem (DiMichele, Kerp, Lucas, 

and Chaney, unpublished data), a small, juvenile specimen that 

Ash and Tidwell (1987) described as Arnophyton kuesii. Seeds 

of several kinds, all relatively small, occur on 9% of the quadrats 

(Figure 10f–i). Unidentifiable woody or striated axes were identified on 25% and 16% of the quadrats, respectively.

Stratigraphic and Environmental Interpretations

The Unit 8 flora is most likely from the upper portion 

of Read’s Abo Pass section. The floral elements, particularly 

Odontopteris (Mixoneura) schlothemi and O. subcrenulata, 

are typical of Late Pennsylvanian floras. The presence of conifer 

remains also is typical of western Pangean floras of this time 

interval; conifers have been identified in strata as old as Middle 

Pennsylvanian in New Mexico (Lucas et al., 2013). Sphenopteris germanica had been considered diagnostic of Permian-aged 

strata (e.g., Pfefferkorn and Resnick, 1980) but also has been 

identified, often in considerable abundance, in strata as old as 

Middle Pennsylvanian (Lucas et al., 2013; see also DiMichele et 

al., 2013b, for discussion of the taxonomic issues surrounding 

the differentiation of Sphenopteris germanica and Sphenopteridium manzanitanum). 

The presence of Sphenopteris germanica, walchian conifers, 

and species of mixoneurid-type Odontopteris and the absence 

of pecopterid ferns and neuropterid pteridosperms suggest a 

seasonally dry climatic setting, perhaps dry subhumid (Cecil, 

2003). This is consistent with the presence of conchostracans 

among the faunal remains. Such seasonally dry environmental 

conditions also are reflected in the generally common occurrence 

of floral elements such as conifers, callipterids, and exotic elements, such as Charliea, Plagiozamites, or even Podozamites, in 

western Pangean floras earlier than their common occurrence in 

floras of coal basins in central Pangea (the U.S. Midcontinent 

through Europe). For example, compare the stratigraphic ranges 

of many of these plants as reported in Kerp and Fichter (1985), 

Blake et al. (2002), Barthel (2009), and Wagner and ÁlvarezVázquez (2010). These plants appear to have been common 

throughout much of western Pangea from at least the Middle 

Pennsylvanian onward and perhaps earlier. Their occasional 

appearances in coal basins of central Pangea as early as Middle 

Pennsylvanian (e.g., Falcon-Lang et al., 2009) most likely reflects 

the increased intensity of periods of seasonal aridity at this time 

during certain portions of glacial-interglacial cycles (Rosenau et 

al., 2013), which were closely synchronized with changes in both 

sea level and tropical climate (Cecil et al., 2003, 2014).

UNIT 10

Unit 10, USGS locality 8975, is the largest collection of the 

Abo Canyon section. The plant fossils are encased in a browntan siltstone. The siltstone does not have notable sedimentary 

structures, such as ripples or other bed forms. Rather, the matrix appears to be massive, and specimens often split irregularly 

across bedding surfaces. The plant fossil remains frequently 

are confined to one surface of the specimen, the reverse sides 

of nearly a third being barren, suggesting pulsed transport and 

intermittent deposition of plant material. This type of deposition is further suggested by abundant CPD in the Unit 10 collection (Figure 11a), including up to 3.5-cm-thick plant-hash 

FIGURE 10. (Facing page) Unit 8, USGS locality 8974, minor elements. Scale bar in image (j) applies to all images without scale bars. (a)

Neuropteroid pinnules, possibly assignable to Neurodontopteris auriculata but also possibly Odontopteris (Mixoneura) schlothemi. USNM 

specimen 619010. (b) Odontopteris (Mixoneura) subcrenulata, free, rounded pinnules at base of pinna. USNM specimen 619011. (c) Odontopteris (Mixoneura) subcrenulata, transition from free pinnules to broad, neuropteroid lamina. USNM specimen 619012. (d) Odontopteris 

(Mixoneura) subcrenulata, free pinnules transitioning to broad, neuropteroid lamina, perhaps near pinna apex. USNM specimen 619013. (e)

Cordaitalean leaf. USNM specimen 619014. (f)–(i) Various shapes and sizes of seeds found in the collection, all singletons and not clearly associated with any particular type of vegetative foliage. USNM specimens (f) 619015, (g) 619016, (h) 619017, and (i) 619018. (j) Calamitalean stem. 

USNM specimen 619019. (k) Possibly a Sphenophyllum specimen, but of uncertain specific affinity. USNM specimen 619020. (l) Sphenopteris

sp., possibly groundcover fern foliage. USNM specimen 619021. (m) Sphenopteris sp., also possibly foliage of a groundcover fern. USNM specimen 619023. (n) Pinnules with general characteristics of Odontopteris (Mixoneura) schlotheimii but with reticulations of the lateral veins in the 

apical portion of the pinnule. USNM specimen 619022. Scale bars = 1 cm.
2 0 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

conglomerate layers within a siltstone matrix that includes small 

flakes of muscovite, various-sized silt grains, and small, rounded 

quartz granules.

Within the CPD layers the silt matrix splits irregularly, 

although the plants mostly appear to be horizontally disposed, 

suggesting a slurry, but one within which the organic matter 

could settle somewhat gently. Whereas muscovite is common in 

the siltstone, it occurs more abundantly in association with the 

CPD deposits, especially in layers just above or below the CPD 

layers. Many identifiable plants are found among the CPD. Despite the abundance of plant debris and small organic clasts, only 

a single, questionable specimen of charcoal was identified in the 

collection.

Small animal remains are found both within the CPD layers 

and preserved within the siltstones. These include spirorbids/microconchids (Figures 11b, 13b) and small snails (Figures 11b, 

13b). The spirorbids occur only in attachment to plant remains, 

including calamitalean stems, cordaitalean foliage, neuropterid 

foliage, and, most rarely, conifer branches. Such remains suggest a brackish-water to freshwater environment of deposition 

(Taylor and Vinn, 2006; we do not accept the argument of Gierlowski-Kordesch and Cassle [2015] for restriction of microconchids to exclusively marine salinities).

Fossil Flora

A total of 384 quadrats were examined; 269 were fossiliferous and were used in the quantitative analysis, whereas the 

remaining 115 were barren. The flora is dominated by conifers, 

which occur on over 60% of the fossiliferous quadrats. 

Walchian conifers account for the bulk of this material, occurring on 56.5% of all fossiliferous quadrats. Most of these 

conform broadly to Morphotype B (Figure 12a,b,d,e); a small 

number of specimens (1% of all quadrats) have relatively triangularly shaped leaves that are more adpressed against the stem 

than in most of the specimens (Figure 12f). A small number of 

specimens (2.6% of all quadrats) fall into Morphotype D in having leaves that are much longer than those of other walchians 

and positioned at a low angle, just above orthogonal with the 

stem (Figure 12c); the leaves are very straight through most of 

their length, making a narrow angle with the stem at their base 

before bending to a lower-angle position and again turning upward abruptly at their tips. Morphotype A (Figure 13a,b) occurs 

at a frequency of approximately 12% of the quadrats. Nearly all 

the remains are of vegetative foliage with its distinctive morphology, the leaves inserted at right angles or even obliquely prior to 

turning up sharply in their outer 1⁄2 to 2

/3. A single specimen was 

found with several compound ovuliferous cones (Figure 13c).

A single stem was found with distinctive leaf scars and attached walchian-type leaves (Figure 14a). In addition, several 

parts of walchian reproductive organs also were found, including a possible cone axis with subtending bracts/leaves (Figure 

13D), buds or small polliniferous cones (Figure 14b), and bracts 

from ovuliferous cones, including Gomphostrobus (Figure 14c) 

and bracts of uncertain affinity (Figure 14d).

Minor elements of this flora include the following presumptive seed plants: Mixoneurid Odontopteris (5.9%) is represented 

by two species, Odontopteris (Mixoneura) schlotheimii (Figure 

15a–d) and Odontopteris (Mixoneura) subcrenulata (Figures 

15e–g, 16a). These may be differentiated by the lower vein density and thus sparser venation of O. schlotheimii, its somewhat 

more neuropteroid terminal pinnules or pinnae, and the more 

rectangular nature of small, individual pinnules. The smaller 

pinnules of O. subcrenulata, by contrast, are more rounded.

A single specimen is possibly attributable to the callipterid 

Rhachiphyllum sp. (Figure 15h,i). This single complete pinnule 

and a fragment of another, likely from the same rachis, have 

an elongate shape with a decurrent base, parallel sides, and a 

rounded apex. The midvein is thin and not sunken; that is to 

say, the laminae of the pinnules are not vaulted. Lateral veins 

are relatively sparse but are steeply ascending and do not show 

inflections toward the margin as they approach, but rather 

maintain their straight, steep path. A single specimen of the callipterid reproductive structure, Peltaspermum, was identified; 

however, the partial preservation of this specimen leaves open 

affinity with Remia (Figure 17l). There is a single occurrence 

of small valvate organs that may be peltasperm pollen organs 

(Figure 14n).

Relatively large, reticulate-veined pinnules with meshes resembling those of Reticulopteris muensteri (5.6%; Figure 16b,c) 

are among the most unusual specimens in this assemblage. Reticulopteris muensteri is typically found in strata of Westphalian/

Moscovian age. In consideration of the rest of this flora, therefore, this seems an unlikely identification. Late Pennsylvanian 

reticulate-veined neuropteroid plants that may be considered 

possible candidates for identity include Barthelopteris germarii

and Linopteris neuropteroides. However, the vein meshes in the 

Unit 10 pinnules are concentrated near the midvein, are very 

large and open, and do not parallel the midvein. The lateral veins 

turn sharply toward the margin of the pinnule and, through an 

elongate, flattened trajectory, meet it at right angles. The number 

of vein anastomoses in the middle and outer areas of the lamina 

is small compared to those near the midrib. This pattern is much 

more like that of Reticulopteris than either Barthelopteris germarii or Linopteris neuropteroides, where the vein meshes are 

smaller, more uniformly distributed across the pinnule lamina, 

and more parallel with the midvein; in addition, the lateral vein 

path to the margin is more arcuate in these latter species (see 

Zodrow and Cleal, 1993; Zodrow et al., 2007). The Abo Pass 

Unit 10 specimens may indicate the existence of a new species of 

Reticulopteris. However, given the age uncertainties of the flora 

in the absence of Charles B. Read’s field notes and given that this 

potentially new species would be known from only a handful 

of isolated pinnules, we feel it best to defer such a description until, and if, more supporting information or additional, 

more complete specimens become available. Isolated pinnules 

of similar shape and venation have been found at a few other 

locations in New Mexico but provide no additional information on possible affinities or gross frond morphology (see further 
number 9 9 • 2 1

FIGURE 11. Unit 10, USGS locality 8975, comminuted plant debris and animal fossils. (a) Comminuted plant debris covering a bedding surface. USNM specimen 619051. (b) Microconchids/spirorbids attached to pinnules of Odontopteris (Mixoneura) schlotheimii. USNM specimen 

619024. (c) Small snail in sediment surrounding walchian conifer branches and microconchids/spirorbids attached to calamitalean stem fragments on right. USNM specimen 619052. Scale bars = 1 cm.
2 2 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

FIGURE 12. Unit 10, USGS locality 8975, walchian conifers. (a) Walchian conifer Morphotype B. USNM specimen 619025. (b) Walchian 

conifer Morphotype D. USNM specimen 619026. (c) Walchian conifer Morphotype D. USNM specimen 619027. (d) Walchian conifer Morphotype B. USNM specimen 619028. (e) Walchian conifer Morphotype B branch fragment. USNM specimen 619029. (f) Walchian conifer 

Morphotype B; however, note that the leaves are shorter and more closely adpressed to the stem than is typical of that morphotype. USNM 

specimen 619030. Scale bars = 1 cm.

comment below in Stratigraphic and Environmental Interpretations subsection).

Several specimens were found of elongate, strap-like foliage 

with parallel veins, likely of cordaitalean origin (3.7%; Figure 

14o). The uniformity, consistent separation, and regular spacing 

of the longitudinal striations in these specimens argue for them 

being veins rather than sclerenchyma strands. Thus, these specimens are likely leaves rather than pteridosperm or fern rachis 

fragments. 

Three specimens have characteristics of the pteridosperm 

genus Alethopteris. Pinnules are relatively short and have an inclined insertion, with subparallel lateral margins and a round to 
number 9 9 • 2 3

FIGURE 13. Unit 10, USGS locality 8975, conifers. (a) Walchian conifer Morphotype A vegetative branches with obliquely inserted leaves. 

USNM specimen 619031. (b) Walchian conifer Morphotype A with obliquely inserted leaves, apex to the left. Calamitalean stem with clearly 

marked node above the conifer branch. USNM specimen 619032. (c) Possible ovuliferous Walchian conifer Morphotype A. USNM specimen 

619033. (d) Walchian conifer vegetative axis or, possibly, remains of a partially disaggregated ovuliferous cone. USNM specimen 619034. 

Scale bars = 1 cm.

bluntly pointed apex. The base of a pinnule is decurrent, but pinnules are not or are only weakly confluent. The midvein extends 

about 2

/3 of the way to the pinnule apex, and the lateral veins are 

not dense, arch slightly in their path, are convex to the apex, and 

meet the margin at an angle of <45°. One of the specimens has 

an elongate terminal pinnule. We identify these specimens as 

most similar to A. schneideri (1.1%; Figure 16d–f). Another 

possible identification is Alethopteris zeilleri; in this case, the 

pinnules would be expected to have somewhat denser venation, oriented more closely to orthogonal to the midvein and 

margins, with somewhat more bluntly acute pinnule apices and less 

inclined pinnule insertions. However, a clear identification 

cannot be made on the basis of the fragmentary material at hand. 

Sphenopteris germanica (2.2%; Figure 16g–i) specimens, 

although not common, are well preserved and show considerable variation in form. Pinnules vary from squared off at the 
2 4 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

FIGURE 14. Unit 10, USGS locality 8975, various specimens. (a) Walchian conifer stem with leaves still in attachment and leaf scars where 

the leaves have been removed during splitting of the rock. USNM specimen 619035. (b) Walchian conifer buds or polliniferous cones. Note 

mold of snail at top of image. USNM specimen 619036. (c) Gomphostrobus bifidus, bract from ovuliferous walchian cone. USNM specimen 

619037. (d) Incertae sedis, possible ovuliferous conifer cone bract. USNM specimen 619038. (e) Numerous seeds of similar kind within a layer 

of comminuted plant debris (CPD), suggesting a synchronous reproductive event. USNM specimen 619039. (f) Close-up of seeds from the layer 

shown in Figure 14e. USNM specimen 619040. (g)–(l) Seeds of various shapes and sizes to demonstrate the range of seed types preserved in Unit 

10 sediments. All, including image (f), are at identical magnifications. USNM specimens (g) 619041, (h) 619042, (i) 619043, (j) 619044, (k)

619045, and (l) 619046. (m) Two seeds of different morphology preserved in a CPD layer. USNM specimen 619047. (n) Valvate organs that 

may be pollen-producing organs shed from a more complex peltasperm reproductive structure. USNM specimen 619048. (o) Cordaitalean leaf 

fragment, broad, strap-like with longitudinally parallel venation. USNM specimen 619050. (p) Charliea sp. cf. C. manzanitanum leaves, at least 

three, the top fragment preserving the distinctive leaf tip. Scale bars = 1 cm. USNM specimen 619049. The scale bar in (m) also applies to (f)–(l).
number 9 9 • 2 5

tips to rounded. Venation varies from prominent, with the veins 

strongly protruding from the lamina, to muted, where veins are 

difficult to identify. 

Several partially preserved pinnules of indeterminate affinity but of neuropteroid aspect occur, in toto, at a frequency of 

nearly 6%.

Various pteridophytes also occur in the flora: Calamitaleans 

include stems (1.1%) (Figures 13b, 17a); a small form of Annularia, possibly A. spicata (0.7%, n = 2) with fusiform, relatively 

short leaves (Figure 17b); and a small cluster of cones similar to 

Calamostachys spicata (0.4%, n = 1; Figure 17c).

Two isolated sphenophyll leaves also are present (Figure 

17d), identified tentatively as Sphenophyllum thonii (0.7%) on 

the basis of rounded distal leaf margins that grade into the lateral 

margins, both of which are lined by small teeth and by veins that 

terminate along the lateral margins rather than strictly at the distal 

margin. Given the intraspecific variation in Sphenophyllum leaves, 

identifying isolated leaves often must be tentative. Possibilities include Sphenophyllum verticillatum and Lilpopia raciborskii (see 

comparisons in Kerp, 1984; Wagner, 1985; Barthel, 2009). 

The ferns include both groundcover and tree fern foliage, 

and all specimens are rare. The small fern Oligocarpia gutbieri

FIGURE 15. Unit 10, USGS locality 8975, seed plant foliage. (a)–(d) Odontopteris (Mixoneura) schlotheimii, different manifestations of pinnae 

showing entire laminae, individuated pinnules, and terminal pinnules where subjacent pinnules have fused. Veins are relatively widely spaced. 

Pinnules are angular. USNM specimens (a) 619053, (b) 619054, (c) 619055, and (d) 619056. (e)–(g) Odontopteris (Mixoneura) subcrenulata. 

Various forms of pinnule and pinna morphology. Venation is dense. Pinnules are rounded. USNM specimens (e) 619057, (f) 619058, and (g)

619059. (h) Rhachiphyllum sp. cf. R. schenkii. (i) Magnification of specimen illustrated in Figure 15h. Note the flat pinnule aspect, nonsunken 

midvein, and steep, straight lateral veins. USNM specimen 619060. Scale bars = 1 cm except as noted.
2 6 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

FIGURE 16. Unit 10, USGS locality 8975, various specimens. (a) Odontopteris (Mixoneura) subcrenulata. Fragments 

of several pinnae. Mixed with walchian conifer fragments. USNM specimen 619061. (b)–(c) Pinnules with reticulate 

venation. Compare with Reticulopteris muensteri or undescribed species. USNM specimens (b) 619062 and (c) 619063. 

(d)–(f) Alethopteris sp. cf. A. schneideri. Relatively short, broad pinnules with bluntly rounded tips, weakly confluent, 

deeply incised apical attachment. USNM specimens (d) 619064, (e) 619065, and (f) 619066. (g)–(i) Sphenopteris germanica. Note variability in the shape of pinnule lobes and the prominence of the venation. USNM specimens (g) 619067, 

(h) 619068, and (i) 619069. Scale bars = 1 cm.
number 9 9 • 2 7

(2.2%; Figure 17i–k) has rounded pinnules, differentiating it 

from O. leptophylla; flexuous rachises; and widely spaced, relatively flexuous pinnule venation. Sphenopteris sp., with small 

pointy pinnule lobes, similar to S. biturica or S. mathetii, also 

likely a small, filicalean fern, was present in one quadrat (0.4%). 

Two forms of marattialean tree fern foliage were identified; one 

is similar to Lobatopteris geinitzii (0.7%, n = 2; Figure 17f), 

and the other, identified by Read (note in collection drawer) 

as Pecopteris oreopteridia, is here identified as more similar to 

Pecopteris densifolia (0.7%, n = 2). A large flabellate leaf may 

be a pecopterid aphlebia (0.4%, n = 1; Figure 17h, but see below 

for further comment).

Noeggerathialeans are represented by a single specimen of 

Charliea sp. cf. C. manzanitanum (0.4%, n = 1; Figure 14p). 

Several pinnules are present, one with the distinctive division of 

the leaf terminal margin into three pointed, toothlike lobes and 

all with strongly parallel pinnule venation.

Unidentifiable material, or plant remains not specifically attributable to a rank of order or below, includes several coarsely 

veined, flabellate objects, possibly aphlebiae or cyclopterid fragments. It is difficult to provide a definitive identification of these 

objects because of their fragmentary preservation. One of these, 

similar to a marattialean aphlebia (Figure 17h), also is similar 

to more typically Angaran foliage, such as Psygmophyllum (see 

Naugolnykh, 1998). The other form, represented by several 

fragmentary specimens (1.1%; Figure 17m), has dense, fine veins 

that spread outward in a palmate manner from the base, which 

appears to be an entry point for multiple veins. Such specimens 

have no clear affinities to which the authors can point. A final 

odd object (Figure 17e) is a small, planar, leaflike or cone-scalelike structure with strongly developed, pointed terminal teeth 

and a wide vein ending in each tooth (0.4%, n = 1). The overlapping nature of what appear to be lobes, their large number, and 

pointed distal teeth suggest walchian cone-scale affinities. 

Seeds of various morphologies (Figure 14e–m) are common in this collection (19%), more so than in any of the other 

Abo Canyon collections; this may reflect the abundance of CPD 

(25.3%) in which many of the seeds were found. Indeterminate 

axes include striate specimens, presumably from pecopterid ferns 

or pteridosperms (9.3%), and woody axes from conifers or other 

large plants (3.7%).

Stratigraphic and Environmental Interpretations

The Unit 10 flora is the largest and most diverse of the floras 

in the Read Abo Canyon collection suite. Considered in toto, the 

composition of the flora suggests strong seasonality at the time 

of plant growth and during the burial of the plant remains. This 

is indicated by conifer dominance and the presence of pteridosperms such as the mixoneurid Odontopteris, Rhachiphyllum,

and the reticulate-veined cf. Reticulopteris sp. and by plants that 

occur only in association with other plants and sedimentological indicators of periodic drought, such as Charliea. However, 

some of the rarer elements of the flora indicate the at least local 

presence of wet substrates, including calamitaleans and marattialean tree ferns, which occurred during the Pennsylvanian and 

early Permian almost anywhere that some form of standing water 

was available. The flora also includes small, groundcover ferns 

and sphenophylls, indicating that the environment was not arid. 

The animal fossils in close association with the plant remains, particularly spirorbids attached to probable floating material, suggest a coastal or estuarine depositional environment. 

Furthermore, the abundance and dense, thick concentrations of 

comminuted plant debris; abundant muscovite, often concentrated in layers; and the lack of distinctive indicators of scour 

or rapid deposition of the larger plant remains (absence of remains crossing bedding planes, jumbled within the sediment, or 

aligned by flow) are consistent with deposition of the plants in a 

quiet-water setting into which sediment and organic matter were 

introduced episodically. This interpretation is further supported 

by the large number of quadrats lacking organic matter, which 

indicates episodic introduction of organics.

Interpretation of the age of this flora is complicated by its 

strongly dryland affinities. The difficulties in determining the age 

of such assemblages from plant composition alone are indicated, 

perhaps in an extreme example, by the study of van Hoof et 

al. (2013), in which a conifer-rich flora thought to be of Permian age was shown to be Middle Pennsylvanian. In the case of 

Unit 10, the flora is unproblematically Late Pennsylvanian, most 

likely Missourian. The presence of a reticulate-veined medullosan of uncertain affinity but similar to Middle Pennsylvanian 

Reticulopteris (Figure 16b,c) is noteworthy. Neither the survival 

of Reticulopteris into the Late Pennsylvanian of western Pangea 

nor the evolution of a convergent form can be ruled out on the 

basis of the specimens at hand. Further, although the evidence is 

too slim for a confident age correlation, isolated pinnules with 

shape and venation similar to those from Abo Pass have been 

identified east of Socorro, New Mexico, from strata of the Tinajas Member of the Atrasado Formation (DiMichele, Kerp, Lucas, 

and Chaney, unpublished data); these strata are confidently 

dated as late Missourian in age on the basis of conodonts (Lucas 

et al., 2013; see their fig. 15.1 for a photograph of the outcrop on 

which the fossiliferous bed is marked by a white arrow).

In terms of the regional stratigraphy, on the basis of the 

above considerations this assemblage probably was collected 

from the upper part of the Atrasado Formation. We suspect it is 

drawn from the Read and Wood (1947) “upper arkosic member 

of the Madera Formation” (Figure 2). This location would most 

likely place it near the Missourian–Virgilian (Kasimovian–Gzhelian) boundary, although we cannot suggest further refinement.

UNIT 35

Unit 35 is USGS locality 8976. The hand specimens consist 

of tan-colored, micaceous siltstone. Thin concentrations of muscovite flakes drape thicker layers of siltstone (~1 cm), suggesting 

periodicity of water flow velocity, with muscovite concentrating 
2 8 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

FIGURE 17. Unit 10, USGS locality 8975, minor elements. (a) Calamitalean stem with node. USNM specimen 619071. (b) Annularia sp. cf. 

A. spicata. Leaves are the correct, fusiform shape for this species but are somewhat larger than is typical. USNM specimen 619072. (c) Calamostachys spicata, calamitalean cones. USNM specimen 619073. (d) Sphenophyllum sp. cf. S. thonii. USNM specimen 619074. (e) Incertae 

sedis, possible coniferous cone scale. USNM specimen 619070. (f) Lobatopteris geinitzii, marattialean fern foliage. USNM specimen 619210. (g)

Pecopteris densifolia, marattialean fern foliage. USNM specimen 619211. (h) Incertae sedis, flabellate organ, possibly a marattialean aphlebia or 

a fragment of a plant of exotic affinity (see text). USNM specimen 619212. (i)–(k) Oligocarpia gutbieri, frond fragments. Note flexuous rachises, 

rounded pinnules, and widely spaced venation with weak midvein development. USNM specimens (i) 619213, (j) 619214, and (k) 619215. (l)

Peltaspermum-like reproductive organ. USNM specimen 619216. (m) Incertae sedis, flabellate organ, possibly leaf of exotic affinity. USNM 

specimen 619217. Scale bars = 1 cm.
number 9 9 • 2 9

during slack-water periods. Plant remains are often somewhat 

aligned and thickly disposed on bedding surfaces; they appear to 

have been allochthonous. The plants are uniformly scrappy, and 

comminuted plant debris is common. As in other Abo Canyon 

plant deposits, charcoal is rare, noted in this collection only on 

a single sampling quadrat. No animal fossils were noted, distinguishing this collection from all others except those from the 

Abo Formation.

Fossil Flora

The flora of Unit 35 has a more Pennsylvanian wetland aspect than any other of the Read Abo Canyon collections, particularly when considered quantitatively and at the generic level. 

However, it also bears a strong imprint of seasonal moisture 

limitation and can be characterized as a mixed flora. There is 

no evidence that the plants generally considered typical of drier 

substrates were transported from uplands and mixed with the 

litter of lowland vegetation. Preservation of all taxa is about the 

same quality and type, indicating local transport, but not of such 

turbulence that all organic matter was obliterated. 

Pteridosperms dominate the flora, followed by walchian 

conifers, calamitaleans, and pecopterid ferns. The equally most 

abundant taxa are Macroneuropteris scheuchzeri and walchian 

conifers, followed in descending order by calamitaleans, neuropterids (represented by numerous isolated pinnules), Sphenophyllum oblongifolium, Odontopteris brardii, and marattialean tree 

ferns. Consequently, there is a significant pteridophyte component to the flora, greater numerically than in any of the other 

collections.

Macroneuropteris scheuchzeri is the most abundant pteridosperm in the flora (17.8%). This distinctive species is easily 

recognized by its large, linguloid pinnules with a thin midvein 

that extends the length of the pinnule and dense, arching lateral 

veins (Figure 18a and possibly 18b). Examined in detail, many 

specimens have small “hairs” that appear to be on the leaf surface 

(Figure 18c, at arrows), although Laveine and Oudoire (2015) 

have suggested that these, in fact, are subepidermal resin ducts.

Walchian conifers are of equal importance to M. scheuchzeri

in abundance (17.8%). Two forms were identified. The most 

common form (Figure 18e) has shoots bearing leaves that were 

bilaterally flattened and strongly directed acroscopically but concave in curvature, held some distance off the stem, and with tips 

that recurve toward the apex; these shoots are most like Morphotype B. The less common form has leaves that are borne at 

about a 40° angle to the stem, are nearly straight and narrow 

(not wide bilaterally), and turn upward at the tips (Figure 18d); 

these specimens conform, more or less, to Morphotype D.

Sphenopteris germanica (Figure 19a) occurs at a frequency 

of 11.9%, making it the third most abundant plant in the collection. As in other Abo Pass collections, S. germanica specimens 

are fragmentary. They show the three- to five-lobed pinnules 

with coarse, sparse venation. Pinnule lobes are spatulate and 

have rounded to nearly flat outermost margins.

The third most common group of pteridosperms is those 

attributable to the genus Odontopteris, including those forms 

sometimes attributed to Mixoneura. The most common of these 

(10%) is O. brardii (Figure 19b,c), characterized by elongate, 

triangular pinnules with a weak midvein and multiple veins entering pinnule lamina from the supporting rachis. The pinnule 

margins may appear to be slightly undulate at the points where 

veins end, but this characteristic is not universal and may reflect 

the effects of natural maceration prior to burial. Rare elongate 

pinnules (5% of quadrats) with dense, relatively coarse venation 

and marginal undulations (Figure 19d) may be terminal portions 

of O. brardii pinnae; however, these also may be attributable to 

O. subcrenulata, among other possibilities.

Three specimens (3%) of odontopterid aspect have widely 

spaced veins in the preserved pinnule fragments (Figure 19e–g). 

Pinnules have weak midveins and steep lateral veins with multiple veins entering the base of a pinnule (Figure 19e,f). In other 

instances, the margin of the pinnule/pinna may be undulatory 

but otherwise of odontopterid aspect, with widely spaced, fine 

veins (Figure 19g). The specimen illustrated in Figure 19g also 

may be of this type, but being apical, marginal undulations are 

muted. We attribute these specimens to Odontopteris (Mixoneura) schlotheimii.

As noted above, at least 5% of odontopteroid specimens 

are elongate, lingulate laminae with dense lateral veins. These 

specimens may be attributable to Odontopteris (Mixoneura) 

subcrenulata. Two are illustrated (Figures 18b, 19d). The specimen illustrated in Figure 18b can be compared to the Macroneuropteris scheuchzeri specimen illustrated next to it in Figure 

18a. The possible O. subcrenulata pinna has much straighter 

secondary veins that turn up sharply as they come into contact 

with the margin, whereas those of M. scheuchzeri arch through 

the lamina.

A single, large cyclopterid-like pinnule was found (Figure 

20d), which may belong to Odontopteris brardii or O. subcrenulata. It has dense, relatively straight veins and a slightly undulatory margin.

Rare pteridosperm taxa, each represented by a single, fragmentary specimen, include Alethopteris sp. cf. A. virginiana, 

Pseudomariopteris cordato-ovata, and the callipterid Autunia 

naumannii. Alethopteris virginiana may be conspecific with 

and supersede the name A. leonensis (Wagner and Lyons, 1997; 

Blake and Gillespie, 2011). The single specimen illustrated here 

(Figure 20a) is similar in having somewhat triangular pinnules 

with little confluence with adjacent pinnule bases. Venation is 

obscure but can be seen to be of alethopteroid aspect. Characteristic aspects of Pseudomariopteris cordato-ovata seen in the Unit 

35 specimen (Figure 20b) include pinnules with thick laminae, 

which obscure secondary venation; a prominent, slightly sunken 

midvein; broad pinnule bases constricted on both the acroscopic 

and basiscopic sides; and spinelike prolongation of the pinna 

tip, consistent with an inferred vine-like habit for the species of 

this genus, which may be of callistophytalean affinity (Krings 

and Kerp, 2000). The attribution of a small foliage fragment to 
3 0 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

FIGURE 18. Unit 35, USGS locality 8976, dominant elements. (a) Macroneuropteris scheuchzeri pinnule fragment. Note possible insect marginal feeding damage at upper right, including reaction rim. USNM specimen 619218. (b) Macroneuropteris scheuchzeri or Odontopteris 

(Mixoneura) subcrenulata pinnule. Differences discussed in text. USNM specimen 619220. (c) Macroneuropteris scheuchzeri pinnule tip with 

distinctive “hairs” (at white arrows), which may be subepidermal resin canals. USNM specimen 619219. (d) Walchian conifer Morphotype B, 

slightly different in leaf shape, angle of insertion, curvature of the lamina. USNM specimen 619221. (e) Walchian conifer Morphotype B. USNM 

specimen 619222. Scale bars = 1 cm.
number 9 9 • 3 1

FIGURE 19. Unit 35, USGS locality 8976, subdominant elements. (a) Sphenopteris germanica. USNM specimen 619223. (b)–(c) Odontopteris 

brardii. USNM specimens (b) 619224 and (c) 610225. (d) Odontopteris brardii or Odontopteris (Mixoneura) subcrenulata terminal pinnule/

pinna. USNM specimen 619226. (e)–(g) Odontopteris (Mixoneura) schlotheimii. Several pinnules or pinnae with widely spaced lateral veins, 

high-angle secondary venation, and angular free pinnule shape. USNM specimens (e) 619227, (f) 619228, and (g) 619229. (h) Pinnule fragment most closely resembling Odontopteris (Mixoneura) schlotheimii. USNM specimen 619230. (i) Pinnule fragment most closely resembling 

Odontopteris (Mixoneura) schlotheimii. USNM specimen 619321. (j) Pinnule fragment most closely resembling Pecopteris. USNM specimen 

619232. Scale bars = 1 cm.
3 2 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

FIGURE 20. Unit 35, USGS locality 8976, minor elements. (a) Alethopteris virginiana. USNM specimen 619242. (b)

Pseudomariopteris cordato-ovata. Note spinelike prolongation of the pinna tip. USNM specimen 619243. (c) Autunia 

naumannii. USNM specimen 619244. (d) Cyclopterid pinnule, possibly of Odontopteris brardii or Odontopteris (Mixoneura) subcrenulata. USNM specimen 619245. (e) Calamitalean stem with node. USNM specimen 619246. (f) Annularia 

spicata. USNM specimen 6192467. (g) Sphenophyllum oblongifolium. USNM specimen 619248. (h) Sphenophyllum oblongifolium and walchian conifer. USNM specimen 619249. (i) Polymorphopteris (Acitheca) polymorpha marattialean 

fern pinna. Venation not fully developed in each pinnule. USNM specimen 619251. (j) Polymorphopteris polymorpha

pinnule. USNM specimen 619250. Scale bars = 1 cm.
number 9 9 • 3 3

Autunia naumannii (Figure 20c) is suggested by its lobed, undulate lamina and small size. However, any identification of such a 

small fragment must be considered tentative. It is possible that it 

represents an isolated pinnule/pinna of Sphenopteris germanica, 

although it is quite small for that and has a larger number of pinnule lobes than would be expected. 

Sphenopsids in the flora include both calamitaleans and 

sphenophylls. The calamitaleans include both stem material 

(Figure 20e; 9.9%) and two foliage species, both attributable to 

Annularia. One, A. spicata, is quite small (Figure 20f; 7.9%), 

composed of whorls of narrow, short leaves of fusiform shape, 

similar to A. spicata var. eimeri of Remy and Remy (1975; see 

also specimens illustrated by Kerp and Fichter, 1985). A single 

specimen with large, spatulate leaves in ovoid whorls is attributed to A. carinata. 

Two forms of sphenophylls occur in the assemblage. Most 

of these (11.9% of all quadrats) are attributable to Sphenophyllum oblongifolium (Figure 20g,h), typified by six leaves per 

whorl, with two pairs borne opposite to one another and the 

third, generally of shorter leaves, borne at right angles below the 

two pairs. A smaller number of specimens (5% of all quadrats) 

consist of isolated, small, triangularly shaped leaves with small 

blunt teeth along the outer margin, possibly attributable to S. 

verticillatum. 

Ferns are common components of the flora of Unit 35, most 

of marattialean tree fern affinities. However, small scraps of 

what are likely filicalean ferns also were found.

Marattialean tree fern foliage is notoriously difficult to 

identify in fragmentary preservation, in part because of high intrafrond morphological variability and in part because of oversplitting taxonomically. Marattialean foliage in the collection 

(approximately 14% frequency) is largely fragmentary and has 

been identified only as Pecopteris sp. (a name that Cleal, 2015, 

recommends abandoning for marattialean foliage for taxonomic 

reasons, and although taxonomically correct, at the present time 

there is no substitute for this long-used name). Some foliage fragments (Figure 20i,j), however, have venation characteristic of 

Polymorphopteris (Acitheca) polymorpha (Wagner, 1959; Zodrow et al., 2006), in which the lateral veins first branch close to 

the pinnule midvein and then each limb of the initial fork may 

branch again to form a symmetrical pair of forked veins.

Small ferns are represented only by tiny fragments of material found among the comminuted plant debris. The most common of these (5% of all quadrats) has small rounded pinnules 

with weakly differentiated midveins; widely spaced, ascending, 

dichotomous venation; and a somewhat flexuous rachis (Figure 

21g). Were these of larger size, they might be attributed to Oligocarpia sp. cf. O. gutbieri, although O. leptophylla is also a possibility. A single, tiny terminal portion of a pinna is attributable to 

Sphenopteris sp. cf. S. biturica (see Wagner, 1985) or S. matheti

(see Barthel, 2009). Another single specimen can be designated 

only as Sphenopteris sp. (Figure 21h), although it may be a very 

small specimen of Oligocarpia.

Lycopsids are represented in the Abo Canyon collection 

only in the Unit 38 flora. These include a number of specimens 

but none that point clearly to whole-plant taxa or that are sufficient to give an indication of the diversity of these plants on the 

landscape. Three small-stem specimens (two illustrated in Figure 21a,b) may be some kind of isoetoid with an upright stem 

and permanently attached short leaves. The leaves are widely 

spaced and helically arranged in both specimens; apparent leaf 

scars are the result of fracture of the specimens during splitting 

of the encasing rock matrix. The leaves are too short to conform to known species of small isoetaleans, such as Polysporia (Chaloner, 1958), but this kind of lycopsid has rarely been 

noted in descriptions of fossil floras. Small specimens of Lepidostrobophyllum, perhaps L. hastatum, are characterized by a 

distinctly triangular distal lamina and a short, keeled proximal 

lamina (Figure 21d,e). Three of these were found in the collection. They are from a small cone of a heterosporous lycopsid, 

one that produces deciduous sporangium-sporophyll units and 

thus likely produced a single functional megaspore. They suggest 

the presence of tree lycopsids of the Diaphorodendraceae or Lepidodendraceae. Finally, two fragments of suspect lycopsid leaves, 

Lepidophylloides, were found (Figure 21c); these are of the typical long, narrow, grasslike form with a single central midvein.

Final components of the flora include small, isolated pinnules of various sizes, mostly fragmentary, thus making specific 

and even generic identification impossible. It is likely that many, 

even most, such specimens are not neuropterids; some may be 

marattialean fern foliage (e.g., Figure 19j), others Odontopteris

(e.g., Figure 19h and perhaps 19i). In total, these occur on 14.8% 

of all fossiliferous quadrats. Seeds (e.g., Figure 21i) are common 

but not abundant, occurring on approximately 10% of quadrats. 

The most common plant remains in the assemblage are striate 

axes, likely from either pteridosperms or marattialean tree ferns, 

which have an occurrence frequency of 70.3%. Small roots were 

noted on 11.9% of quadrats, suggesting postdepositional rooting of the sediment (the roots do not appear to have been washed 

in with the aerial litter).

Stratigraphic and Environmental Interpretations

The Unit 38 flora contains few taxa diagnostic of stratigraphic location. Most of the species-level taxa are long ranging and cross through the Middle–Late Pennsylvanian boundary. 

The most that can be said with confidence is that the flora is either late Middle Pennsylvanian or Late Pennsylvanian, probably 

Missourian. The presence of Sphenophyllum oblongifolium is 

considered diagnostic for that time interval and is not known to 

occur earlier (Blake et al., 2002; Wagner and Álvarez-Vázquez, 

2010), setting a lower boundary to somewhere in the middle 

Desmoinesian. If the identification of a specimen as Sphenopteris 

biturica/S. mathetii is correct, it would indicate a middle Late 

Pennsylvanian age (Wagner and Álvarez-Vázquez, 2010), but it 

seems overly speculative to base an age determination on such 
3 4 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

a scrap. A confounding factor for a middle Late Pennsylvanian 

age is the presence of lycopsid remains such as Lepidostrobophyllum hastatum. Although this species is long ranging, into 

the Late Pennsylvanian in Europe, its likely source plants in the 

Diaphorodendraceae sensu DiMichele and Bateman (1996) are 

not known to extend beyond the Middle Pennsylvanian in North 

America, although they do so in western Europe and in China 

(Wagner and Talens, 1985; Wang, 2004), and survival in more 

peripheral areas of the western Pangean equatorial region cannot 

be ruled out without better, independent stratigraphic placement.

The flora of Unit 38 is a mixture of plants with different 

substrate moisture preferences. Some are characteristic of high 

substrate moisture, such as calamitaleans, tree ferns, and the 

pteridosperm Macroneuropteris scheuchzeri (Stull et al., 2012). 

Others are characteristically drawn from what are often inferred 

to be mesic but seasonally dry settings, such as the odontopterids and mixoneurids (Rees et al., 2002; Ziegler et al., 2002; 

Šimůnek and Martínek, 2009), whereas others are considered to 

be indicators of more strongly seasonally dry settings, such as 

the conifers (DiMichele and Aronson, 1992; Šimůnek and Martínek, 2009; Opluštil et al., 2013). This kind of mixture is often 

attributed to vegetational zonation on a landscape, reflective of 

soil drainage (e.g., Dimitrova et al., 2011). In principle, we agree 

with this interpretation. However, we would not attribute the 

floristic composition to long-distance transport of the more xeric 

elements from uplands, an interpretation that suggests soil-moisture restrictions result from drainage alone, under an otherwise 

generally mesic to humid climate. This interpretation, in effect, 

sets the default climate at the wet end of the spectrum. 

Rather, the taphonomic setting of these plant remains and 

of most mixed floras suggests that all plants were growing in the 

immediate area under a climate that was seasonally dry, but with 

sufficient moisture to create and support patches of wetlands or 

corridors where the most robust wetland taxa could survive, even 

through periodic intervals of reduced rainfall. Thus, we believe 

the presence of dryland plants, so rare within stratigraphic sequences of the Pennsylvanian coal measures but present in these 

western Pangean basins as part of floras that are compositionally 

and sedimentologically distinct from the wetland floras (e.g., Galtier et al., 1992; Falcon-Lang et al., 2009; Plotnick et al., 2009; 

Bashforth et al., 2014, 2016), calls for setting the default climate 

at the dry end of the spectrum represented by the plant groups 

FIGURE 21. Unit 35, USGS locality 8976, minor elements. (a)–(b) Stems, possibly of small lycopsids of uncertain affinity. USNM specimens (a)

619233 and (b) 619234. (c) Lepidophylloides, leaf of an arborescent lycopsid. USNM specimen 619235. (d)–(e) Lepidostrobophyllum sp. cf. L. 

hastatum, dispersal units of megasporangiate (female) cones. USNM specimens (d) 619236 and (e) 619237. (f) Sphenopteris sp. cf. S. biturica

filicalean fern foliage. USNM specimen 619238. (g) Possible foliage of Oligocarpia gutbieri but too minute for a confident identification as that 

species. USNM specimen 619239. (h) Sphenopteris sp. USNM specimen 619240. (i) Seed of indeterminate affinity. USNM specimen 619241. 

Scale bars = 1 cm. Scale bar in (h) applies also to (f), (g), and (i).
number 9 9 • 3 5

that are present. The remainder of the flora indicates a heterogeneous landscape of the kind that is more typical of settings with 

seasonal rainfall than of strongly humid to perhumid climates.

DISCUSSION

In this work we have examined a stratigraphic sequence of 

five Pennsylvanian through Permian age floras collected in Abo 

Canyon, primarily by Charles B. Read of the U.S. Geological 

Survey in 1940 and 1941. Because Read’s field notes, which 

are referred to on handwritten notes stored with the collections 

themselves, are lost or not available, we have had to determine 

for ourselves the most likely temporal order of these floras. 

However, from a combination of Read’s bed-numbering scheme, 

notes in the collections, and the composition of the floras, the 

correct stratigraphic order seems apparent.

Three of the floras are arguably of Pennsylvanian age, most 

likely from Missourian and perhaps Virgilian strata (Kasimovian and Gzhelian). These are floras from Read’s Units 38, 10, 

and 8 in ascending order. In present terminology, these collections 

would be from the upper Atrasado Formation. All are in gray 

to buff siltstones. A fourth, collected in 1941 and described as 

“base of the Red Magdalena,” appears to be from near the Pennsylvanian–Permian boundary, what currently would be called the 

Bursum Formation. The fossils are preserved in brown to red 

and yellow mottled siltstones. The final set of collections comes 

from the Abo Formation, which is of early Permian age, corresponding to the Wolfcampian and early Leonardian in American 

usage (Sakamarian through early Kungurian). The Abo plants are 

preserved in well-cemented, light-red to deep-red siltstones and 

sandstones.

In terms of depositional environments, collections from 

below the Abo Formation are mostly from quiet-water settings, 

in which the plants appear to have been carried in from the surrounding landscape and to have settled to the bottom under lowenergy conditions. Muscovite concentrations and comminuted 

plant debris drape many bedding surfaces, which is also indicative of episodic low-energy conditions with little water movement. In most of the collections, the organic matter appears to 

have been deposited episodically, given the significant number 

of surfaces that do not contain plant fossils or any sort of comminuted plant debris, suggesting that the collections were made 

from one or several discrete layers, around which the siltstones 

were barren. Three of the five collections, Units 10 and 8 and 

base of Red Magdalena, all from the middle of the sequence, 

also contain various numbers of invertebrate animal remains, 

principally microconchids/spirorbids, small snails, ostracods, 

and occasional conchostracans. A single linguloid occurrence 

was noted. The animals associated with these three collections 

suggest proximity to marine conditions and possible deposition 

of the plant material in water of brackish salinities (although 

that does not indicate that the living plants were growing in saline water). Unit 35, at the base of the sequence, and the Abo 

Formation, at the top, are devoid of any animal fossils (although 

a terrestrial fauna is known from the Abo Formation as macrofossils and trace fossils from numerous sites throughout New 

Mexico). 

The floras all appear to be derived from vegetation that 

grew under some degree of climatic seasonality. In general, the 

extremity of this seasonality appears to increase through the 

section from the oldest to the youngest assemblage. However, 

it must be kept in mind that, on the basis of the character of 

the enclosing sediments, these floras appear to have been buried 

and preserved under subtly to significantly different depositional 

conditions. Such differences also may be correlative with different degrees of landscape microhabitat heterogeneity, elevational 

variability, and vegetational zonation around the site of deposition of organic matter. 

The oldest assemblage contains the highest concentration, 

based on quantitative compositional assessments, of wetland 

plants, such as pteridosperms, calamitaleans, sphenophylls, 

groundcover ferns, and marattialean tree ferns. However, plants 

generally considered to be typical of seasonally dry settings, such 

as conifers, Sphenopteris germanica, and some pteridosperms 

(mixoneurids), also are abundant in this flora. We argue that the 

climatic setting of this assemblage was mostly likely seasonally 

dry but to the least extent of all the assemblages reported herein. 

The other siltstone assemblages, those with indicators of nearby 

marine or brackish water bodies, are dominated by taxa typical 

of seasonally dry settings, primarily conifers of several types but 

also Sphenopteris germanica and two mixoneurid species, Odontopteris (Mixoneura) schlotheimii and Odontopteris (Mixoneura)

subcrenulata. Small numbers of such plants as Charliea, Taeniopteris, and various callipterids (mainly Rhachiphyllum), also 

typical of dryland settings, characterize these intermediate floras. 

However, all three of these also contain wetland elements, most 

commonly calamitalean stems and foliage (most often Annularia 

spicata) and small amounts of marattialean fern foliage, most 

likely indicating that wet patches remained on the landscape—

these plants are widespread through the early Permian in western 

equatorial Pangea and appear to have been able to locate and 

grow in even small areas of wet ground, as long as regional rainfall was sufficient to keep such habitats among the landscape mixture. The youngest floras, from the Abo Formation, are typically 

dominated by either walchian conifers or the peltaspermous seed 

fern Supaia. The Abo Canyon floras are Supaia-dominated for 

the most part, although conifers are among the floral elements; in 

effect, these floras are more or less identical to those found in the 

Abo elsewhere in the state. The only unique feature is the presence of Sphenophyllum gilmorei, described from the lower Permian of the Supai Formation in the Grand Canyon, which we have 

not found elsewhere in the Abo, nor are we aware of reports of it.

In summary, the Abo Pass floras are of significance because 

they record vegetational changes through the Pennsylvanian–Permian transition in the western equatorial reaches of the Pangean supercontinent. This area had been much drier for much longer than 

the central parts of the continent at low latitudes; the last western 
3 6 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

Pangean coals, which are ash rich, were in the earlier parts of the 

Middle Pennsylvanian, whereas peat and coal formation continued 

through the Pennsylvanian and into the Permian in west central 

Pangea and into the Permian in Cathaysia, along the margin of the 

Tethys Ocean. Consequently, the study samples are an ecological 

record of the same progressive global climatic changes that were occurring across the much better known and more intensively studied 

central Pangean equatorial regions during this time period. However, the western floras lived within a background climatic context 

that was significantly different from that usually depicted for the 

change from the Pennsylvanian to the Permian in the more humid 

parts of central Pangea. Nonetheless, as noted by many authors 

(e.g., Gillespie and Pfefferkorn, 1979; Pfefferkorn and Gillespie, 

1980; Blake et al., 2002; Bashforth et al., 2016), most of the same 

species known from central Pangea can be found in these western 

settings, bespeaking large biogeographic ranges and pointing to migratory pathways or other means (e.g., long-distance dispersal) of 

maintaining species continuity across these vast areas.

ACKNOWLEDGMENTS

We thank Arden R. Bashforth, Natural History Museum of 

Denmark, and B. Mitchell Blake, West Virginia Geological Survey, for critical reviews of the paper. Jean-Pierre Laveine, Université des Sciences et Technologies de Lille, is kindly acknowledged 

for sharing his opinion on the identity of the cf. Reticulopteris

specimens. We thank Bruce Wardlaw (deceased); Kevin McKinney, U.S. Geological Survey; and Roberta Sirmons, National Museum of Natural History, Smithsonian Institution, for assisting in 

our search for C. B. Read’s original field notebooks.

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Index of Scientific Names

Alethopteris, 22

Alethopteris leonensis, 29

Alethopteris schneideri, 5, 23, 26

Alethopteris sp., 26, 29

Alethopteris virginiana, 5, 29, 32, 33

Alethopteris zeilleri, 23

Annularia, 25

Annularia carinata, 6, 33

Annularia sp., 10, 19, 28

Annularia spicata, 6, 19, 25, 28, 32, 35

Annularia spicata var. eimeri, 33

Annularia spp., 6

Arnophyton kuesii, 14, 19

Autunia, 14

Autunia conferta, 5, 7, 8

Autunia naumannii, 5, 29, 32, 33

Barthelopteris germarii, 20

Calamostachys spicata, 6, 25, 28

Callipteris, 14

Callipteris lyratifolia, 16

Charliea, 19, 27, 35

Charliea manzanitanum, 14, 24, 27

Charliea sp., 5, 24, 27

Cordaites, 5, 19

Diaphorodendraceae, 33

Ernestiodendron, 4

Ernestiodendron filiciforme, 14

Gomphostrobus, 20

Gomphostrobus bifidus, 24

Lepidodendraceae, 33

Lepidophylloides, 6, 33, 34

Lepidostrobophyllum, 33, 34

Lepidostrobophyllum hastatum, 6, 33, 34

Lilpopia raciborskii, 25

Linopteris neuropteroides, 20

Lobatopteris geinitzii, 5, 27, 28

Macroneuropteris scheuchzeri, 5, 29, 30, 34

Mixoneura, 14, 29

Neurodontopteris auriculata, 16, 19

Neuropteris pseudoblissii, 14

Odontopteris, 5, 19, 20, 27, 29, 33

Odontopteris brardii, 5, 29, 31, 32

Odontopteris lingulata. See Odontopteris 

(Mixoneura) lingulata

Odontopteris (Mixoneura) lingulata, 16, 19

Odontopteris (Mixoneura) schlotheimii, 5

from Red Magdalena, 10, 14, 15

from Unit 8, 16, 17, 19

from Unit 10, 20, 21, 25

from Unit 35, 29, 31, 35

Odontopteris (Mixoneura) sp., 10, 14

Odontopteris (Mixoneura) subcrenulata, 5

from Unit 8, 16, 19

from Unit 10, 20, 25, 26

from Unit 35, 29–32, 35

Odontopteris obtusa, 16

Odontopteris schlotheimii. See Odontopteris 

(Mixoneura) schlotheimii

Odontopteris subcrenulata. See Odontopteris 

(Mixoneura) subcrenulata

Oligocarpia, 33

Oligocarpia gutbieri, 5, 25, 28, 33, 34

Oligocarpia leptophylla, 27, 33

Pecopteris, 14, 31, 33

Pecopteris densifolia, 5, 27, 28

Pecopteris oreopteridia, 27
4 0 • SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY

Peltaspermum, 5, 14, 15, 20, 28

Phasmatocycas, 14

Plagiozamites, 19

Podozamites, 19

Polymorphopteris (Acitheca) polymorpha, 5, 32, 33

Polysporia, 33

Pseudomariopteris cordato-ovata, 5, 29, 32

Psygmophyllum, 27

Remia, 20

Reticulopteris, 20, 27

Reticulopteris muensteri, 20, 26

Reticulopteris sp., 5, 27

Rhachiphyllum, 5, 14, 20, 25, 27, 35

Rhachiphyllum schenkii, 14, 15, 25

Sphenophyllum, 6, 19, 25, 28

Sphenophyllum gilmorei, 6–8, 35

Sphenophyllum oblongifolium, 6, 29, 32, 33

Sphenophyllum thonii, 6, 25, 28

Sphenophyllum verticillatum, 25, 33

Sphenopteridium manzanitanum, 14, 16, 19

Sphenopteris biturica, 27, 33, 34

Sphenopteris germanica, 5

from Red Magdalena, 14

from Unit 8, 16, 17, 19

from Unit 10, 23, 26

from Unit 35, 29, 31, 33, 35

Sphenopteris matheti, 5, 33

Sphenopteris mathetii, 27, 33

Sphenopteris, 5, 19, 27, 33, 34

Supaia, 5, 7, 9, 35

Supaia thinnfeldioides, 7, 9

Taeniopteris, 5, 10, 12, 14, 35

Taeniopteris abnormis, 10, 12

Walchia Morphotypes A–D, 5

from Red Magdalena, 10–13

from Unit 8, 16, 17

from Unit 10, 20, 22, 23

from Unit 35, 29, 30

Walchia piniformis, 4, 14

Walchia schneideri, 4, 14

Walchia sp., 5
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spaced; and have 1" margins. 

REQUIRED ELEMENTS are title page, abstract, table of contents, main text, and references.

FIGURES must be numbered sequentially (1, 2, 3, etc.) in the order called out; have components lettered consistently (in 

size, font, and style) and described in captions; include a scale bar or scale description, if appropriate; include any legends 

in or on figures rather than in captions. Figures must be original and must be submitted as individual TIF or EPS files.

FIGURE FILES must meet all required specifications in the Digital Art Preparation Guide. Color images should be 

requested only if required. 

TAXONOMIC KEYS in natural history manuscripts should use the aligned-couplet form for zoology. If cross referencing 

is required between key and text, do not include page references within the key, but number the keyed-out taxa, using the 

same numbers with their corresponding heads in the text.

SYNONYMY IN ZOOLOGY must use the short form (taxon, author, year:page), with full reference at the end of the 

manuscript under “References.”

REFERENCES should be in alphabetical order, and in chronological order for same-author entries. Each reference should 

be cited at least once in main text. Complete bibliographic information must be included in all citations. Examples of the 

most common types of citations can be found at SISP’s website under Resources/Guidelines.