The southwesternmost record of late Silurian (Pridolian) early land plants of Gondwana

Abstract

The Silurian-Devonian transition played a crucial role in the development of early terrestrial ecosystems due to the rapid diversification of early vascular plants. However, records of Pridolian plants in western Gondwana are scarce, limited to outcrops located in southern Bolivia. In this contribution, an association of fossil plants housed in the Rinconada Formation is presented. This association corresponds to primitive fossil flora with reproductive structures and sterile axes linked to basal tracheophytes. The fossil assemblage is composed of Aberlemnia caledonica, Caia langii Cooksonia cf. cambrensis, C. paranensis, C. cf. pertoni, Hostinella sp, Cf. Isidrophyton sp, Salopella marcensis, Steganoteca striata, two morphotypes of doubtful taxonomy, and graptolites colonies. The association between flora remains and graptolites, represents a parautochthonous assemblage in an inner marine platform, dominated by gravity flows. This record has paleophytogeographic importance indicating the extension of the northwest Gondwana-southern Laurusia unit to more southern areas of Gondwana. This expansion would have been favored by the post-glacial climatic improvement of the Late Silurian, together with a great radiation capacity and environmental flexibility of the flora. Furthermore, the biochron is extended of three taxa (A. caledonica, C. paranensis and Cf. Isidrophyton sp) first known from the Lochkovian, to the Pridoli.

Keywords: Early land plants; Gondwana; Paleophytogeography; Pridoli; Rinconada formation.

Figure 1

Map of the distribution of the Rinconada Formation at the Chica de Zonda Range showing its geographical distribution, the upper and lower contacts and the distribution of the olistoliths. Red star shows the place where samples were collected. Satellite images were obtained using the free software SAS.Planet v.221122.10312 Nightly, source Bing Maps. QGIS 3.30.0-'s-Hertogenbosch was used for image processing and the layout of the geological features. The final assembly of the figure was carried out using the free software Inkscape v.1.3.2.

Figure 2

Detailed stratigraphic column of the Rinconada Formation where the plant fossils were found, at the Chica de Zonda Range locality. PD, piedmont deposits. Scale in the graphic.

Figure 3

(a–c) Aberlemnia caledonica. (a) Specimen PBSJ-1770. (c) Specimen PBSJ-1771. (b) detail drawing of (a) showing the last section of the subtending axe and the sporangium-axis concave junction (yellow arrow). (d, e) Cooksonia paranensis. (d) Specimen PBSJ-1766 showing the compression. (e) Drawing of (d), showing the sporangium-axis junction (red arrow) and the sporangial chamber (yellow arrow). (f–h) Cooksonia cf. pertoni. (f–g) specimens PBSJ1767 and PBSJ-1768 showing a coalified trumpet-shape sporangium. (h) Drawing of (g), showing the sporangial chamber (red arrow) and the operculum (yellow arrow). (i, j) Cooksonia cf. cambrensis. (i) specimen PBSJ-1769 showing the junction sporangium-axis (red arrow). (j) Drawing of the specimen PBSJ-1769 reconstructing the missing section of the sporangium (dotted line). Scale bars: 1 mm.

Figure 4

(a, b) Salopella marcensis. a, coalified compression of specimen PBSJ-1763. (b) Detail of sporangia. (c–e) Steganotheca striata. (c) General aspect of specimen PBSJ-1765. (d) Detail of the top of the sporangia. (e) drawing of d showing the apical plateu broken (yellow arrow) and two possible striations (red arrow). (f) Isidrophyton sp. coalified impression of specimen PBSJ-1772 with two terminal sporangia (yellow arrow). Scale bars: 1 mm.

Figure 5

(a–c) Caia langi. (a) General aspect of the specimen PBSJ-1773 showing the isotomously bifurcation (yellow arrow). (b) Detail of the sporangium. (c) Drawing of (b), showing the sporangium and four conical emergencies at the apex (yellow arrows). (d) Hostinella sp. specimen PBSJ-1764, notice the isostomously branching axes (yellow arrow). (e, f) Morphotype A. (e), coalified compression of specimen PBSJ-1775. (f) Drawing of (e), showing the two sporangial-shaped structures (yellow arrows). (g, h) Morphotype B. (g), coalified compression of specimen PBSJ-1774. h, drawing of g, showing the bifurcation (red arrow) and the conical emergence at the apex of the sporangium (yellow arrow).

Figure 6

(a) General view of the Rinconada Formation outcrops, showing the matrix (red arrow) and the olistoliths (white arrows). (b) Detail of (a), showing the bearing fossil strata (yellow scheme) and the olistolith over the matrix (red arrow). (c) Detail of synsedimentary deformation (red arrow). (d) orientation of the fossil plants (yellow arrows). (e–f) Skalograptus parultimus found in the same strata of the fossil plants. (g) Detail of the degree of fragmentation of the plant fossil assemblage and mica size (yellow arrows). (h) Trace fossils found in the same facies of the fossil plant strata, classified as Gordia isp. (i) Interpretation of the process that preserved together plants fossil and graptolites.

Figure 7

Paleogeographic reconstruction of the continents during the interval Ludlow-Pridoli. Map showing the distribution during, Ludlow (red star) and Pridoli (blue star), fossil assemblages and the Rinconada Formation fossil plants strata. NL = North Laurussian unit; K = Kazakhstan unit; NEG = Northeast Gondwana unit; SL-NWG = South Laurassia-Northwest Gondwana unit. Modified from and.