Research Article |
Corresponding author: Roberto R. Pujana ( rpujana@gmail.com ) Academic editor: Dennis Stevenson
© 2020 Roberto R. Pujana, Peter Wilf, Maria A. Gandolfo.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Pujana RR, Wilf P, Gandolfo MA (2020) Conifer wood assemblage dominated by Podocarpaceae, early Eocene of Laguna del Hunco, central Argentinean Patagonia. PhytoKeys 156: 81-102. https://doi.org/10.3897/phytokeys.156.54175
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During the early Eocene, Patagonia had highly diverse floras that are primarily known from compression and pollen fossils. Fossil wood studies from this epoch are scarce in the region and largely absent from the Laguna del Hunco flora, which has a highly diverse and excellently preserved compression assemblage. A collection of 26 conifer woods from the Laguna del Hunco fossil-lake beds (early Eocene, ca. 52 Ma) from central-western Patagonia was studied, of which 12 could be identified to genus. The dominant species is Phyllocladoxylon antarcticum, which has affinity with early-diverging Podocarpaceae such as Phyllocladus and Prumnnopitys. A single specimen of Protophyllocladoxylon francisiae probably represents an extinct group of Podocarpaceae. In addition, two taxonomic units of cf. Cupressinoxylon with putative affinity to Podocarpaceae were found. Diverse Podocarpaceae taxa consistent with the affinities of these woods were previously reported from vegetative and reproductive macrofossils as well as pollen grains from the same source unit. Some of the woods have galleries filled with frass. Distinct growth ring boundaries indicate seasonality, inferred to represent seasonal light availability. Growth ring widths suggest that the woods came from mature trees, whereas the widths and types of some rings denote near-uniform temperature and water availability conditions.
fossil forests, Huitrera Formation, Paleogene, Podocarpaceae, South America, wood anatomy
The early Eocene Earth had warm climates worldwide (e.g.,
The volcanic-lacustrine strata of the Tufolitas Laguna del Hunco of the Eocene Huitrera Formation exposed at Laguna del Hunco in northwestern Chubut, central Patagonia, have long been known for their diverse and superbly preserved plant fossils (e.g.,
Although fossil leaves, reproductive structures, and pollen are well documented, a comprehensive study of the woods from Laguna del Hunco is lacking.
During a recent field season, we collected a large sample of fossil woods from the Tufolitas Laguna del Hunco, including both angiosperms and conifers. In this contribution, we treat the conifer woods from Laguna del Hunco for the first time. This work comprises the largest study to date of the Laguna del Hunco flora from permineralized wood specimens, otherwise known almost entirely from compression and palynomorph remains.
Fossil-wood samples were collected 3–5 December 2016 from 10 localities in the exposures of the Tufolitas Laguna del Hunco, Huitrera Formation, at Laguna del Hunco (Fig.
Geographical coordinates of the localities where the fossils were collected.
Sampling locality | Geographical coordinates (DDM) | n° of conifer woods studied (identified to genus) |
---|---|---|
LU2 | 42°27.53'S, 70°02.26'W | 1 (1) |
LU4 | 42°27.51'S, 70°02.43'W | 2 (1) |
LU6 | 42°27.54'S; 70°02.48'W | 4 (2) |
LU8 | 42°27.68'S; 70°02.56'W | 1 (1) |
LU9 | 42°27.60'S; 70°02.52'W | 2 (0) |
LU12 | 42°27.67'S; 70°02.28'W | 1 (0) |
LU15 | 42°28.28'S; 70°02.92'W | 1 (1) |
LU16 | 42°28.23'S; 70°02.76'W | 6 (1) |
LU22 | 42°27.42'S; 70°02.09'W | 7 (4) |
LU23 | 42°27.63'S; 70°02.25'W | 1 (1) |
The 26 conifer fossil woods studied here (Table
All specimens are housed at the Museo Paleontológico Egidio Feruglio, Trelew, Chubut Province, Argentina, repository acronym MPEF-Pb (Table
Wood anatomy of studied conifer samples. Locality (LU); Seriation index (Si); Contiguity percentage (Cp) [%]; Mean vertical diameter of radial pits (VDRP) [μm]; Mean tracheid tangential diameter (TTD) [μm]; Mean pits per cross-field (PxCF); Mean vertical diameter of cross-field pits (VDCP) [μm]; Mean ray height (RH) [cells]; Mean rays per mm (R×M). * indicates fewer than 15 measurements.
MPEF-Pb | Taxonomic unit | LU | Si | Cp | VDRP | TTD | PxCF | VDCP | RH | RxM |
---|---|---|---|---|---|---|---|---|---|---|
10694 | Protophyllocladoxylon francisiae | 2 | 1.25 | 88.1 | 19.2 | 44.5 | 1.97 | 14.8 | 5.6 | 3.5 |
10697 | cf. Phyllocladoxylon | 4 | 1.00* | 5.6* | 15.6 | ? | 1.00* | 11.4* | ? | ? |
10700 | Indeterminate conifer | 4 | 1.00* | 7.1* | 14.6 | ? | ? | ? | ? | ? |
10707 | Phyllocladoxylon antarcticum | 6 | 1.00 | 12.5 | 19.3 | 33.1 | 1.07 | 10.8 | 10.1 | 8.2 |
10710 | Phyllocladoxylon antarcticum | 6 | 1.03 | 8.5 | 20.9 | 29.5 | 1.11 | 12.0 | 7.9 | 8.4 |
10724 | Indeterminate conifer | 6 | ? | ? | ? | ? | ? | ? | ? | ? |
10725 | Indeterminate conifer | 6 | ? | ? | ? | ? | ? | ? | ? | ? |
10733 | cf. Cupressinoxylon sp. 1 | 8 | 1.01 | 7.2 | 13.9 | 24.2 | 1.15 | 7.4 | 4.0 | 6.7 |
10736 | Indeterminate conifer | 9 | ? | ? | ? | ? | ? | ? | ? | ? |
10739 | Indeterminate conifer | 9 | ? | ? | ? | ? | ? | ? | ? | ? |
10742 | Indeterminate conifer | 12 | ? | ? | ? | ? | ? | ? | ? | ? |
10747 | Phyllocladoxylon antarcticum | 15 | 1.00 | 9.6 | 17.5 | 32.1 | 1.03 | 14.0 | 5.9 | 5.7 |
10748 | Indeterminate conifer | 16 | ? | ? | ? | ? | 1.00* | ? | ? | ? |
10749 | cf. Phyllocladoxylon | 16 | 1.00* | 9.5* | 16.8* | ? | 1.00* | 13.7* | 9.5 | 9.6* |
10750 | Indeterminate conifer | 16 | 1.00* | 12.5* | ? | ? | ? | ? | ? | ? |
10751 | Indeterminate conifer | 16 | ? | ? | ? | ? | 1,00* | ? | ? | ? |
10753 | Indeterminate conifer | 16 | ? | ? | ? | ? | ? | ? | ? | ? |
10754 | Indeterminate conifer | 16 | ? | ? | ? | 30.7 | ? | ? | 5.8 | 5.8* |
10765 | Phyllocladoxylon antarcticum | 22 | 1.00 | 13.4 | 16.4 | 31.7 | 1.00 | 13.6 | 5.6 | 4.0 |
10766 | Indeterminate conifer | 22 | ? | ? | ? | ? | ? | ? | ? | ? |
10767 | Phyllocladoxylon antarcticum | 22 | 1.17 | 23.1 | 19.1 | 42.1 | 1.00 | 14.6 | 11.6 | 5.1 |
10771 | Indeterminate conifer | 22 | ? | ? | ? | ? | ? | ? | ? | ? |
10773 | Phyllocladoxylon antarcticum | 22 | 1.00* | 5.3* | 17.1* | 31.2 | 1.00 | 12.9 | 6.2 | 8.1* |
10775 | Indeterminate conifer | 22 | 1.00* | 16.3* | 15.1 | ? | 1.00* | ? | ? | ? |
10776 | Phyllocladoxylon antarcticum | 22 | 1.00 | 10.6 | 17.5 | 32.5 | 1.06 | 13.2 | 9.9 | 6.3 |
10778 | cf. Cupressinoxylon sp. 2 | 23 | 1.06 | 6.3 | 18.4 | 32.7 | 1.09 | 11.7 | 5.9 | 6.6 |
We followed the terminology of the
For growth ring classification, we followed the scheme of
MPEF-Pb 10694.
LU2 at Laguna del Hunco (Fig.
Tufolitas Laguna del Hunco, Huitrera Formation (Ypresian, early Eocene).
Growth ring boundaries are distinct (Fig.
Protophyllocladoxylon francisiae, MPEF-Pb 10694: A Growth rings of type D (transverse section, TS) B detail of a growth ring of type D boundary (TS) C opposite (arrowheads) intertracheary radial pits (radial longitudinal section, RLS) D alternate (arrowheads) intertracheary radial pits (RLS) E and F cross-fields (RLS) G wall alteration of the secondary walls of tracheids (tangential longitudinal section, TLS) H uniseriate rays (TLS) I uniseriate rays (TLS). Scale bars: 5 mm (A); 500 μm (B); 50 μm (C, D, E, F, G); 100 μm (H); 200 μm (I).
This specimen is characterized by its distinct growth ring boundaries, uni- to biseriate mixed intertracheary radial pitting, cross-fields usually with one or two mostly simple pits, relatively wide tracheids, uniseriate rays, and absence of resin-plugs and axial parenchyma. These characters indicate that this wood belongs to the fossil-genus Protophyllocladoxylon, because of the mixed radial pitting, simple large pits in the cross-fields, uniseriate rays, and smooth ray cell walls (
Protophyllocladoxylon francisiae was first described by
The fossil-genus Protophyllocladoxylon is quite controversial.
MPEF-Pb 10707, 10710, 10747, 10765, 10767, 10773 and 10776.
LU6, LU15 and LU22 at Laguna del Hunco (Fig.
Tufolitas Laguna del Hunco, Huitrera Formation (Ypresian, early Eocene).
Growth ring boundaries are distinct (Fig.
Phyllocladoxylon antarcticum: A Growth rings of type D (TS), MPEF-Pb 10747 B detail of a growth ring of type D boundary (TS), MPEF-Pb 10776 C detail of roundish tracheids (TS), MPEF-Pb 10765 D opposite contiguous biseriate intertracheary radial pits (arrowheads) (RLS), MPEF-Pb 10767 E uniseriate non contiguous intertracheary radial pits (scanning electron microscope, SEM), MPEF-Pb 10776 F uniseriate contiguous (arrowheads) and non contiguous intertracheary radial pits (SEM), MPEF-Pb 10776 G cross-fields with large simple pits (RLS), MPEF-Pb 10707 H cross-fields with large pointed and narrow-bordered pits (RLS), MPEF-Pb 10765 I cross-fields with large simple pits (SEM), MPEF-Pb 10710 J wall alteration of the secondary walls of tracheids (RLS), MPEF-Pb 10767 K uniseriate rays (TLS), MPEF-Pb 10767 L uniseriate rays (TLS), MPEF-Pb 10747. Scale bars: 5 mm (A); 200 μm (B, K); 100 μm (C, L); 50 μm (D, F, G, H, I, J); 20 μm (E).
Abietinean intertracheary radial pitting and cross-fields with usually one large simple pit (
Specimen MPEF-Pb 10767 frequently has biseriate opposite pits (Fig.
Phyllocladoxylon antarcticum is the most common species in our sample of conifer woods from Laguna del Hunco. In Patagonia, it was previously recorded in the Cretaceous (
Recently, a compressed branch bearing phylloclades from Laguna del Hunco was assigned to the newly described fossil-genus Huncocladus Andruchow-Colombo et al., a stem relative of Phyllocladus (
MPEF-Pb 10733.
LU8 at Laguna del Hunco (Fig.
Tufolitas Laguna del Hunco, Huitrera Formation (Ypresian, early Eocene).
Growth ring boundaries are distinct (Fig.
cf. Cupressinoxylon sp. 1, MPEF-Pb 10733: A Growth rings (TS) B detail of a growth ring boundary (TS) C Detail of roundish tracheids and axial parenchyma (arrowheads) (TS) D uniseriate non contiguous intertracheary radial pits and cross-field pit (arrowhead) (SEM) E and F cross-fields with bordered pits (SEM) G axial parenchyma (arrowhead) and cross-fields with bordered pits (RLS) H uniseriate rays (TLS) I uniseriate rays (TLS). Scale bars: 5 mm (A); 500 μm (B); 100 μm (C, I); 20 μm (D, E, F, G); 200 μm (H).
Abietinean intertracheary radial pitting and apparently cupressoid pits in the cross-fields (bordered pits with the border wider than the aperture) indicate that this specimen belongs to the genus Cupressinoxylon, following
Cross-field pit border width is a key character of Cupressinoxylon. The poor preservation of this specimen prevents clear observation of the cross-field pits and also of the axial parenchyma walls; consequently, this specimen cannot be assigned with confidence to this fossil-genus and it is left as cf. Cupressinoxylon.
The particular specimen studied here seems to be more similar to Podocarpaceae than to Cupressaceae because of the number of pits per cross-field. One, rarely two, pits per cross-field is common in the Podocarpaceae, whereas it is rarely observed in Cupressaceae (
MPEF-Pb 10778.
LU23 at Laguna del Hunco (Fig.
Tufolitas Laguna del Hunco, Huitrera Formation (Ypresian, early Eocene).
Growth ring boundaries are distinct (Fig.
cf. Cupressinoxylon sp. 2, MPEF-Pb 10778: A Growth rings of type B (TS) B detail of a growth ring of type B boundary (TS) C uniseriate non contiguous intertracheary radial pits D uni- and biseriate intertracheary radial pits, opposite when biseriate (arrowheads) E uniseriate non contiguous intertracheary radial pits F cross-fields with bordered pits (RLS) G and H cross-fields with bordered pits (SEM) I uniseriate rays (TLS). Scale bars: 5 mm (A); 200 μm (B, I); 50 μm (C, D, E); 20 μm (F, G, H).
This specimen shares with cf. Cupressinoxylon sp. 1 the abietinean intertracheary radial pitting and bordered cross-field pits, and it also conforms to Cupressinoxylon according to
Due to preservation, complete growth rings were only observed in a few specimens. Nonetheless, all the samples have distinct, well-defined growth ring boundaries (e.g., Figs
Growth ring analysis of selected samples. Growth ring type (GRT); Mean width (MW) [μm]; Minimum and maximum growth ring width (Min.-Max.) [μm]; Standard deviation (SD) [μm]; Number of rings counted (n); Minimum estimated diameter (MED) [cm]; Minimum age (MA) [years], MA= (MED*10000/MW)/2.
MPEF-Pb | Taxonomic unit | GRT | MW | Min.–Max. | SD | n | MED | MA |
---|---|---|---|---|---|---|---|---|
10694 | Protophyllocladoxylon francisiae | D | 681 | 262–1314 | 327 | 21 | 15 | 110 |
10707 | Phyllocladoxylon antarcticum | D | 816 | 300–1324 | 307 | 19 | 12 | 73 |
10747 | Phyllocladoxylon antarcticum | D | 1400 | 371–3056 | 678 | 15 | 15 | 54 |
10778 | cf. Cupressinoxylon sp. 2 | B | 2786 | 2258–3742 | 517 | 7 | 50 | 90 |
Two samples of indeterminate conifers (Fig.
The conifers represent about 30% of the total collected wood assemblage, indicating an important presence of this clade within the forest canopy. Even though not all the conifer woods could be identified, two taxa belong undoubtedly to Podocarpaceae, Protophyllocladoxylon francisiae and Phyllocladoxylon antarcticum, while the other two, cf. Cupressinoxylon sp. 1 and cf. Cupressinoxylon sp. 2, are putative Podocarpaceae. Consequently, we found a significant species richness (four types of woods) from only twelve samples that could be identified to genus.
The family Podocarpaceae was apparently the dominant conifer component within the Laguna del Hunco wood flora, with the caveat of small sample size. The family is also abundant and strikingly diverse at generic level in the intensively collected compression flora (>7,000 specimens), including remains of Acmopyle Pilg., Dacrycarpus (Endl.) de Laub., Podocarpus, and Retrophyllum CN Page as well as the extinct phyllocladoid genus Huncocladus (
Nevertheless, Araucariaceae, which are not yet known in the wood flora, are the most abundant conifer compression fossils at Laguna del Hunco, where Agathis Salisb. (formerly “Zamia”) and Araucaria Juss. compressions are each more common than any podocarp genus (
This discrepancy in the family proportional abundances between the woods and compression macrofossils could result from many factors, most likely including the number of fossils studied, local variations of the source flora in time and space, and well-known differences in the taphonomic pathways for wood vs. other plant parts (e.g.,
Podocarpaceae are today distributed mainly in the Southern Hemisphere and Malesia and are almost entirely restricted to rainforest or wet montane environments (
Growth ring boundaries of all samples are usually distinct (although some boundaries are not very well-defined), which indicates seasonality. The growth rings (type D; e.g., Figs
The estimated minimum ages based on growth ring widths suggests that the specimens were mature trees at the time of deposition. We infer that the remains of Protophyllocladoxylon francisiae came from a tree older than 100 years (Table
Galleries found in the woods were apparently produced by arthropod borers. They are filled with coprolites and particulate frass. This type of fill is produced by many types of arthropods (e.g., Platypodidae beetles,
Herein, we report the first taxonomic study of conifer fossil woods from the highly fossiliferous Laguna del Hunco exposures. The proportion of conifers in this fossil wood assemblage (ca. 30%) indicates a significant presence of this group within the paleoflora. We document the family Podocarpaceae with confidence as the dominant component of the Laguna del Hunco wood paleoflora. The family is represented by two fossil species, Protophyllocladoxylon francisiae and Phyllocladoxylon antarcticum. Additionally, two species assigned to the genus Cupressinoxylon (cf. Cupressinoxylon sp.1 and sp.2) are probably representatives of the family as well. Although sample size is small compared with the compression flora, these data strongly indicate that Podocarpaceae were important components of the Laguna del Hunco flora.
Podocarpaceae dominance in the fossil woods is consistent with diverse, abundant podocarpaceous macrofossil compressions and pollen grains previously described from the same section, including vegetative and reproductive structures related to several extant podocarp genera. However, in the Laguna del Hunco compressions, Araucariaceae are the dominant conifers, Cupressaceae are also well represented, and both families are found in the palynoflora. Neither family is yet known from the wood flora, presumably a result of lower sample size available for the woods by two orders of magnitude (10s of wood fossils vs. 1000s of compression fossils) or unknown taphonomic factors.
Growth rings indicate seasonality, probably because of the seasonal light regime at paleolatitude ca. 47°S, and mature tree development. The galleries found in two woods indicate arthropod interactions.
We thank the MEF staff, including P. Puerta, M. Caffa, M. Krause, L. Reiner, and E. Ruigomez, for logistical support on the 2016 field trip, stratigraphic discussions, and collections support; H. Padula and C. Greppi for field assistance; S. Mirabelli for preparing the thin sections; and Secretaría de Cultura del Chubut for access permits.
This research was supported by National Science Foundation grants DEB-1556666, DEB-1556136, EAR-1925755, and EAR-1925481; CONICET grant PIP 2014-0259; and ANPCyT grant PICT 2017-0671.