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1975-02-01
Conifer wood from the Upper Jurassic of Utah; I,
Xenoxylon morrisonense sp. nov.
William D. Tidwell
David A. Medlyn
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Original Publication Citation
Conifer wood from the Upper Jurassic of Utah I, Xenoxylon morrisonense sp. nov. David A. Medlyn
and William D. Tidwell American Journal of Botany (February 1975), 62(2):23-28
BYU ScholarsArchive Citation
Tidwell, William D. and Medlyn, David A., "Conifer wood from the Upper Jurassic of Utah; I, Xenoxylon morrisonense sp. nov."
(1975). All Faculty Publications. Paper 1452.
http://scholarsarchive.byu.edu/facpub/1452
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Amer. J. Bot. 62(2):
203-208.
1975.
CONIFER WOOD FROM THE UPPER JURASSIC OF UTAH
PART I: XENOXYLON MORRISONENSE SP. NOV.1
DAVID
Department
A. MEDLYN AND WILLIAM D. TIDWELL
of Botany, Brigham Young University, Provo, Utah 84602
ABSTRACT
A new species of conifer wood, Xenoxylon morrisonense, is described from the Morrison
Formation on the Colorado Plateau. It is compared with other species of Xenoxylon, with X.
latiporosum being the closest. Xenoxylon morrisonense differs from X. latiporosum in its
marked indentations, simple pits on the horizontal and tangential walls of ray cells, absence
of crassulae, presence of wood parenchyma, and thin borders on podocarpoid type crossfield
pits. The origin of the septa in the tracheids is summarized, and the possible affinity of Xenoxylon with the Podocarpaceae is considered.
THE PETRIFIEDWOOD considered in this report
was collected from the Upper Jurassic Morrison
Formation on the Colorado Plateau in southcentral Utah. The locality, near Clay Point, Garfield
Co., Utah2, was shown to us by Mr. and Mrs.
Thomas Hopkins of Hanksville, Utah. The site
is relatively undisturbed and contains several wellpreserved specimens. Petrified woods are 10caUy
abundant in the Morrison Formation. Many of
these woods are highly siliceous and variously
colored and are often poorly preserved. However,
some specimens are well preserved and are generically comparable to previously reported fossilwoods of similar age.
The specimen, a trunk measuring approximately
twenty-two in. at its widest diam, was embedded
in a pebble conglomerate (Fig. 1). The axis is
partly silicified and contains areas of structurally
preserved cells favorable for study. However, the
pith, primary xylem, and phloem are not preserved.
DIAGNOSIS: Xenoxylon
morrisonensesp. n.Transverse section-Growth rings narrow, sometimesindistinct (Fig. 2), 2-3 cells wide, late wood
tracheids tangentially flattened,
angular to
rounded, 20 fLm in diam, lumens round to elliptic;
early wood tracheid size varies from 25-50 fLm in
diam, tracheids more or less regularly aligned, occasionaUy with smaUer tracheids interspersed
among the larger; cell walls 5-6 fLm thick in both
early and late wood; wood parenchyma present
but not readily observable in cross section; rays
uniseriate,occasionally biseriate, separated by one
to five rows of tracheids, commonly four; horizontal walls of ray parenchyma pitted with 1-2
simplepits.
Received for publication 16 January 1974.
'U.S. Geo!. Surv. Map, Hall Mesa Quadrangle,
S, R 10 E, N. E. 1,4 Sec. 24.
1
T 35
Tangential-Rays
2-35 cells high, commonly
10-15 (Fig. 3), cells squarish, largest about
25 fLm; rays mostly uniseriate, frequently partiaUy
biseriate, rarely entirely biseriate; tracheids with
numerous septa (Fig. 3, 7), tangential pitting
absent.
Radial- Tracheary pits uniseriate, round, commonly vertically flattened (Fig. 4, 9), mostly
contiguous with adjacent pits, rarely separate,
pit diam varies from 16 fLm in late wood to 27-30
fLm in early wood, pit aperture circular 5-7 fLm,
pit border nearly fills entire width of the lumen;
crassulae absent, contact line of adjacent pits commonly appear dark brown; tracheids with numerous septa (Fig. 3, 4, 7, 8); ray crossfields with
one (Fig. 5), often two (Fig. 10), large podocarpoid pits 10-16fLm in diam, with elliptic apertures (Fig. 5); pit borders thin, crescent shaped,
some crossfields appear to be occupied by a large
Pinus silvestris-type pit (although the lack of
border could be a feature of preservation); horizontal and tangential waUs of ray parenchyma
pitted with numerous indentations; tangential walls
of ray parenchyma typically meet horizontal
waUs mostly at right angles; ray tracheids absent;
axial parenchyma throughout stem and commonly
appear as swollen bulbous cells (Fig. 6, 8).
Holotype:
tory 926.
Brigham Young University Reposi-
Horizon: Upper Jurassic Morrison Formation.
DISCUSSION-Xenoxylon was proposed by Gothan (1905) for specimens of wood previously described by Cramer (1868) as Pinites latiporosus
from Green Harbor, Spitzbergen. The diagnostic
characteristics of this genus are (1) the occurrence of at least some vertically flattened and
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204
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travers ely elongated pits on the radial walls of
tracheids; (2) the absence of pitting on the horizontal and tangential walls of rays; and (3) large
oval pits on the radial walls of the ray cells.
Xenoxylon spans a comparatively narrow geological range (Middle Triassic to Lower Cretaceous). However, geographically it is widely
distributed in northern latitudes. Since 1905,
an increasing number of occurrences have been
cited. In 1906, Gothan reported Xenoxylon from
the Jurassic of Poland, and Holden (1913) described it from the Jurassic of Yorkshire, England. The oldest occurrence was cited by Fliche
( 1910) from Middle Triassic strata of France.
Xenoxylon has been reported from the Jurassic of
China (Chang, 1929; Gothan and Sze, 1933),
Korea (Ogura, 1931), Japan and Manchuria
(Shimakura, 1936; Watari, 1960), Jurassic of
France (Grambast, 1953), and also the Cretaceous of Alaska (Arnold, 1952).
Five previously described species of Xenoxylon
are X. latiporosum (Cramer) Gothan, X. phyllocladoides Gothan, X. conchylalianum Fliche, X.
hopeiense Chang, and X. barberi (Seward) Kdiusel. Xenoxylon morrisonense shows a close affinity to X. latiporosum on the basis of the diagnostic tracheid septation which is lacking in the
other four species. These thin, transverse septations were figured by Gothan (1905), Ogura
(1944), Arnold (1952), and Watari (1960) in
their reports of X. latiporosum. Xenoxylon morrisonense differs from X. latiporosum in the absence of crassulae, and in having marked indentations and simple pits on both the horizontal
and tangential walls of the rays (Fig. 5). The
latter condition is not compatible with the generic
designation, but it is not inconsistent with the
admixture of characters associated with transitional conifers. Shimakura (1936) and Watari
(1960) has stated that true crassulae, as seen in
abietinean wood, are never present in X. laiiporosum, although the contact between the borders of two adjacent pits are often dark brown.
However, Arnold (1952) pointed out distinct
crassulae that had apparently been overlooked
by authors who previously described this species.
He described these crassulae as being narrow because of the crowded condition of the pits. The
tracheal pitting of X. morrisonense is always contiguous (Fig. 9), but the pits are not always vertically flattened and horizontally elongated as in
X. latiporosum. The conspicuous presence of a
OF BOTANY
[Vol. 62
thin border on some of the crossfield pits of our
specimen is also a notable difference.
Wood parenchyma is present in Xenoxylon
morrisonense but not in X. latiporosum. The latter species has small oval or circular pits on the
tangential walls of the late wood tracheids (Shimakura, 1936), which are missing in the former.
Seward (1919) stated there are no resin canals
or xylem parenchyma in Xenoxylon. However,
X. hopeiense Chang, recorded from China in
1929, is said to differ from other described species of Xenoxylon in having crassulae, wood parenchyma, occasional biseriate rays, and resinous
cells in the rays.
Thus, on the basis of marked indentations, pitted horizontal and tangential walls of ray cells,
absence of crassulae, thin borders on podocarpoid-type crossfield pits, and wood parenchyma,
X. morrisonense is proposed as a new species.
The origin of the septa in tracheids has been
reviewed and postulated by several authors. Penhallow (1907) attributed similar structures to
resin plates which divided the tracheids. Conrad (1910) and Record (1918) proposed that
these septa are of parenchymatous origin. Thomson (1913) described septate tracheids from the
xylem of the Araucarineae. Some of these are
partial septations composed of secondary walls,
whereas others are complete septations which
Thomson described from Agathis bornensis and
A. alba. In connection with the latter, Thomson
(1913, p. 25) stated " ... there are often parenchyma cells replacing some of the septated parts.
This may go so far that the whole tracheid is replaced parenchyma, but usually there is some
vestige of the origin of these vertical series of cells
from the tracheary elements." He also mentioned
that septate tracheids and parenchyma cells replace tracheids in vertical rows in the wood of
Abies and also in association with the resin canals
of pines. Jeffery (1925) considered septate tracheids to be intermediate stages between tracheids
and wood parenchyma, as viewed in Picea, and
that they represent a primitive form of axillary
parenchyma. He further mentioned that tracheids
in the roots and cones of pine are occluded by ingrowing parenchyma cells (Jeffery, 1917), although this condition is not normal to the vegetative axis of living pines. Jane (1956) discussed
horizontal biconcave resin plates characteristic of
the genera Agathis and Araucaria. He explained
..,)
Fig. 1-6. Xenoxylon morrisonense. BYU 926 holotype.
1. Holotype specimen imbedded in pebble conglomerate. 2. Transverse section, illustrating the homogenity of xylem and a narrow growth ring. X 145. 3. Tangential section, showing the septa in the tracheids. X 125. 4. Radial section, demonstrating
pitting and tracheal
septation.
X 195. 5. Radial section, showing indentations and crossfield pitting. X 575. 6. Radial section, showing swollen axillary parenchyma.
X 285.
February, 1975]
MEDLYN AND TIDWELL-WOOD
OF XENOXYLON
205
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OF BOTA
Y
Fig. 7-10.
Xenoxylon morrisonense.
BYU 926 holotype.
7. Tangential section, showing
epta.
Radial section. Notice septa and swollen axillary parenchyma.
X 155. 9. Radial section,
howing
flattened pits. X 490. 10. Radial section, showing crossfield pitting. X 490.
that if these plates were cut medianly, the tracheid
would appear to be full of resin; whereas if cut
near the upper boundary, there would appear to
be a resinous lining to the tracheid lumen.
Both Ogura (1944) and Watari (1960) discussed the origin of septate tracheids in Xenoxylon latiporosum and concluded they were tylosoid. The septa were formed by the fusion of
membranes of adjacent tyloses which originated
by the projecting of the parenchymatous ray cells
through the ray pit. Watari (1960) demonstrated
the development of the tylose from its origin as a
bladder until it formed the septum. In Araucaroxylon mineense and A. huzinamiense Ogura
(1960) observed the projection of the contents of
[Vol.
62
X 490. 8.
contiguous,
wood parenchyma into some tracheids. The presence of tylose formation in some species of living
conifers (Batton and Boureau, 1965; Ogura,
1944) makes the presence of these structures in
fossil woods entirely possible.
Arnold (1952), commenting on the tylosoid
nature of the septa in his specimen of Xenoxylon
latiporosum, remarked, " ... they are present in
such great numbers that connection of all them
with ray cells does not seem quite possible." This
is also the case in Xenoxylon morrisonense; although some septa definitely appear to have
formed from tyloses, others appear as transverse
walls (Fig. 7). Ogura (1944) postulates a similar condition as being due to the lateral mem-
February, 1975]
MEDL Y
AND TIDWELL-WOOD
branes of the tylo es coming in close contact with
the tracheid walls and thereby obscuring the tylose-like structure. The horizontal portion of the
membranes then appear as thin, flattened plates
crossing the tracheid lumen. Ogura (1960) concluded that differences in tracheid septation may
be due to species variation and the age of the
wood.
Affinities-As
is the case with many Mesozoic
conifers, placing Xenoxylon in an extant family
or genus is rather problematical. In tracing conifers through the geologic past, the Mesozoic,
especially Middle Mesozoic, would have to be
termed "The Age of Conifers." It was at this
time that conifers were undergoing rapid divergence. The species of this Era had retained many
ancestral characteristics, but at the same time
many of the characteristics which denote modern
forms were evolving. Hence some authors have
called them "transitional conifers." If classification of these species were based only on primitive
characters, which some authors have done, all
would be diagnosed as araucarian or, probably
more accurately, cordaitalean conifers.
Arnold (1952) reviewed the previous concepts
of araucarian affinities proposed for Xenoxylon
by Holden (1913) and others. He pointed out
discrepancies in using some criteria, particularly
presence or absence of crassulae, as a fundamental difference between araucarian and the "more
advanced" conifers. As stated by Arnold (1952),
"Considerable confusion and misunderstanding
about the affinities of fossil conifers has resulted
from an 'either-or' attitude on the part of investigators who have endeavored to force misfits into modern families where they do not belong." Perhaps by using a combination of ancestral
and living characters with special emphasis on the
"more advanced" features, a semiorderly system
of classification of these fossil conifers may yet be
established.
A combination of large podocarpoid pits in the
crossfields, smooth horizontal and tangential walls
on the ray cells, and the absence of resin canals
suggests a possible phyletic affinity of Xenoxylon
with the modern family Podocarpaceae. Some
Xenoxylon species are said to be indistinguishable from Sciadopitys and Podocarpus (Arnold,
1952). The occurrence of pitted horizontal and
tangential walls of the rays of X. morrisonense
also suggests a possible ancestral tie to some
members of the Pinaceae.
There are three established fossil genera, Mesembrioxylon, Protophyllocladoxylon, and Xenoxylon, for petrified woods which show possible
affinities to the Podocarpaceae.
In addition,
Embergerixylon is similar but has not yet been
allied with woods related to this family.
Mesembrioxylon is an artificial genus established by Seward (1919) to include those extinct
OF XENOXYLON
207
genera which show affinity to the modern family
Podocarpaceae. It was primarily intended to replace Gothan's two genera Podocarpoxylon and
Phyllocladoxylon. Earlier Stopes' (1915), recognizing the inconsistencies between these two
genera, lumped them into the single genus Podocarpoxylon. Seward in his original diagnosis defined Mesembrioxylon as a coniferous wood in
which the features are similar to those associated
with Cupressinoxylon, although the xylem parenchyma mayor
may not be present. Another
feature that Seward described for the former
genus is the presence of one or two large simple
pits, or in some species two or more smaller bordered pits, in the crossfields. In either case, the
apertures are more vertical than horizontal.
When Gothan established Xenoxylon he did
not mention its affinities with the Podocarpaceae
but alluded to some relationship by naming one
species X. phyllocladoides. Seward (1919) also
recognized a relationship when he said that Xenoxylon showed a close resemblance to Gothan's
genus Phyllocladoxylon (=M esembrioxylon).
Krausel (1939) proposed Protophyllocladoxylon for woods similar to that of the Podocarpaceae
but which differ by having araucaria-type pitting
on the radial walls of the tracheids. The previously described species of this genus range stratigraphically from the Upper Carboniferous (?) to
the Tertiary. All species are characterized by the
presence of araucaria-type crossfield pitting. Septate tracheids are present in some species. Species of Protophyllocladoxylon show many striking
similarities to Xenoxylon morrisonense, especially
those having septate tracheids. However, the
araucarian pitting on the radial walls of the tracheids of Protophyllocladoxylon excludes our
specimen.
Lemoigne (1966) and Demarcq and Lemoigne
( 1967) placed two new species in Dadoxylon,
but Lemoigne (1968) later removed them by assigning them to a new genus, Embergerixylon,
which features septate tracheids with uniseriate
and, more rarely, biseriate (araucarian) tracheary
pitting on the radial walls. The crossfield pitting
is also of the araucarian type, although poorly preserved, which is probably why any possible affinity of this genus with the Podocarpaceae was
not mentioned. Xenoxylon morrisonense and Embergerixylon alpinum are very similar but differ
in their crossfield pitting.
All of the above genera tend to merge into one
another, part of the problem of affinities being the
great range of structural variability in both living
and extinct conifers. The rest of the enigma lies
in the lack of precise generic delineation.
The foliage of Xenoxylon is unknown although
Nathorst (1897) suggested a possible relationship
between this genus and Elatides. Arnold (1952)
noted the association of his specimen with Podo-
AMERICAN
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JOURNAL
zamites and Baiera. Watari (1960) mentioned
that Podozamites occurs in the same strata as his
specimens of Xenoxylon latiporosum. No fossil
foliage has yet been collected in the area where
Xenoxylon morrisonense was found. However,
Brown (1972) reported Podozamites with the
conifers Pagiophyllum and Pityophyllum from the
Morrison Formation in Montana. If an organic
connection could be proven between the supposed
araucarian Podozamites and Xenoxylon, which
appears to be unlikely, the evidence would be
against the placement of Xenoxylon in the Podocarpaceae.
Xenoxylon has a wide geographical distribution
in the Northern Hemisphere. It has been reported
from Spitzbergen, King Charles Land, England,
Poland, France, Germany, Korea, Manchuria,
western Canada, and northern Alaska (Arnold,
1952). The occurrence of Xenoxylon morrisonense in Utah is the southernmost extension of the
genus in North America.
Xenoxylon is not known from the Southern
Hemisphere. In contrast, the Podocarpaceae is
the most important family of Coniferales in the
Southern Hemisphere today. Species of this family grown mainly in mountain tropical forests, although some occur in lowland forests (Florin,
1958).
Xenoxylon and other allied genera seem to be
widespread in the Mesozoic. Of the described
species of Xenoxylon, Protophyllocladoxylon,
Mesembrioxylon, and Embergerixylon, approximately 75 % are found in the Mesozoic, thus
showing that these possible representatives of the
Podocarpaceae were an important aspect of the
Mesozoic flora.
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