A new species of Carlquistoxylon from the Early Cretaceous of Patagonia (Chubut province, Argentina): the oldest record of angiosperm wood from South America

In: IAWA Journal

ABSTRACT

An angiospermous wood from the Lower Cretaceous (upper Albian) of the Cerro Barcino Formation, Chubut Group, central Patagonia, Argentina, is described. Its estimated minimum diameter is 40 cm and it is significant as the oldest known angiosperm wood for South America.

It has indistinct growth ring boundaries, vessels solitary and in radial multiples, simple perforation plates, alternate intervessel pits, vessel-ray parenchyma pits oval to horizontally elongated, heterocellular rays, non-septate fibres, axial parenchyma absent, and abundant tyloses. Because this Albian wood has non-septate fibres we assign it to Carlquistoxylon, even though it has a general combination of characters similar to that of Paraphyllanthoxylon, which has septate fibres. The number of vessels per radial multiple, vessel tangential diameter and frequency, vessel-ray parenchyma pitting, and absence of axial parenchyma distinguish the fossil described here from the only previously known species of Carlquistoxylon: Carlquistoxylon nacimientense; therefore, a new species is erected. Because of the close similarities between this new specimen and Paraphyllanthoxylon species, comparisons with all the species included in both genera are provided. Systematic affinities for this wood are discussed considering previous discussions for both Paraphyllanthoxylon and Carlquistoxylon affinities. As the oldest described angiosperm wood in South America to date, this specimen provides critical information on the diversity and growth habit of Cretaceous angiosperms from the Southern Hemisphere.

ABSTRACT

An angiospermous wood from the Lower Cretaceous (upper Albian) of the Cerro Barcino Formation, Chubut Group, central Patagonia, Argentina, is described. Its estimated minimum diameter is 40 cm and it is significant as the oldest known angiosperm wood for South America.

It has indistinct growth ring boundaries, vessels solitary and in radial multiples, simple perforation plates, alternate intervessel pits, vessel-ray parenchyma pits oval to horizontally elongated, heterocellular rays, non-septate fibres, axial parenchyma absent, and abundant tyloses. Because this Albian wood has non-septate fibres we assign it to Carlquistoxylon, even though it has a general combination of characters similar to that of Paraphyllanthoxylon, which has septate fibres. The number of vessels per radial multiple, vessel tangential diameter and frequency, vessel-ray parenchyma pitting, and absence of axial parenchyma distinguish the fossil described here from the only previously known species of Carlquistoxylon: Carlquistoxylon nacimientense; therefore, a new species is erected. Because of the close similarities between this new specimen and Paraphyllanthoxylon species, comparisons with all the species included in both genera are provided. Systematic affinities for this wood are discussed considering previous discussions for both Paraphyllanthoxylon and Carlquistoxylon affinities. As the oldest described angiosperm wood in South America to date, this specimen provides critical information on the diversity and growth habit of Cretaceous angiosperms from the Southern Hemisphere.

INTRODUCTION

Flowering plants are the most diverse and abundant group of seed plants. Their fossil record provides direct evidence of their rapid diversification during the middle part of the Cretaceous in both Northern and Southern hemispheres, resulting in their extraordinary diversity today (see Simpson 2006; Taylor et al. 2009; Herendeen et al. 2017 and citations therein). However, understanding the events related to this radiation is still a difficult task in a global context, therefore, the finding of new mid-Cretaceous fossil localities is crucial for broadening our knowledge about the early evolution of the angiosperms and how they developed into the dominant group in most of the tropical and temperate forests worldwide (see Taylor & Hickey 1996; Doyle & Endress 2000; Archangelsky et al. 2009; Herendeen et al. 2017 and citations therein).

Although the record of Cretaceous angiosperms in the Southern Hemisphere is sparse compared to the Northern Hemisphere, what is known for the southernmost part of South America indicates that paleofloristic patterns associated with the radiation of flowering plants were similar to those reported for the Northern Hemisphere (see Archangelsky et al. 2009 and citations therein). This conclusion is based on fossil pollen, compressions, and impressions. Fossil angiosperm woods in South America are scarce until the Cenozoic (Archangelsky et al. 2009; InsideWood 2004-onwards; Wheeler 2011; Gregory et al. 2008); and only a few angiosperm fossil woods have been described from its Late Cretaceous (i.e., Milanez 1935; Torres & Rallo 1981; Nishida & Nishida 1987; Mourier et al. 1988; Nishida et al. 1990; Franco et al. 2015; Egerton et al. 2016). In particular, the fossil record of angiosperm woods in central Patagonia is restricted to a few Cenozoic and Late Cretaceous occurrences (i.e., Romero 1970; Petriella 1972; Ragonese 1980; Brea 1998; Brea & Zucol 2006; Raigemborn et al. 2009).

Recently, an abundant megaflora associated with sauropod remains has been discovered in Lower Cretaceous deposits of the Chubut Group (see Carballido et al. 2017). These sediments belong to the Cerro Castaño Member of the Cerro Barcino Formation and dating of associated tuffs indicates they are upper Albian (101.6 ± 0.11 Ma; Carballido et al. 2017). The flora consists of impressions and compressions of fern pinnules, conifer leaves, angiosperm leaves and flowers, and silicified woods (Nunes et al. 2015). Among the impressions and compressions, the angiosperm remains are the most abundant. In contrast, gymnospermous wood remains dominate the fossil wood assemblage, with only one specimen being an angiosperm.

MATERIALS AND METHODS

The specimen herein described was collected at the La Flecha ranch, from deposits of the Chubut Group, in a site referred as “La Flecha Pb 1”, Chubut Province, central Patagonia (Fig. 1). The Chubut group comprises the lower Los Adobes and the upper Cerro Barcino formations, both composed mainly of fluvial and volcanoclastic sediments (Rauhut et al. 2003). The Cerro Barcino Formation is divided into four members, from bottom to top: La Paloma, Cerro Castaño, Las Plumas, and Puesto Manuel Arce. (Marveggio & Llorens 2013; Figari et al. 2015; Carballido et al. 2017). The sample described herein was found at the Cerro Castaño member of the Cerro Barcino Formation. The Cerro Castaño Member was dated at 101.6 ± 0.11 Ma (upper Albian, Lower Cretaceous) at the sauropod excavation (Carballido et al. 2017) from an ash layer a few metres below the plant horizon. The fossiliferous deposits are siltstones and fine to medium-grained sandstones that represent sandy floodplains and meandering channels (Carmona et al. 2017), rich in plant and dinosaur remains, including a giant sauropod (Carballido et al. 2017). Plant impressions and compressions were collected from the La Flecha ranch, at two stratigraphically equivalent sites known as “La Flecha Pb 1” (FLPb 1) and “La Flecha Pb 2” (FLPb 2). Most of the permineralized logs, which are largely dominated by gymnosperms, are dispersed on a conglomerate that lies on top of the clay and sandstone layers. Angiosperm diversity at FLPb 1 and FLPb 2 comprises six leaf morphotypes, well preserved pentamerous flowers, and a small permineralized fragment of wood found among the gymnospermous woods (Nunes et al. 2015).

Figure 1
Figure 1

Location map of the La Flecha ranch (star), Chubut province, Argentina. References of previously described South America Cretaceous angiosperm woods localities: 1. Mourier et al. 1988. – 2. Torres & Rallo 1981. – 3. Nishida & Nishida 1987. – 4. Nishida et al. 1990. – 5. Egerton et al. 2016. – 6. Franco et al. 2015. – 7. Milanez 1935.

Citation: IAWA Journal 39, 4 (2018) ; 10.1163/22941932-20170206

The permineralized angiosperm wood specimen is a decorticated fragment, of 4 × 5.5 cm approximately (Fig. 213). Transverse, tangential longitudinal, and radial longitudinal thin sections were prepared following standard methodology (Archangelsky 1962; Jones & Rowe 1999). Slides were observed using transmitted light and epifluorescence microscopy, and small fragments of the wood were observed with a scanning electron microscope as well. All images were taken with a Nikon DS-Fi1 camera attached to a Nikon Eclipse 80i microscope. In addition, general views of the transverse sections were taken using a Canon 7D camera with a Canon macro lens of 60 mm, magnified with 50 mm extension tubes, in a light box with fluorescent tubes. We used an image-stacking technique to obtain greater depth of focus for high magnification images (Bercovici et al. 2009). Helicon Focus software (http://www.heliconsoft .com/heliconfocus.html) was used, with the “Method B” (Depth Map) parameter, and the resulting image was carefully checked for the presence of artifacts. Several smaller, partially overlapping images were merged to create high-quality images of critical features. This technique was applied both manually and using the Adobe Photoshop CS5 Photomerge macro. A minimum of 25 measurements or observations of each character were obtained. Measurements are expressed as the mean followed by the range between brackets. The terminology of IAWA Committee (1989) was followed for describing the wood anatomy.

The macro-specimen and its sections are housed at the Paleobotanical Collection of the Museo Paleontológico Egidio Feruglio, Trelew, Chubut, Argentina, under accession number MPEF-Pb 7018.

SYSTEMATIC PALEONTOLOGY

Genus: Carlquistoxylon Wheeler, McClammer & LaPasha

Type species: Carlquistoxylon nacimientense Wheeler, McClammer & LaPasha 1995

Remarks

The fossil here described shows most characters present in Carlquistoxylon. In the original diagnosis of the fossil-genus the value ranges of continuous features are highly restricted. Minor differences in continuous traits are not enough to justify the creation of a new fossil-genus; therefore, the diagnosis was expanded for those features. Bold letters indicate the amended characters.

CARLQUISTOXYLON Wheeler, McClammer & LaPasha emend. Nunes, Pujana, Escapa, Gandolfo, Cúneo

Amended generic diagnosis

Wood diffuse-porous. Vessels solitary and in radial multiples normally of 2–3, and up to 8; mean tangential diameter 50–150 μm; generally, fewer than 40 vessels per mm 2 , sometimes up to 80 per mm 2; perforation plates exclusively simple, vessel element length between 500 and 800 μm; intervessel pits crowded alternate; vessel-ray parenchyma pits with reduced borders; axial parenchyma absent or rare, if present, scanty paratracheal; non-septate fibres; pits not obvious; rays 4 or fewer cells wide; uniseriate rays rare.

CARLQUISTOXYLON AUSTRALE Nunes, Pujana, Escapa, Gandolfo & Cúneo, sp. nov.Fig. 213.

Specific diagnosis

Growth rings boundaries indistinct. Vessels solitary and in radial multiples of 2–4, sometimes more; tangential diameter usually smaller than 100 μm; normally 30–80 vessels per mm2. Vessel-ray parenchyma pits opposite to scalariform, when opposite oval to horizontally elongated. Axial parenchyma absent. Rays 1–4 cells wide (uniseriate rays very rare), heterocellular, with procumbent and square cells mixed through the ray, upright cells occasionally present at the ray margin and throughout the ray.

Figure 2–7
Figure 2–7

Carlquistoxylon australe sp. nov. Holotype: MPEF-Pb 7018. – 2: Macroscopic view of transverse section (TS) showing indistinct to absent growth ring boundaries and diffuse porosity. – 3: Vessel arrangement, TS. – 4: Radial longitudinal section (RLS) showing a simple perforation plate. – 5: Tangential longitudinal section (TLS) showing intervessel pits arrangement and morphology. – 6 & 7: Vessel-ray parenchyma pitting, scalariform to opposite, RLS — Scale bar for 3 = 50 μm, for 4–6 = 10 μm, for 7 = 25 μm.

Citation: IAWA Journal 39, 4 (2018) ; 10.1163/22941932-20170206

Figure 8–13
Figure 8–13

Carlquistoxylon australe sp. nov. Holotype: MPEF-Pb 7018. – 8: Developing tyloses, TS. – 9: Fibre details, TLS – 10: Rays, TLS. – 11: Rays and a simple perforation plate (arrowhead), TLS. – 12 & 13: Heterocellular rays, RLS. — Scale bar for 8 & 13 = 30 μm, for 9 = 20 μm, for 10 & 12 = 150 μm, for 11 = 100 μm.

Citation: IAWA Journal 39, 4 (2018) ; 10.1163/22941932-20170206

Stratigraphic horizon: Cerro Castaño Member, Cerro Barcino Formation, Chubut Group.

Age: late Albian (Early Cretaceous).

Type locality: “La Flecha Pb 1”, La Flecha ranch, Chubut province, Argentina.

Holotype: MPEF-Pb 7018.

Etymology: the specific epithet australe refers to the Southern Hemisphere provenance of the fossil.

Description

The specimen is a fragment of 4 × 5.5 cm (minimum estimated diameter based on the curvature of the growth rings is 40 cm). Growth ring boundaries indistinct to absent, hardly observed macroscopically, slightly curved (Fig. 2).

Wood is diffuse-porous. Vessels are solitary (21%), and in radial multiples of 2 (36 %), 3 (33%) or 4–8 (10%), average 49 (29–76) per mm2. Vessels have a tangential diameter of 62 (31–100) μm and are circular to oval in outline in transverse section, sometimes radially flattened when in radial multiples (Fig. 3). Perforation plates are exclusively simple and oblique (Fig. 4, 11). Intervessel pits are crowded, oval, sometimes polygonal, alternate, medium, 7.95 (5.5–11.5) μm in horizontal diameter (Fig. 5). Vessel-ray parenchyma pitting is scalariform to opposite, with narrow borders to apparently simple, circular to horizontal (gash-like) (Fig. 6, 7). Tyloses are common (Fig. 8).

Fibres are non-septate and thin-walled, pits not observed on either tangential or radial walls (Fig. 9).

Axial parenchyma is absent.

Rays are up to 4 cells wide, mostly 2–3 cells wide, and 110–860 μm high (generally 350–380 μm), uniseriate rays very rarely observed (Fig. 10, 11). Rays are heterocellular, with procumbent, square and upright cells mixed through the ray, occasionally with one or two marginal rows of upright cells (Fig. 12, 13). Ray frequency is 14 (11–18) rays per mm.

Comparisons with fossil woods – Genera (Table 1)

Fossil woods assigned to Paraphyllanthoxylon Bailey 1924 are characterized by diffuse porosity, vessels solitary and in radial multiples, simple perforation plates, alternate intervessel pits, vessel-ray pits circular to horizontal, heterocellular rays, axial parenchyma rare or absent and, if present, scanty paratracheal, and abundant tyloses (e.g., Bailey 1924; Mädel 1962; Thayn & Tidwell 1984; Herendeen 1991). This fossil-genus was diagnosed by Bailey (1924) when he described the type species Paraphyllanthoxylon arizonense Bailey 1924, from the Upper Cretaceous (Cenomanian) of Arizona, USA. The presence of septate fibres was mentioned as a distinctive feature in the original description; however, this feature is absent in some species subsequently assigned to the genus (e.g., P. yvardi Koeniguer 1970; P. obiraense Takahashi & Suzuki 2003). Afterwards, Wheeler et al. (1995) proposed using the name Carlquistoxylon for fossil woods sharing the basic combination of features present in Paraphyllanthoxylon, but lacking septate fibres. The Patagonian specimen lacks septate fibres, therefore, it is assigned to Carlquistoxylon. Nevertheless, we consider it important to compare it with Paraphyllanthoxylon.

The general combination of characters mentioned above partially overlaps the diagnoses of additional Mesozoic and Cenozoic wood fossil-genera (see Thayn & Tidwell 1984; Wheeler et al. 1995; Gryc et al. 2009 and citations therein). Differences between Paraphyllanthoxylon, Carlquistoxylon and other genera are often subtle (Table 1). They are discussed below by the orders they were assigned.

Bridelioxylon, Putranjivoxylon — Bridelioxylon Ramanujam 1956, widely distributed in Cretaceous and Tertiary sediments of Europe, Africa, Central South Asia, Oceania, and South America (Ramanujam 1956; Mädel 1962; Koeniguer 1967; Petriella 1972; Bamford & McLoughlin 2000) and Putranjivoxylon Ramanujam 1956, from the Tertiary of India (Ramanujam 1956), are similar to Carlquistoxylon australe in having vessels in radial multiples, presence of simple perforation plates, heterocellular rays, and alternate intervessel pits. However, Bridelioxylon differs from Carlquistoxylon in showing distinct growth rings, septate fibres, rays weakly heterocellular, parenchyma paratracheal scanty or diffuse, and absence of tyloses (Ramanujam 1956). Putranjivoxylon also lacks tyloses and has both simple and scalariform perforation plates, rays markedly heterocellular and abundant apotracheal parenchyma (Ramanujam 1956).

Burseroxylon — Burseroxylon Prakash & Tripathi 1975, from the Cretaceous and Tertiary of India and Africa (Prakash & Tripathi 1975; Bande & Prakash 1983; Bamford 2003), shares with the Patagonian specimen the presence of indistinct growth rings, diffuse porosity, small to medium vessel diameters, simple perforation plates, alternate intervessel pitting, heterocellular rays, and presence of tyloses. However, Burseroxylon has large intervessel pits and scanty to vasicentric axial parenchyma (Prakash & Tripathi 1975).

Elaeocarpoxylon — Elaeocarpoxylon Prakash & Dayal 1964 found in the Cretaceous and Tertiary of India (Prakash & Dayal 1964; Prakash & Tripathi 1975) and South America (Petriella 1972; Nishida et al. 1988) also shares a high number of features with the specimen described here. However, it has septate fibres, large intervessel pits, and differences in ray structure.

Aplectotremas — Aplectotremas Serlin 1982, from the Early Cretaceous of North America (Serlin 1982), has poor preservation and lacks a detailed description. It shares with our specimen a similar wood pattern (diffuse porosity, short radial multiples, simple perforation plates, heterocellular rays 1–4 cells wide). However, it differs from Carlquistoxylon australe in its large vessels and its vasicentric paratracheal axial parenchyma.

Lauraceae – The specimen described here is similar to the lauraceous genera Laurinoxylon Felix emend. Dupéron, Dupéron-Laudoueneix, Sakala & De Franceschi 2008, and Beilschmiedioxylon Dupéron-Laudoueneix & Dupéron 2005. The shared features include the arrangement and size of vessels, presence of simple perforation plates, alternate intervessel pitting, and ray width and composition. However, Carlquistoxylon lacks oil cells, a distinctive feature of the aforementioned genera.

Comparisons with fossil woods – Species (Table 2)

Until now, only one species of Carlquistoxylon has been described: Carlquistoxylon nacimientense Wheeler, McClammer & LaPasha, and it differs from Carlquistoxylon australe mostly in quantitative features. Carlquistoxylon nacimientense has wider vessels and fewer vessels per mm2; the radial multiples are shorter and the intervessel pits are larger than those observed in Carlquistoxylon australe. Another difference is the lack of axial parenchyma in C. australe, while C. nacimientense has scanty paratracheal parenchyma. On the base of these anatomical differences, the erection of a new species is justified.

Table 1

Comparison of Carlquistoxylon with similar wood fossil-genera.

Bold letters indicate characters shared with Carlquistoxylon. GR = growth rings, A = absent, I = indistinct, D = distinct — Po. = porosity, D = diffuse — VA = vessel arrangement, S = solitary, M = radial multiples, C = clusters — VTD = vessel diameter, S = small, M = medium, L = large — PP = perforation plates, Si. = simple, Sc. = scalariform — IP = intervessel pits, S = small, M = medium, L = large, VL = very large — VRP = vesselray parenchyma pits — RWC = ray width and composition — Ty. = tyloses, A = absent, C = common — Fi. = fibres, S = septate, N = non-septate — DS = distinctive structures — Age, K = Cretaceous, Cz = Cenozoic — Dis. = distribution — ? = missing information.

Table 1Table 1
Table 2

Comparisons among Paraphyllanthoxylon and Carlquistoxylon species.

Bold letters indicate characters shared with Carlquistoxylon australe. GR = growth rings, A = absent, I = indistinct — VF = vessels per mm2 — VA = vessel arrangement, S = solitary, M = radial multiples, C = clusters, Rar. = rarely, Occ. = occasionally — VTD = vessel diameter — RW = ray width — UR = uniseriate rays, A = absent, C = common, R = rare — RF = ray frequency — MURC = margin of up-right cells — IPS = intervessel pit size, S = small, M = medium, L = large — VRP = vessel-ray parenchyma pits — Fi. = Fibres, S = septate, N = non-septate, rar. = rarely — AP = axial parenchyma — AL = Age and Location — Obs. /Ref. = Observations/References — ? = missing information.

Table 2Table 2Table 2

Although this wood is assigned to Carlquistoxylon, it also resembles several species assigned to Paraphyllanthoxylon that, as mentioned earlier, differs from Carlquistoxylon mainly in the absence/presence of septa in the fibres. Given the similarities shared by Paraphyllanthoxylon and Carlquistoxylon, comparisons among the species of both genera are informative (Table 2). Species assigned to Paraphyllanthoxylon that lack septate fibres are P. yvardi Koeniguer 1967 and P. obiraense Takahashi & Suzuki 2003. Paraphyllanthoxylon obiraense differs from C. australe in vessel diameter and frequency, intervessel pits size, and presence of axial parenchyma. Paraphyllanthoxylon yvardi is more similar to C. australe in vessel arrangement, diameter and frequency, and intervessel pit size (Koeniguer 1970). According to the diagnoses of Carlquistoxylon and Paraphyllanthoxylon, these two fossil-species, Paraphyllanthoxylon obiraense and Paraphyllanthoxylon yvardi, should not be placed in Paraphyllanthoxylon, and could be transferred to Carlquistoxylon. However, P. yvardi has rays up to seven cells wide and P. obiraense has moderately abundant axial parenchyma and they do not conform with the generic diagnosis of Carlquistoxylon. Consequently, we suggest that these fossil-species should be revised and a transference to Carlquistoxylon should be considered.

We noticed that most similarities are shared especially with the species grouped in Herendeen’s (1991) Paraphyllanthoxylon Anatomical Group A. This group is characterized by having a few marginal rows of upright cells on the multiseriate rays and few or rare uniseriate rays (Herendeen 1991; Martínez-Cabrera et al. 2006; Gryc et al. 2009). In C. australe, marginal rows of upright cells in the multiseriate rays are occasional, and the square to upright cells are mostly mixed with the procumbent cells through the ray body, which are consistent with all species from Anatomical Group A. Of the Paraphyllanthoxylon species included in Group A, the Patagonian specimen is similar to the Cretaceous North American species P. anazasii Wheeler, McClammer & LaPasha 1995 and P. marylandense Herendeen 1991 in vessel diameters and frequency, and lack of axial parenchyma, but these species have septate fibres; also axial parenchyma can be rarely observed in P. anazasii (Table 2).

DISCUSSION

The new Patagonian specimen is characterized by a combination of characters that occurs in some woods assigned to Paraphyllanthoxylon. However, given the absence of septate fibres, this wood is assigned to Carlquistoxylon. Due to the anatomical similarity of these genera, the specimen was compared with all Paraphyllanthoxylon and Carlquistoxylon species. Based on these comparisons we conclude that it represents a new species of Carlquistoxylon.

Systematic affinities

The combination of anatomical features present in Paraphyllanthoxylon and Carlquistoxylon has been referred to as the phyllanthoid xylotype, one of the earliest patterns known for angiosperm woods (Thayn & Tidwell 1984; Wheeler & Baas 1991; Wheeler et al. 1995; Oakley & Falcon-Lang 2009; Jud et al. 2017). The anatomical pattern of Paraphyllanthoxylon occurs in more than one order and in multiple families: Laurales – Lauraceae; Malpighiales – Euphorbiaceae, Salicaceae; Rosales – Cannabaceae; Oxalidales – Elaeocarpaceae; Sapindales – Burseraceae, Anacardiaceae; and Lamiales – Verbenaceae (Mädel 1962; Thayn & Tidwell 1984; Wheeler et al. 1987; Herendeen 1991; Wheeler et al. 1995; Martínez-Cabrera et al. 2006; Gryc et al. 2009; APG IV 2016), indicating the genus does not represent a natural group. Similarly, due to the anatomical similarities with Paraphyllanthoxylon, Carlquistoxylon nacimientense was compared to some of those families. Wheeler et al. (1995) noticed that the general pattern of this fossil-genus occurs in families from two separate clades, equivalent to Cronquist (1981) subclasses, Magnoliidae and Rosidae. These authors also suggested that Carlquistoxylon could be related to the Lauraceae (Magnoliidae) or to the Anacardiaceae, Burseraceae, Melastomataceae, and Euphorbiaceae (Rosidae).

Most comparisons of the so-called phyllanthoid fossil woods including Paraphyllanthoxylon and Carlquistoxylon, have been with the Lauraceae and Euphorbiaceae (Thayn & Tidwell 1984; Prakash et al. 1986; Wheeler et al. 1987; Herendeen 1991; Wheeler 1991; Wheeler et al. 1995; Martínez-Cabrera et al. 2006; Gryc et al. 2009). Carlquistoxylon nacimientense was compared with Euphorbiaceae because of its similarity to Euphorbioxylon Felix emend. Mädel 1962 (Mädel 1962; Wheeler et al. 1995). However, Wheeler et al. (1995) pointed out that Euphorbioxylon needs to be revised because it has been used for woods with different combinations of features, which occur in the large family Euphorbiaceae. Thus, they hesitated about the relationship between their specimen and this genus.

They noted that among all the species of Euphorbioxylon, Carlquistoxylon nacimientense is most similar to Euphorbioxylon saggarense Mahabale & Deshpande 1963, a species whose affinities with the Euphorbiaceae are questionable (Mahabale & Deshpande 1963). Wheeler et al. (1995) also compared C. nacimientense with Laurinoxylon (syn. Ulminium Felix emend. Dupéron, Dupéron-Laudoueneix, Sakala & De Franceschi 2008), a fossil-genus for Lauraceae woods, but were not sure about this possible affinity because C. nacimientense lacks idioblasts so could not be assigned to this genus (Wheeler et al. 1995). Even though oil cells are an extremely common feature of Lauraceae wood, they are not always present in extant genera (Metcalfe & Chalk 1950; Stern 1954). Paraphyllanthoxylon marylandense, a species that resembles Carlquistoxylon australe and is unambiguously related to Lauraceae due to its attachment to a lauraceous fruit, lacks idioblasts (Herendeen 1991). There is variation in the distinctiveness of oil cells in modern woods (Richter 1987). Paraphyllanthoxylon vancouverense from the Turonian of British Columbia, Canada, has some enlarged ray cells suggesting oil cells (Jud et al. 2017) and Campanian-Maastrichtian woods from Texas show a continuum from rays with well-defined oil cells to slightly enlarged cells (Jud et al. 2017). Jud et al. (2017) conclude that the idioblasts’ distinctiveness in Lauraceae is variable enough to support considering their fossil wood a Lauraceae.

As explained before, the main feature that separates Carlquistoxylon species from Paraphyllanthoxylon is the presence/absence of septa in the fibres. Septate fibres seem to be a common character in the woods of the Euphorbiaceae sensu lato, at least in the morphological Group B or Glochidion type of Metcalfe and Chalk (Metcalfe & Chalk 1950), which Paraphyllanthoxylon has been related to (Prakash et al. 1986). In the other morphological group defined by Metcalfe and Chalk (1950), Group A or Aporosa type, fibres are mostly non-septate, but it is difficult to relate our wood to them due to the lack of axial parenchyma in C. australe. In Lauraceae, the presence of septa in the fibres is variable among the genera and species, but axial parenchyma is constantly present, ranging from scanty paratracheal to abundant (Metcalfe & Chalk 1950; Stern 1954; Herendeen 1991); Paraphyllanthoxylon marylandense also lacks axial parenchyma (Herendeen 1991).

Herendeen (1991) focused his attention on the variation of ray structure among the Paraphyllanthoxylon species and suggested there are two distinct Paraphyllanthoxylon anatomical groups probably with different affinities: Anatomical Group A and Anatomical Group B. Anatomical Group A, characterized by a few short marginal rows of vertical upright cells in the multiseriate rays and few or rare uniseriate rays, has features seen in the Lauraceae, Elaeocarpaceae, Anacardiaceae, Burseraceae, and Verbenaceae. The Anatomical Group B, characterized by abundant uniseriate rays and long marginal rows of upright cells in the multiseriate rays, has features seen in the Euphorbiaceae, Cannabaceae, Salicaceae, and Simaroubaceae. Herendeen (1991) pointed out that, given the variation in ray structure and oil cell occurrence, it is possible that Paraphyllanthoxylon Group A species were produced by extinct members of Lauraceae, perhaps one widespread genus. Nonetheless, he was circumspect in pointing that it is still necessary to have information from reproductive structures to validate such woods’ familial relationships (Herendeen 1991).

Additional phenetic grouping analyses have included phyllanthoid woods; however, no definitive conclusions were reached in terms of internal relationships within the groups, and their results are similar to those of previous reports (e. g. Oakley & Falcon-Lang 2009; Méndez-Cárdenas et al. 2013).

Unfortunately, the systematic affinities of Carlquistoxylon and Paraphyllanthoxylon are unresolved. Future studies on the flowers preserved at the Cerro Barcino Formation and six (or more) angiosperm leaf morphotypes from the same strata where this wood was collected may help with suggesting affinities. However, given the absence of organic connections between flowers, leaves, and wood, it is difficult to establish an unambiguous reconstruction for this plant. As Jud et al. (2017) and Manchester and Tiffney (2001) have pointed out, for establishing affinities it is important to find repeated co-occurrences of different plant parts for whole plant reconstructions.

Biogeography implications

Carlquistoxylon australe represents the first mention of the genus in South America, but considering that this is a genus of relatively recent creation (Wheeler et al. 1995), it is not surprising that more species have not been described yet. However, Carlquistoxylon australe also represents the first mention in South America of a phyllanthoid xylotype wood. The other phyllanthoid wood reported for the Southern Hemisphere is Paraphyllanthoxylon capense Mädel 1962, from the Upper Cretaceous of South Africa. The anatomical pattern shared by Carlquistoxylon and Paraphyllanthoxylon is now also represented in another region of the Southern Hemisphere, indicating that these wood types were more widespread than previously known.

With a late Albian absolute age obtained for the La Flecha ranch, this fossil represents the oldest record of angiosperm wood for South America, and one of the oldest for the Southern Hemisphere. Previously described Cretaceous angiosperm woods from South America are of Upper Cretaceous age (Milanez 1935; Ragonese 1977; Torres & Rallo 1981; Nishida & Nishida 1987; Mourier et al. 1988; Nishida et al. 1990; Franco et al. 2015; Egerton et al. 2016). Thus, this finding increases our knowledge of the diversity of angiosperm organs in the Cretaceous of South America (see Archangelsky et al. 2009). In addition, the minimum estimated diameter for this specimen (at least 40 cm) suggests its habit was arboreal rather that shrubby. This adds useful information about the possible life habit that early flowering plants had in the Southern Hemisphere.

CONCLUSIONS

This is the first direct evidence of the presence of woody angiosperms in the Lower Cretaceous of South America and is important for documenting that by the middle part of the Cretaceous woody flowering plants were of global occurrence. Additionally, the estimated minimum diameter for this tree suggests an arboreal habit for this early flowering plant.

The generalized anatomical patterns found in Carlquistoxylon and Paraphyllanthoxylon will continue to generate debates about the lineages these woods represent. Their combination of characters is found in multiple unrelated families (e.g. Lauraceae, Euphorbiaceae, Anacardiaceae, and Burseraceae) and this suggests that their anatomical pattern may be homoplastic. However, it is also possible that some of these woods could represent basal nodes in angiosperm history.

The finding of new Cretaceous localities from different regions of the world, with repeated association of angiosperm woods and leaves is critical for determining the affinities of the so-called phyllanthoid fossil woods.

ACKNOWLEDGEMENTS

The authors thank the Editor and the reviewers for their comments, which helped to improve the manuscript. We also thank Drs. Jose Luis Carballido and Diego Pol, Technician Pablo Puerta and members of their team, Dr. Luis Miguel Sender, Dr. Alberto Garrido, Dr. Cecilia Apaldetti, Technician Ariel Aresti and B. Sc. Kevin Gomez for field trip assistance; Technician Mariano Caffa for assisting in sample preparation; Drs. Stanila Iamandei and Eugenia Iamandei, and B.Sc. Ana Andruchow Colombo and Andres Elgorriaga for bibliography access; Dr. Nathan Jud for valuable personal communication and bibliography access. This contribution was founded by ANPCyT grant PICT 2014-2433 (to NRC), NSF DEB 0918932 and DEB 1556136 (to MAG) and EVC-CIN grant EVC3-UNPSJB3805 (to CIN).

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*Corresponding author: e-mail: cnunes@mef.org.ar

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  • APG IV . 2016. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Bot. J. Linn. Soc. 181: 1–20, doi:.10.1111/boj.2016.181.issue-1

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    • Search Google Scholar
    • Export Citation
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    • Crossref
    • Search Google Scholar
    • Export Citation
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    • Crossref
    • Search Google Scholar
    • Export Citation
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    • Crossref
    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Export Citation
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    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Export Citation
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    • Search Google Scholar
    • Export Citation
  • Doyle JA , Endress PK . 2000. Morphological phylogenetic analysis of basal angiosperms: comparison and combination with molecular data. Intern. J. Plant Sci. 161 (S6): S121– S153, doi:.10.1086/317578

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    • Search Google Scholar
    • Export Citation
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    • Crossref
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  • View in gallery

    Location map of the La Flecha ranch (star), Chubut province, Argentina. References of previously described South America Cretaceous angiosperm woods localities: 1. Mourier et al. 1988. – 2. Torres & Rallo 1981. – 3. Nishida & Nishida 1987. – 4. Nishida et al. 1990. – 5. Egerton et al. 2016. – 6. Franco et al. 2015. – 7. Milanez 1935.

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    Carlquistoxylon australe sp. nov. Holotype: MPEF-Pb 7018. – 2: Macroscopic view of transverse section (TS) showing indistinct to absent growth ring boundaries and diffuse porosity. – 3: Vessel arrangement, TS. – 4: Radial longitudinal section (RLS) showing a simple perforation plate. – 5: Tangential longitudinal section (TLS) showing intervessel pits arrangement and morphology. – 6 & 7: Vessel-ray parenchyma pitting, scalariform to opposite, RLS — Scale bar for 3 = 50 μm, for 4–6 = 10 μm, for 7 = 25 μm.

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    Carlquistoxylon australe sp. nov. Holotype: MPEF-Pb 7018. – 8: Developing tyloses, TS. – 9: Fibre details, TLS – 10: Rays, TLS. – 11: Rays and a simple perforation plate (arrowhead), TLS. – 12 & 13: Heterocellular rays, RLS. — Scale bar for 8 & 13 = 30 μm, for 9 = 20 μm, for 10 & 12 = 150 μm, for 11 = 100 μm.

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