Two new petrified cycad stems, Brunoa gen. nov. and Worsdellia gen. nov., from the Cretaceous of patagonia (Bajo de Santa Rosa, Rio Negro Province), Argentina.
I. Abstract II. Introduction III. Locality, Materials, and Methods IV. Systematics V. Discussion VI. Conclusions VII. Literature Cited
Cycadales are the most primitive order within gymnosperms with extant representatives. The evolutionary history as known from fossils begins in the Upper Paleozoic (Mamay, 1969, 1976; Taylor, 1969; Zhu & Du, 1981) and reaches maximum distribution during the Mesozoic, with decline beginning toward the end of the same period. Presently, Cycadales are a very well circumscribed group consisting of two suborders with three families: Cycadaceae, Stangeriaceae, and Zamiaceae (Johnson, 1959; Petriella & Crisci, 1975, 1979; Stevenson, 1990, 1992), with 11 genera and approximately 281 species (Hill et al., 2004).
The formal classification of extant genera was carried out taking into account 52 morphological, anatomical, karyological, physiological, and phytochemical characters (Stevenson, 1990, 1992). The advances in this sense contrast with what happened with fossil taxa, where it has not been possible to follow one unique classification criterion.
The fossil record of anatomically preserved stems of Cycadales is quite rare, and only ten taxa are known so far. The oldest genera are Triassic in age, with Michelilloa (Archangelsky & Brett, 1963) from Argentina, an unnamed cycad genus from Chile (Lutz & Crisafulli, 1998), Charmorgia (Ash, 1985) and Lyssoxylon (Gould, 1971) from western North America, and Antarcticycas (Smoot et al., 1985) from Antarctica. Fascivarioxylon (Jain, 1962) is from the Jurassic of India. Cretaceous forms include Sanchucycas (Nishida et al., 1991) from Japan and Centricycas (Cantrill, 2000) from Antarctica. Two genera, Bororoa (Petriella, 1972) and Menucoa (Petriella, 1969), are known from the Tertiary of Argentina.
The finding in Argentina of new Cretaceous trunks (Andreis et al., 1991; Artabe & Stevenson, 1999; Artabe & Zamuner, 1999; Artabe, 2001) assignable to the Cycadales reinforces the importance this group had in the geological past of Argentina and stands in marked contrast to the absence of cycads in that nation's extant flora.
III. Locality, Materials, and Methods
The fossil stems described below represent new additions to the fossil cycad flora of Argentina and come from the Allen Formation at the Bajo de Santa Rosa locality (ca. 40[degrees] S-50[degrees] W), north of the locality of Valcheta, Rio Negro Province, Argentina (Fig. 1). Stratigraphic studies by Andreis et al. (1991) and Andreis (1996) show the Allen Formation to be a continental to deltaic, shallow, marine succession. The petrified trunks studied were found at the base of the sequence, which corresponds to a meandriform fluvial system with channel facies and floodplains.
[FIGURE 1 OMITED]
The trunks were fossilized as silicifications in a good state of preservation. The material of the new species, Brunoa santarrosensis, is deposited in the Paleobotanical Collection of the Paleobotany Department of the Facultad de Ciencias Naturales y Museo de La Plata (LPPB). In order to study the anatomy of the trunks, thin sections of B. santarrosensis were made using traditional techniques. Worsdellia bonettiae gen. nov. et sp. nov. was described on the basis of another specimen deposited in the Entomology Collection of the Museo Argentina "Bernardino Rivadavia." This holotype specimen (53818) was studied only in cross-section as represented by the specimen ends and in external surface views.
Order: Cycadales Dumortier
Suborder: Cycadineae Stevenson
Family: Zamiaceae Reichenbach
Subfamily: Encephlartoideae Stevenson
Brunoa Artabe, Zamuner, and D. Stevenson, gen. nov.
Figures: Figs. 2-9.
[FIGURES 2-9 OMITTED]
Generic diagnosis: Cataphyll and leaf bases in alternating cycles. Polyxylic stem covered by cataphylls and persistent rhomboidal foliar bases. Vascular cylinder with six rings of collateral vascular bundles with phloem toward the outside and divided into fanlike sectors by multiseriate, parenchymatous rays. Foliar traces run irregularly, and as they pass through wide rays to the outer cortex they curve, producing a girdling configuration.
Type species: Brunoa santarrosensis Artabe, Zamuner, and D. Stevenson.
Eponymy: The genus is named in honor of Dr. Bruno Petriella, the late outstanding Argentinian paleobotanist who made many significant contributions to our knowledge of fossil cycads.
Brunoa santarrosensis Artabe, Zamuner, and D. Stevenson, sp. nov.
Figures: Figs. 2-9.
Holotype: LPPB 12968a-k; LPPB-Pm 1545 to 1555.
Paratype: LPPB-Pm 1545 to 1555.
Species diagnosis: Parenchymatous pith with idioblasts, mucilage cavities, and anastomosing medullary bundles, and vascular bundles forming rings related to cone domes. Tracheids of secondary xylem have scalariform or araucaroid, uniseriate to triseriate, bordered pitting. Phloem consisting primarily of fibers with few sieve cells. Cortex parenchymatous with idioblasts and mucilage cavities. Foliar bases with monostratified epidermis and a noncontinuous sclerenchyma band below a thin parenchymatous zone and with collateral vascular bundles without any associated mucilage canals or cavities.
Age: Upper Cretaceous (Allen Formation; Andreis, 1996).
Type locality: Bajo de Santa Rosa, Rio Negro Province, Argentina.
Etymology: The specific epithet is in reference to the type locality.
Description: Trunk columnar, to 32 cm in diameter, covered by alternating cycles of cataphylls and persistent foliar bases (Fig. 7). Leaf bases rhomboidal with the transversal axis wider than the vertical axis, 3.1 cm (2.4-3.6 cm) wide x 1.76 cm (1.2-2.2 cm) high. Cataphyll bases 2.66 cm (2.5-3 cm) wide x 0.9 cm (0.8-1 cm) high.
The pith is approximately 12 cm in diameter. Fundamental tissue is composed of parenchymatous cells with thin walls, more or less isodiametric, polygonal, and mostly 143 [micro]m (112-180 [micro]m) in diameter. Idioblasts of similar form and size also present. Mucilage cavities quite abundant, ellipsoidal, 376 [micro]m (225-585 [micro]m) in diameter, grouped in rows (Fig. 6). Numerous medullary vascular rings, corresponding to reproductive cone domes (Fig. 6), are present.
Vascular cylinder, 6.5 cm thick, composed of six concentric rings of secondary xylem and phloem varying between 1 cm and 1.5 cm in radial thickness, separated by parenchymatous zones that are 0.1-0.2 cm wide and with abundant mucilage cavities (Figs. 2, 5). Vascular bundles of each ring fanlike in shape. Tracheids of the secondary xylem polygonal or quadrangular in transection, with a rounded lumen (Fig. 2) 28 [micro]m in tangential diameter (10-51 [micro]m) and 30 [micro]m in radial diameter (10-41 [micro]m), and with a wall 3-9 [micro]m (average, 6 [micro]m) thick. Radial wall pitting scalariform or bordered, uniseriate to triseriate (generally biseriate), alternate araucaroid with inconspicuous crossfield pitting, and 18.6 [micro]m (12-24) x 14.2 [micro]m (12-16) (Fig. 3). Secondary rays uniseriate, up to 228 [micro]m tall, often greater than 1848 [micro]m in radial length, and 100-370 [micro]m wide. Most of the preserved secondary phloem composed of fibers that are somewhat radially flattened with a tangential diameter of 19-28 [micro]m (average 22 [micro]m). Transectionally round fibers also present, 16-22 [micro]m (average 19 [micro]m) in diameter.
Cortex parenchymatous, to 2 cm thick, with idioblasts and mucilage cavitiels (Figs. 2, 5). Foliar traces semigirdling, following a centrifugal oblique course in the inner zone of the cortex (Fig. 6) and becoming horizontal in the external cortical zone.
Leaf bases with a monostratified epidermis, a parenchymatous zone of 4-5 rows of cells, followed by a discontinuous layer of sclerenchyma. Vascular bundles collateral, lacking an associated mucilage canal, arranged without any order within the parenchyma; thus, the typical inverted omega pattern of other Cycadales is absent.
Comparisons: Brunoa is characterized by polyxyly, cone domes, and mucilage cavities. Brunoa can be compared with other polyxylic fossil forms, Sanchucycas, Worsdellia, Bororoa, and Menucoa, as summarized in Table I. Characters common to Brunoa and Sanchucycas are the presence of mucilage cavities, more than three vascular rings, and secondary xylem with uniseriate and biseriate, alternate, araucaroid, tracheid pitting. However, there are significant differences, in that Sanchucycas develops up to 90 vascular rings, the radial pitting of the tracheids is dominantly uniseriate, the conspicuous crossfield pitting is composed of 1-2 large pits, and the secondary rays are uniseriate to hexaseriate.
Bororoa and Menucoa have subovoid and columnar trunks, respectively. Like Brunoa, they develop medullary vascular bundles. These medullary vascular bundles are related to the vasculature of reproductive structures (i.e., cone domes) in Bororoa, while they are not related to cone domes in Menucoa. The secondary xylem of Bororoa and Menucoa differs from that in Brunoa by having araucaroid multiseriate, bordered, intervascular pitting and mucilage canals. The length/width ratio of the foliar bases of Brunoa is similar to that observed in Bororoa.
With respect to monoxylic forms, Charmorgia differs from Brunoa in the presence of mucilage canals; medullary bundles; alternate, bordered, uniseriate to biseriate, intervascular pitting and a few tracheids with scalariform pitting; and the length/width ratio of the foliar bases is greater (oscillating between 3 and 5).
Lyssoxylon more closely resembles Brunoa because its stem is covered by persistent foliar bases, and the tracheids of the intervascular pitting can be uniseriate to triseriate but generally are araucaroid biseriate. Lyssoxylon can be distinguished from Brunoa by its elaborate crossfields, which have up to 9 pits and a parenchymatous pith with mucilage canals instead of mucilage cavities. In most respects Brunoa is most similar to Lyssoxylon among monoxylic taxa.
Fascivarioxylon has stems with diameters much smaller (only up to 2.3 cm) than those of Brunoa (up to 32 cm), with persistent foliar bases and a width/height relationship of 3.5:1. In Fascivarioxylon, radial pitting of the secondary xylem is uniseriate to triseriate and alternate to opposite, and the pith is heterogeneous, with medullary vascular bundles, mucilage cavities, and sclerotic nests. It is interesting to note that the presence of sclerotic nests composed of sclereids is not a generalized character in extant cycads (Stevenson, 1990) and is present only in Encephalartos, Macrozamia, and Lepidozamia among extant cycad genera.
Antarcticycas has a delicate habit. The stems reach 3.7 cm in diameter, have no persistent foliar bases, and have a well-developed periderm (Smoot et al., 1985). The pith and the cortex have mucilage canals that form concentric cycles, and the intervascular pitting of the secondary xylem is alternate, multiseriate, and bordered.
Michelilloa has a stem up to 10 cm in diameter without persistent foliar bases or a well-developed periderm. The radial intervascular pitting of the secondary xylem is alternate, multiseriate (up to 6 series), and bordered, and the crossfields have many small pits. The pith and the cortex have idioblasts and mucilage canals.
Centricycas has a central pith and the innermost portion of the xylem cylinder. The pith is parenchymatous, with frequent dark staining, thick-walled cells, and numerous medullary vascular bundles randomly arranged and not related to cone domes. Radial intervascular pitting of the secondary xylem is alternate, multiseriate, and bordered with poorly preserved crossfields.
In summary, Brunoa shares polyxyly with Sanchucycas, Worsdellia, Bororoa, and Menucoa; cone domes with Bororoa; mucilage cavities with Sanchucycas and Fascivarioxylon; and intervascular pitting that is uniseriate to triseriate and araucaroid with Sanchucycas, Charmorgia, Lyssoxylon, Fascivarioxylon, and Centricycas.
Worsdellia Artabe, Zamuner, and D. Stevenson, gen. nov.
Figures: Figs. 10-17.
[FIGURES 10-17 OMITTED]
Generic diagnosis: Stem arborescent, covered by alternating cycles of cataphylls and persistent rhomboidal foliar bases, polyxylic. Manoxylic vascular cylinder with two concentric rings of collateral vascular bundles and centripetal vascular bundles with respect to the inner ring. Foliar traces run irregularly, and as they pass through wide rays to the outer cortex they curve, producing a girdling configuration. Medullary bundles and mucilage canals occur in the pith.
Type species: Worsdellia bonettiae Artabe, Zamuner, and D. Stevenson.
Eponymy: The genus is named in honor of the late W. C. Worsdell, who made many outstanding contributions to the clarification of the anatomy, morphology, systematics, and phylogeny of extant and fossil cycads.
Worsdellia bonettiae Artabe, Zamuner, and D. Stevenson, sp. nov.
Figures: Figs. 10-17.
Holotype: 53818 in the Entomology Division, Museo Argentino "Bernardino Rivadavia."
Species diagnosis: Polyxylic stem with a wide, central, parenchymatous pith containing mucilage canals and medullary bundles. Vascular cylinder composed of two rings of collateral vascular strands and centripetal vascular bundles with respect to the inner ring. Cortex parenchymatous with extrafascicular concentric bundles and girdling leaf traces in the external zone. Persistent leaf bases are of small dimensions.
Age: Upper Cretaceous (Allen Formation; Andreis, 1996).
Type locality: Bajo de Santa Rosa, Rio Negro Province, Argentina.
Eponymy: The specific epithet is dedicated to Dr. Maria Bonetti in recognition of her important contributions to Argentinian paleobotany.
Description: Stem columnar with persistent leaf bases, polyxylic, and laterally compressed, resulting in one of its diameters reaching 27 cm, whereas the other reaches only 13.5 cm. In cross-section, four zones are differentiated: pith, vascular cylinder, cortex, and persistent leaf bases (Figs. 11, 12).
The pith is parenchymatous, with mucilage canals (Fig. 17) and peripheral medullary vascular bundles (Fig. 16). Like the stem in general, the pith is elliptic and 13.8 cm x 9.9 cm. The parenchyma cells vary between 67.5 and 135 [micro]m in diameter. Mucilage canals are 225-450 [micro]m (average 329 [micro]m) in diameter, random in their course, and frequently branched. Medullary vascular bundles are located in the outer regions of the pith and are 450-810 [micro]m (average 679.5 [micro]m) in radial thickness and 675-1080 [micro]m (average 886.5 [micro]m) in width. In many cases the medullary bundles are branching. However, no medullary bundles appear to be related to reproductive axes and form no organized rings. Thus, the medullary bundles are not related to cone domes.
The vascular cylinder is made up of two collateral rings of secondary xylem and phloem (Figs. 11, 12) and an incomplete ring of centripetal vascular bundles at the periphery of the pith (Figs. 12, 15). Each normal vascular ring is composed of triangular vascular segments separated by wide secondary rays that are up to 0.1 cm wide. The inner vascular ring is more developed than the outer vascular ring. The segments of the inner ring are 0.09-0.3 cm wide and 0.5 cm in radial thickness, whereas the segments of the outer ring are 0.08-0.33 cm wide and 0.3 cm in radial thickness. The tracheids of the secondary xylem are 18-54 [micro]m (average, 33 [micro]m) in tangential diameter, and the phloem has fibers that are 36-54 [micro]m (average, 44 [micro]m) in tangential diameter.
The parenchymatous cortex is composed of isodiametrical parenchyma cells that are 45-135 [micro]m (average, 87.3 [micro]m) in diameter, concentric extrafascicular vascular bundles (Fig. 13), and girdling leaf traces in the outer region (Fig. 14). The leaf bases (Fig. 10) are persistent, rhomboidal, transversely flattened, and 0.8 cm wide x 0.3 cm high.
Comparisons: Worsdellia bonettiae n. sp. is characterized by polyxyly; medullary vascular bundles; mucilage canals; extraxylary, cortical bundles; and centripetal vascular bundles with respect to the inner ring. Because Worsdellia is a polyxylic genus it may be compared with the other fossil genera known to have polyxylic trunks; namely, Sanchucycas, Brunoa, Bororoa, and Menucoa. Trunks of Sanchucycas have neither the pith nor the cortex preserved and, thus, are known only on the basis of their wood structure. The wood of Sanchucycas is more or less dense, with scanty parenchyma, and is similar to that of the fossils of Brunoa, Lyssoxylon, Michelilloa, Bororoa, and the extant genus Dioon (Chamberlain, 1909). In contrast, Worsdellia, like Menucoa, has a more parenchymatous secondary wood.
Among the polyxylic genera, both Worsdellia and Menucoa have true medullary vascular bundles, in contrast to Brunoa and Bororoa, where the medullary bundles are associated with cone domes. Among the monoxylic forms, Charmorgia and Fascivarioxylon have anastomosing and branching medullary bundles that are distributed in the outer region of the pith in the same fashion as in Worsdellia. However, the medullary bundles of Fascivarioxylon are concentric or collateral and often have an irregular course throughout the pith rather than being located only in the outer region as compared with those of Charmorgia and Worsdellia.
The presence of inverse xylem also links Worsdellia to Fascivarioxylon. The latter has been considered a cycad even though it shows important similarities with corystosperms. Supporting this is the presence in both of secretory canals and a ring of inverse xylem with development equal to that of the normal xylem. These characters make it more similar to juvenile stems of Rhexoxylon.
The presence of centripetal xylem links Worsdellia to Fascivarioxylon and Menucoa. In Fascivarioxylon the centripetal xylem forms a ring that is as equally developed as the normal ring. Worsdellia has small centripetal bundles with respect to the first normal vascular ring. In Menucoa the presence of centripetal xylem is rare, and only one inverted bundle was observed (LPPB 9033, Holotype; LPpm 664).
The extrafascicular vascular bundles present in the cortex of Worsdellia have not been seen in the stems of other fossil genera but have been observed in some extant genera of cycads, such as Cycas, Bowenia, Encephalartos, Macrozamia, and Lepidozamia (Worsdell, 1901; Stevenson, 1990). Dorety (1909) also reported the presence of such vasculature in Ceratozamia, but Stevenson (1990) was unable to confirm this. This is a generalized character of the Medullosales that has often been overlooked in the cycads but would appear to be pleisiomorphic as it occurs only in the lower clades of the cycads (Stevenson, 1990).
In summary, Worsdellia shares the presence of mucilage canals with all of the fossil forms except Sanchucycas, Fascivarioxylon, and Brunoa; polyxyly with Sanchucycas, Brunoa, Bororoa, and Menucoa; the presence of anastomosing and branching medullary vascular bundles with Menucoa, Charmorgia, and Fascivarioxylon; and the presence of centripetal xylem with Fascivarioxylon and Menucoa.
The assignation of the genera of fossil trunks to the different taxonomic categories currently recognized in the Cycadales is in need of reassessment. This is due mainly to the presence of a combination of characters that determine structural and anatomical patterns different from those appearing in extant genera and indicate that the latter represent terminal forms of a group that formerly exhibited greater anatomical diversity. According to this, and in order to clarify the systematic position of the fossil forms and its links with the extant genera, some characters that could have a systematic and phylogenetic relevance were analyzed.
The presence of polyxylic steles has been observed in Cycas, Encephalartos, Lepidozamia, and Macrozamia. This is a character of large adult stems and has not been, as yet, observed in all species of those genera. However, all of the polyxylic forms that have been observed show very little variation in the anatomy and organization of the polyxylic vascular cylinder, cortex, and foliar bases. In fossil genera, polyxyly appears in the Cretaceous in Sanchucycas, Brunoa, and Worsdellia and continues in the Tertiary in Bororoa and Menucoa. Polyxylic steles have been interpreted as derived with respect to monoxylic steles (Crane, 1988; Stevenson, 1990); this interpretation is supported by the fossil record because the oldest fossil stems from the Triassic (Antarcticycas, Lyssoxylon, and Michelilloa) and Jurassic (Fascivarioxylon) assigned to Cycadales are monoxylic.
Centripetal xylem has been observed in extant forms and are present in the basal region of the stems of some species of Macrozamia, Lepidozamia, Encephalartos, and Cycas. They also appear in the seedlings of Bowenia spectabilis (Worsdell, 1906). Among fossil forms, Fascivarioxylon has a vascular cylinder of double centrifugal-centripetal collateral bundles, with a uniform development of secondary xylem in both directions. In contrast, Worsdellia has small centripetal bundles with respect to the first normal vascular ring and in this respect is more similar to extant genera than to Fascivarioxylon. The capacity to develop centripetal xylem appears in some Paleozoic pteridosperms and is established as a character in Medullosales, persisting during the Mesozoic in Corystospermales, Pentoxylales, and Cycadales. Worsdell (1896, 1906) suggested that the centripetal xylem of the Cycadales is derived from that of the Medullosales by a progressive reduction of the centripetal portions of the centripetal xylem but that presence of this character in Cycadales is evidence of a relationship between the two groups.
In the Corystospermales, Rhexoxylon exhibits centripetal-centrifugal xylem in perimedullary bundles. Remnant and supernumerary cambia originate new bundles and centripetal polyxyly in the stems and cable-like structure in rhizomes (Zamuner, 1992).
Comparing the Medullosales, Corystospermales, Pentoxylales, and Cycadales, a conspicuous regression in the degree of expression of this character in Cycadales is observed. In fossil forms the quantity of centripetal xylem diminishes from Fascivarioxylon (Jurassic), to Worsdellia (Cretaceous), to Menucoa (Tertiary), to extant forms, where it becomes relictual.
The presence of medullary vascular bundles that are not associated with cone domes has been observed in Lepidozamia, Encephalartos, and Macrozamia (Worsdell, 1896; Greguss, 1968; Stevenson, 1990), though not all species of these genera have been studied anatomically. In fossils, it appears in the Medullosales, Corystospermales, and Cycadales.
In Permian Medullosans two vascular systems exist, the cylindrical one and the medullary one. According to Worsdell (1906), both are variants of one plan, and the cylindrical system is the most primitive one, with the medullary one being derived. This hypothesis seems to find support in the stems and rhizomes of corystosperms (Zamuner, 1992). In this sense, the organization and secondary structure of the wood in Rhexoxylon develops from the simultaneous action of three cambia: normal, supernumerary, and remnant. The supernumerary cambium produces centripetal xylem in the internal zone of the xylem wedges and gives rise to bundles and centripetal polyxyly. The remnant cambium gives rise to the formation of abundant parenchyma, resulting in the fragmentation of perimedullary bundles. With respect to their origin, the cycles of perimedullary bundles in Rhexoxylon can be compared with the system of medullary bundles of Medullosales and Cycadales. The major difference is that, in the trunks of corystosperms, this system follows a regular pattern, which produces--depending on the species--from one to several cycles of vascular bundles to the center of the pith. This pattern of growth, unique among gymnosperms, can be defined as centripetal polyxyly. In contrast, there is no regularity to the production of medullary bundles in either the Medullosales or the Cycadales.
In Cycadales, the medullary bundle character has not been sufficiently evaluated. In fossil forms it appears in Charmorgia, Fascivarioxylon, Menucoa, and Worsdellia. In the latter, the presence of centripetal bundles in the internal section of the first vascular ring is observed. These bundles give rise to medullary bundles that divide because of the action of remnant parenchyma, as in Rhexoxylon. In this case, medullary bundles originate from the main vascular system by a supernumerary cambium, and the anastomosis or divisions of the medullary bundles are related to a remnant cambium.
A cone dome (Chamberlain, 1911; Stevenson, 1988, 1990; Norstog & Nicholls, 1997) is a normal, though diminutive, vascular ring that is internal to the main vascular system within the pith. Cone domes are the vascular supply to the cones that are borne terminally on the main stem and become displaced laterally through time. A series of cone domes can be observed in longitudinal sections of trunks or in serial transverse sections of trunks. In longitudinal sections a convex diaphragm of vascular bundles originating from the main vascular system and running up to the strobilar peduncle is observed. Thus, unlike the free, random medullary bundles described above, cone domes have a regular, circular arrangement of bundles that appear as a diminutive stele within the pith. When multiple cones are produced in a growing season, several cones are observed in any given transverse section. Extant genera with cones are Bowenia, Ceratozamia, Chigua, Cycas, Dioon, Microcycas, Stangeria, and Zamia. Those extant genera lacking cone domes, Encephalartos, Lepidozamia, and Macrozamia, form the clade Encephalarteae (Stevenson, 1992)
In fossil cycad taxa, cone domes are present in Brunoa and Bororoa. In Brunoa, concentric cycles of bundles of different diameters in the same cross section are observed, as if it were a plant with multiple strobili. In Menucoa and Bororoa there are successive vascular diaphragms in longitudinal section, suggesting the existence of terminal solitary strobili.
Extrafascicular vascular bundles are concentric or anphiphloic cortical bundles in the stems of some extant Cycadales (Stevenson, 1990), such as Cycas, Bowenia, Encephalartos, Macrozamia, and Lepidozamia (Worsdell, 1901), and in seedlings of Ceratozamia (Dorety, 1909). When this character appears, the stem has a central stele surrounded by small peripheral vascular cylinders. The resulting anatomy is similar to that of Medullosales (Worsdell, 1901). According to Stevenson (1990), polyxyly and the presence of extrafascicular vascular bundles are not causally related characters, nor are extrafascicular bundles related to the presence of cone domes. The only fossil taxon known to have extrafascicular vascular bundles is Worsdellia.
Even though polyxyly is not a generalized character in the Cycadales, it does itself permit a systematic assignation at the familial level because of its occurrence in Cycadaceae (Cycas) and Zamiaceae (Encephalartos, Macrozamia, and Lepidozamia), but not in the Stangeriaceae. However, when found in association with true medullary bundles--a synapomorphy of some Encephalartoideae; namely, the Encephalarteae (Encephalartos, Macrozamia and Lepidozamia)--then Worsdellia may be assigned to this subfamily. Brunoa also is included in the Encephalartoideae because its dense secondary xylem resembles genera like Michelilloa, Lyssoxylon, and the extant Dioon. Moreover, Dioon is the only genus in Encephalartoideae with cone domes like Brunoa.
Thus, the fossil Zamiaceae-Encephalartoideae would include Brunoa, Worsdellia, Bororoa, and Menucoa. These genera show some primitive characters that appear in related groups, such as the pteridosperms. These characters include mucilage cavities, centripetal xylem, anastomosing medullary bundles, and concentric extraxylary bundles. In extant forms these characters have become relictual.
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ANALIA E. ARTABE, (1) ALBA B. ZAMUNER, (1) AND DENNIS WM. STEVENSON (2)
(1) Division Paleobotanica Facultad de Ciencias Naturales y Museo de La Plata 1900 La Plata, Argentina
(2) New York Botanical Garden, Bronx, NY 10458, U.S.A.
Table I Comparison among the different genera of fossil Cycadales Columns: 1 = Age (Tr = Triassic; J = Jurassic; K = Cretaceous; Tc = Tertiary); 2 = Ratio of pith/vascular tissue; 3 = Medullary vascular bundles; 4 = Presence of sclerotic nests (sn) or sclerotic cells (sc); 5 = Intervascular pitting (1 = araucaroid uniseriate to triseriate; 2 = araucaroid multiseriate (tetraseriate to polyseriate); 6 = Presence of cone domes; 7 = Mucilage canals; 8 = Mucilage cavities; 9 = Polyxyly (p), monoxyly (m); 10 = Centripetal xylem; 11 = Dense secondary xylem; 12 = Abundant parenchyma in secondary xylem; 13 = Extraxylary bundles; 14 = Girdling leaf traces. Genus 1 2 3 4 5 6 7 8 9 10 Michelilloa Tr 4 -- -- 2 -- x -- m -- Charmorgia Tr 8 x -- 1 -- x -- m -- Lyssoxylon Tr 2 - sc? 1 -- x -- m -- Antarcticycas Tr 12 -- -- 2 -- x -- m -- Undescribed genus Tr x? x -- m Fascivarioxylon J 5 x sn 1 -- - x m x Sanchucycas K ? -- -- I -- - x p -- Brunoa K I -- -- 1 x - x p -- Worsdellia K 12 x -- ? -- x -- p x Centricycas K ? x sc? 1 x -- m? ? Bororoa Tc 4 -- sc 2 x x -- p -- Menucoa Tc 5 x -- 2 -- x -- p x Genus 11 12 13 14 Michelilloa x -- -- Charmorgia -- x -- x Lyssoxylon x -- -- x Antarcticycas -- x -- x Undescribed genus ? Fascivarioxylon -- x -- x Sanchucycas x -- -- ? Brunoa x -- -- x Worsdellia -- x x x Centricycas ? Bororoa x -- x Menucoa -- x -- x
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|Author:||Artabe, Analia E.; Zamuner, Alba B.; Stevenson, Dennis Wm.|
|Publication:||The Botanical Review|
|Date:||Apr 1, 2004|
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