Diversity in fossil Araucaria Juss.: new species from the Middle Jurassic Jaramillo Petrified Forests in Santa Cruz province, Argentina

Abstract

Araucariaceae were widespread and diverse in South America during the Jurassic and Cretaceous. However, usually, the fossil record indicates a rather low species diversity in a particular fossil site. This was also the case in the well-known Middle Jurassic Bosques Petrificados de Jaramillo National Park (also known as Cerro Cuadrado / Cerro Madre y Hija Petrified Forest) in the Santa Cruz province, Patagonia, Argentina, from which only a single taxon based on seed cones, foliage and wood, respectively, Araucaria mirabilis, Araucaria sanctaecrucis and Agathoxylon matildense, have been described since the beginning of the 20th century. However, these species are represented by a considerable number of fossils. Two distinct types of growth architecture of araucarian trees were reconstructed from the La Matilde Formation in previous studies motivating us to look for more Araucaria species diversity in this locality. A careful examination of petrified seed cones in several collections revealed specimens confirming a hitherto partly unknown diversity in external morphology and anatomy that is taxonomically important. A first consequence is that we re-evaluate Proaraucaria mirabilis var. minima Wieland 1935 is a separate species and recombine it to Araucaria minina comb. nov. et stat. nov. Furthermore, cutting and sectioning of cone material opened new insights into internal morphological and anatomical features. The overall morphological diversity cannot be ascribed to the single known species compared to the characteristics of extant and fossil Araucaria species but leads to the proposal of two new fossil-species, Araucaria cuneoi sp. nov., and Araucaria stockeyana sp. nov. The new species are defined based on a combination of the following features: cone shape, shape and direction of the distal ovuliferous complex tip, the proportion between ovuliferous complex and seed, shape and proportion of the ligulate extension, and characteristics of the lateral wing or wing-like fringe. Parastichy numbers also support the separation of these species. Whenever possible, different ontogenetic stages were recognised, and the xylem of peduncles or cone-supporting shoots are described. Except for Araucaria minima comb. nov. et stat. nov., wood anatomy fits Agathoxylon sp. aff. Agathoxylon matildense. The proposed fossil-species are compared both to the known fossil record of ovuliferous cones and to extant species of Araucariaceae. The palaeoecological aspect of seed dispersal, as far as it can be deduced from the ovuliferous complex characteristics, is briefly discussed as well as the issue that the petrified forest site constitutes an extraordinary mass occurrence of completely preserved araucarian cones. Based on these studies, the vegetation buried in the volcanic rocks of the La Matilde Formation by catastrophic volcanic events is more diverse in conifers than previously known.