Literature review: Impact of Chilean needle grass ... - Weeds Australia

to Bouchenak-Khelladi et al. 2009). The earliest fossil grass pollen has been found in the early Tertiary, with doubtful earlier
records from the Cretaceous (Thomasson 1986), and presumed records (Monoporites) from 70-60 mybp (Prasad et al. 2005).
Grass pollen first appears in the Australian record in the Paleocene (Macphail et al. 1994), at the end of the Paleocene (c. 58
mybp) (Keith 2004), or about 50 mybp (Keith 2004).
Undoubted fossils, with seeds very similar to modern Stipa, Piptochaetium and Phalaris, occur in rocks dated to the mid-Tertiary
(c. 35 mybp) long after the family first evolved (Stebbins 1972). Bouchenak-Khelladi et al. (2009) indicate that the BEP clade
(Bambusoideae, Ehrhartoideae and Pooideae), which includes Stipeae, originated in the Paleocene (57 mypb), while the other
major clade, including most C 4 species, is younger, originating in the late Eocene (40 mybp). C 4 physiology was probably well
developed by the Miocene (Thomasson 1986), the oldest origin in grasses being approximately 30.9 mybp (Christin et al. 2009,
Bouchenak-Khelladi et al. 2009).
Tsvelev (1977 1984) convincingly argued that Poaceae first evolved in response to colder and drier conditions in mountainous
areas with an increasingly continental climate. Stebbins (1986) argued a probable origin from Joinvillea-like ancestors
(Joinvilleaceae) and differentiation into major families in lowland, open tropical savannahs with seasonal droughts; the non
sequitur of a grass-less savannah going unremarked. A South American (Gondwanan) origin for the family has been suggested
on the basis of the distribution of extant taxa, with diversification of subclades in Gondwana by the late Cretaceous (Prasad et al.
2005). Although considerable diversification within the family took place in the mid-Miocene (Piperno and Sues 2005), most
modern tribes probably existed by the Paleocene (early Tertiary, c. 50 mybp), probably along with many modern genera
(Tsvelev 1984, Jones 1999a). The Stipeae probably arose from primitive Pooideae (Stebbins 1986) which existed at least as early
as 70 mybp (Prasad et al. 2005).
Jones (1999a) cited authors who argued the tropical origin of grasslands in areas that were cooling and developing seasonal
aridity, in forest-savannah ecotone. Poaceae are uncommon in the fossil record until the mid-Miocene (16-11 mybp) (Piperno
and Sues 2005) and “only became widespread 25 to 15 million years ago when cool, dry conditions kicked in” (O’Donoghue
2008 p. 39). The unimportance of mammals, except for South American gonwanatherians, with typical grazing adaptations such
as hypsodont teeth until the Oligocene and Miocene also indicates that grasses were a minor component of vegetation before this
time (Prasad et al. 2005).
Barkworth and Everett (1986 p. 261) noted that the character sets that define non-Australian supra-specific stipoid taxa do not
occur in Australian taxa, even though Australian stipoids in aggregate possess almost all of these characters. They believed that
Stipeae originated in Gondwana, suggesting a late Jurassic or Cretaceous (c. 135 mybp) origin for the tribe (Jones 1999a).
Tsvelev (1977) more or less agreed, noting that the impoverished stipoid flora of Africa (excluding the Mediterranean areas)
probably resulted from repeated long dry periods and the absence of high mountain refugia. Africa began to split from the South
American section of Gondwana in the mid to late Mesozoic (c. 165-70 mybp) leaving Australia and South America still linked
through Antarctica by the early Tertiary (c. 65 mybp) (Barlow 1981). Separation of Antarctica and Australia occurred in the
Paleocene (53-50 mybp) and accelerated in the middle-late Eocene (c. 43-36 mybp) (McGowran et al. 2000). Several authors
have favoured the Gondwanan centre of origin of Stipa (sens. lat.) including Moraldo (1986 p. 205) who placed it in the ‘suture
zones between South American, Antarctica and Australia’.
South America has a very rich stipoid flora, paralleled only by that of Eurasia (Tsvelev 1977). The current centre of diversity of
Nassella is Argentina with c. 72 species, with greatest diversity in the north west, and 26 indigenous species. Uruguay has 23+
spp., and the greatest diversity in Bolivia and Chile is in the central Andes, adjacent to Argentina (Reyna and Barkworth 1994,
Barkworth and Torres 2001, Barkworth 2006). The pampas or Rio de la Plata grasslands have 25 Nassella species (Gardener et
al. 1996b).
Whether the genus evolved in the Pampas region is not known. There are no macrofossil Stipeae known from South America
(Barkworth 2006). Tsvelev (1977) considered Oligocene (36-25 mybp) fossil panicles from Colorado to be Stipa florissanti
(Knowlt.) MacGinitie, and noted that the American grass specialist Agnes Chase considered them identical with the extant
species Stipa mucronata (now Nassella mucronata (Kunth) R.W. Pohl. Barkworth). Everett (1986) accept the Miocene North
American Berriochloa primaeva Thomasson to be the earliest stipoid fossil. According to Barkworth (1990) “nasselloid” fossils
are present in Late Miocene-Early Pliocene (c. 13-5 mybp) deposits in the USA. These have Nassella-like lemma epidermal
patterns, and were considered to be Nassella by Thomasson (1986). Other described fossil stipoids from the USA include
Oligocene and Miocene Stipidium and Stipa and Oligocene Piptochaetium (Thomasson 1986). Barkworth and Torres (2001)
appeared to accept four North American fossil Nassella species, but point out that only a single Nassella sp. is today present in
the areas of Colorado, Kansas and Nebraska where the fossils were found. Tsvelev (1977) accepted lower Miocene fossils as
Nassella and Piptochaetium spp. along with other grasses, indicating that prairie grasslands existed at that time. Johnson (1972)
prematurely considered this fossil record provided geological evidence of a stipoid evolutionary ‘hot-spot’ in the high plains of
Nebraska during the Tertiary and argued for a North American focal point of polyploidisation.
Barkworth and Everett (1986) considered that American stipoids consisted of groups derived directly from Gondwana, such as
Nassella (in the narrow sense) and Piptochaetium, and from a separate independent group that initially occupied Eurasia, notably
Achnatherum. According to Barkworth (1990) citing Tsvelev (1977), the North American Stipa sens. lat. evolved from South
American taxa, and European taxa from North American, however this is not evident in my reading of Tselev (1977), who
argued that stipoids in South America evolved in parallel with those in North America and Eurasia over a very long period prior
to the Pliocene, and that they first evolved around the Tethys sea between Africa and Eurasia when all the continental masses
were joined in Pangaea (i.e. in the Triassic period c. 200 mybp). Tsvelev (1977) thought that evolution of the Stipeae in all areas
simultaneously involved elongation of the spikelet and all its parts, and the lengthening of the awns, usually correlated with
elongation of the glume apices to prevent premature shedding of the floret. Based on anatomical and distributional data, he had
no doubt that Stipa sens. lat. evolved before the formation of lowland grasslands and considered Achnatherum to be the most
primitive stipoid genus.
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