The palaeobotanical literature contains a fair number of assignments of fossil leaves to Annonaceae, but few (if any) stand up to critical phylogenetic scrutiny and most are from the early Tertiary, postdating more diagnostic seed and floral fossils discussed below. Although Doyle & Le Thomas (1996) considered the evolution of secondary and tertiary venation in a phylogenetic context, there have been no comprehensive studies aimed at the recognition of diagnostic differences in leaf architecture between Annonaceae and other Magnoliales, or among clades within the family. Annonaceae resemble other Magnoliales in having pinnately veined, entire-margined leaves that are usually elliptical to obovate in shape (i.e. widest near or above the midpoint of the blade), but, consistent with their greater systematic diversity, Annonaceae vary more than the other families in spacing, angle and course (brochidodromous to eucamptodromous) of the secondary veins and in tertiary and finer venation (Klucking, 1986; Doyle & Le Thomas, 1996). More primitive annonaceous leaves, with reticulate tertiary venation, as in the basal genus Anaxagorea (the sister group of all other Annonaceae), are similar to leaves of Eupomatia R.Br. (the sister group of Annonaceae; Doyle & Le Thomas, 1996; Scharaschkin & Doyle, 2006), whereas others with derived percurrent tertiary venation approach some members of Magnoliaceae and Myristicaceae (Doyle & Le Thomas, 1996; Doyle et al., 2004). Dilcher & Lott (2005) stated that the leaves of Magnoliaceae and Annonaceae can be distinguished on the basis of moderate (< 55°) secondary vein angles and closed ultimate venation in Magnoliaceae and higher (> 55°) secondary angles and open ultimate venation in Annonaceae. Many members of Annonaceae have higher secondary angles (illustrated by figures of Anaxagorea and Duguetia in Doyle & Le Thomas, 1996) and, because both Anaxagorea and Eupomatia have high secondary angles (Scharaschkin & Doyle, 2006), this condition may be ancestral for the family. However, other members of Annonaceae have lower secondary angles, comparable with those of Magnoliaceae (e.g. Asimina Adans. and Annickia Setten & Maas in Doyle & Le Thomas, 1996), and so this character is not always sufficient to distinguish between leaves of the two families.
With a few exceptions (e.g. Wolfe, 1977; Roth, 1981; Dilcher & Lott, 2005), most reports of fossil annonaceous leaves are from what has been called the ‘picture matching’ phase of angiosperm palaeobotany, which predated more rigorous and consistent methods of analysis of leaf architectural characters (Hickey, 1973; Dilcher, 1974; Hickey & Wolfe, 1975; Ellis et al., 2009), to say nothing of principles for the appropriate analysis of their phylogenetic significance. Many identifications from this phase are notorious for overlooking equally close or closer matches with other extant taxa, and many have been shown to be incorrect by the analysis of cuticle structure. In a preliminary analysis of venation and epidermal anatomy, Dilcher (1971, 1974) estimated that about 60% of Berry's identifications of Eocene leaves in the Mississippi Embayment were erroneous at the generic and often the familial level. Late Cretaceous leaves from Massachusetts and New York that were described by Hollick (1906) as Guatteria cretacea Hollick are doubtful because of their lack of higher order venation; they also differ from most Annonaceae, including most, although not all, species of Guatteria Ruiz & Pav. (Erkens, 2007), in their lanceolate rather than obovate or elliptical shape. Berry (1916, 1941), Chaney & Sanborn (1933), Sanborn (1935) and Lakhanpal (1969) identified leaves from Eocene and Oligocene floras of the south-eastern USA (Mississippi Embayment) and Oregon as Annona L. Some of these may represent Annonaceae, but they have not been evaluated with modern leaf architectural or cuticle analysis. Similar considerations apply to Berry's identifications of leaves from the Miocene of Central and South America as Annona and Guatteria (Berry, 1918, 1920, 1922a, b, 1923, 1937). Many of these differ from most members of Annonaceae in their asymmetrical and ovate shape.
Probably the most secure leaf record of Annonaceae consists of fossils described from the middle Eocene of the Mississippi Embayment by Berry (1916) as Magnolia leei Knowlton, but reassigned to the Neotropical annonaceous genus Duguetia (as Duguetia sp.) by Roth (1981) and Dilcher & Lott (2005), based on venation, paracytic stomata and the presence of stellate-peltate trichomes. In Annonaceae, such trichomes appear to be restricted to Duguetia, the related African genus Pachypodanthium Engl. & Diels (synonymized with Duguetia by Chatrou, 1998; Chatrou, Koek-Noorman & Maas, 2000) and Meiocarpidium Engl. & Diels (Fries, 1959; Le Thomas, 1969), but Meiocarpidium differs in having conspicuously sunken stomata (Roth, 1981). Elsewhere in Magnoliales, peltate scales occur in Galbulimima F.M.Bailey (Himantandraceae), but this genus differs in its distinctive radial arrangement of stomata below the scale (Roth, 1981). Considering only epidermal characters, the fossil might be a stem relative of Meiocarpidium or Galbulimima that lacked the derived features of the living genera, but its leaf architecture is more like that of Duguetia. Stellate-peltate trichomes were inferred to be a synapomorphy of Duguetia and ‘Pachypodanthium’ in the analysis of Doyle & Le Thomas (1996), which did not include Meiocarpidium. Doyle & Le Thomas (1996) defined their stellate-peltate state as also including tufts of simple trichomes of the sort found in Tetrameranthus R.E.Fr., Uvaria L. and some Annona spp. (Jovet-Ast, 1942; Van Setten & Koek-Noorman, 1986), in order to allow for the possibility that stellate-peltate trichomes were derived from the tufted type, but, judging from their phylogenetic results (and more recent molecular studies), the two types arose independently.
Based on a morphological phylogenetic analysis, Chatrou (1998) concluded that ‘Pachypodanthium’ was nested within Duguetia, but more recent molecular analyses indicate that the two taxa are sister groups (L. Chatrou, pers. comm.). If stellate and peltate trichomes are not distinguished, their presence in the middle Eocene fossil may be a synapomorphy linking it with the Duguetia–‘Pachypodanthium’ clade, which would provide a minimum age of c. 38 Mya for the node connecting Duguetia (including ‘Pachypodanthium’) with its sister group, Fusaea. However, trichomes in Neotropical Duguetia vary from stellate hairs to peltate scales with variously fused radiating cells (Maas, Westra & Chatrou, 2003), whereas ‘Pachypodanthium’ has only stellate hairs (Le Thomas, 1969), which resemble those of Duguetia uniflora (DC.) Mart. (L. Chatrou, pers. comm.). This might suggest that peltate scales are a synapomorphy of the Neotropical clade, or some large portion of it, to the exclusion of ‘Pachypodanthium’. If so, as the fossil has peltate scales consisting of cells fused about one-third of their length, it could be more closely related to Neotropical Duguetia, and 38 Mya would be a minimum age for the crown node of Duguetia (including ‘Pachypodanthium’). However, better information on the systematic distribution of trichome types is needed to decide between these alternatives. Hopefully, more comprehensive surveys of leaf architecture and epidermal anatomy in a phylogenetic framework will allow more accurate assignment of fossil leaves to Annonaceae and to clades within the family.
In the fossil pollen record, there are reports of morphological types now restricted to the two major sister clades that make up most of Annonaceae. These were called the SBC (short branch clade) and LBC (long branch clade) by Richardson et al. (2004) and the malmeoid-piptostigmoid-miliusoid (MPM) and inaperturate clades by Doyle, Bygrave & Le Thomas (2000) and Doyle et al. (2004), but are now formally classified as subfamilies Malmeoideae and Annonoideae (Chatrou et al., 2012).
Reticulate-columellar monosulcate pollen, described by Sole de Porta (1971) from the Maastrichtian (latest Cretaceous) of Colombia as Foveomorphomonocolpites humbertoides, was compared by Muller (1981) with the Malmea tribe of Walker (1971), which corresponds closely to the malmeoid clade of Doyle & Le Thomas (1996), the South American Centred clade of Pirie et al. (2006) and tribe Malmeeae of Chatrou et al. (2012). Muller considered this the oldest accepted pollen record of Annonaceae. The phylogenetic implications of this pollen are sensitive to relationships among the early branches of Malmeoideae, especially the incompletely resolved arrangement of several granular monosulcate genera and the columellar monosulcate genus Annickia, which form a clade treated as tribe Piptostigmateae in the tree of Chatrou et al. (2012). If Annickia is linked with the granular members of Piptostigmateae, or basal in the Malmeoideae, parsimony optimization indicates that columellar monosulcate pollen is basic for the combined Malmeoideae and Annonoideae, but if the granular Piptostigmateae are sister to the rest of Malmeoideae, such pollen may be basic for either the combined Malmeoideae and Annonoideae or all of Malmeoideae except the granular Piptostigmateae. As such pollen is more derived than the granular monosulcate pollen of Anaxagorea and Ambavioideae (Doyle & Le Thomas, 2012), the Colombian fossil may therefore provide a minimum age of c. 68 Mya for either the split between Ambavioideae and the combined Malmeoideae and Annonoideae, or for the crown node of Malmeoideae.
Tetrads of inaperturate, reticulate-columellar grains from the early Eocene of the US Gulf Coast [Annona (?) foveoreticulatus: Elsik, 1974] and the Oligocene of Cameroon (Inaperturotetradites reticulatus: Salard-Cheboldaeff, 1978) have been compared with Annona, but similar pollen also occurs elsewhere in Annonoideae. Because loss of the aperture (or its modification to a round thin area that becomes proximal during development: Tsou & Fu, 2002) is a synapomorphy of Annonoideae, and tetrads arose at several points within this clade (Doyle & Le Thomas, 2012), these tetrads provide a minimum age of c. 50 Mya for the split of Malmeoideae and Annonoideae, but this would probably be a substantial underestimate.
Fossil wood identified with Annonaceae is also known from the Palaeocene–Eocene of England (Polyalthioxylon oldhavenense: Crawley, 2001), the Eocene of Oregon (Annonoxylon bonesii: Wheeler & Manchester, 2002) and more poorly dated but probably post-Eocene horizons in Sudan (Annonoxylon striatum and A. edengense: Boureau, 1950, 1954). These authors made tentative comparisons of these wood types with modern genera. However, although the relationship of the fossils to Annonaceae seems well established, based on the diagnostic combination of characters, such as simple vessel perforations, reduced fibre pits and apotracheal parenchyma bands, a more detailed analysis of the distribution of wood characters in a phylogenetic framework is needed to assess their position within the family. There have been extensive surveys of wood anatomy in Annonaceae (notably Vander Wyk & Canright, 1956), but Doyle & Le Thomas (1996) found that the two characters that seemed most likely to be informative, vessel density and ray width, were highly homoplastic across the family.
Fossil seeds related to Annonaceae are more distinctive and extend back into the Cretaceous. The most diagnostic seed characters are the perichalazal ring (where the raphe runs most of the way around the seed as a result of differential growth) and ruminate endosperm formed by ingrowths of both the tegmen and testa (derived from both inner and outer integuments) of the sides of the seed (Corner, 1976; Van Setten & Koek-Noorman, 1992). These testal ruminations contrast with the tegminal and/or chalazal ruminations of Myristicaceae (Doyle et al., 2004). Within Magnoliales, testal ruminations are shared with Eupomatia and Degeneria A.C.Sm., but Annonaceae are the only group with a perichalazal ring. Doyle & Le Thomas (1996) and Doyle & Endress (2000) scored Galbulimima as lacking ruminations, but it is better scored as unknown (?) based on the report of probable reduced ruminations by Doweld & Shevyryova (1998), as in Sauquet et al. (2003) and Endress & Doyle (2009). Perichalazal seeds with ruminate endosperm also occur in Austrobaileya C.T.White in the basal ‘ANITA’ grade, but the ruminations differ in their finely ramified form (Endress, 1980).
Within Annonaceae, ruminations show significant variation among taxa. The two most distinctive types are lamelliform, with four (sometimes two) thin transverse plates, and spiniform, with fine spines penetrating the endosperm like pins in a pincushion, but these intergrade via forms with radially subdivided plates (Van Setten & Koek-Noorman, 1992). Other seeds differ in having thicker and more irregular processes, variously described as stout or woody pegs or plates, here called irregular ruminations; this is the type found in the outgroups Eupomatia and Degeneria. The implications of ruminations for systematic placement of fossil seeds can be evaluated in the context of Figure 8, which shows a parsimony reconstruction of the course of evolution of rumination types on a pruned version of the Bayesian tree of Chatrou et al. (2012).
Figure 8. Composite cladogram of Annonaceae and outgroups based on published molecular analyses, showing the evolution of the endosperm rumination character inferred from parsimony optimization with MacClade (Maddison & Maddison, 2003). See text for sources of data on tree topology and ruminations. ‘Uncertain’ indicates that the taxon was scored as having either of two states (here either spiniform or lamelliform, i.e. 2/3), ‘equivocal’ that the reconstructed state on a branch is ambiguous. Figures of fossil seeds illustrate the three major types of rumination: irregular in Anonaspermum ovale from the London Clay (early Eocene, reproduced from Reid & Chandler, 1933, c. 2×), spiniform in A. punctatum (London Clay, c. 2×) and lamelliform in A. commune (London Clay, c. 1.4× in 14 and 15, 2× in 17) and A. gilbediense (Maastrichtian of Nigeria, reproduced from Chesters, 1955, c. 2×). LBC, long branch clade; SBC, short branch clade.
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The oldest reported seeds of Annonaceae are Anonaspermum gilbediense and A. phytoniscus, described from the Maastrichtian (latest Cretaceous) of Nigeria by Chesters (1955; Fig. 8). These seeds have a perichalazal ring, a synapomorphy of Annonaceae as a whole, and lamelliform ruminations, which are characteristic of most members of Annonoideae and some Malmeoideae (Fig. 8). Similar ruminations occur in a seed (Unonaspermum corneri) described by Bonde (1993) from the Deccan intertrappean beds of India, near the Cretaceous–Tertiary boundary. Most members of Malmeoideae have spiniform ruminations, which appear to be ancestral for this clade (Doyle et al., 2000, 2004; Fig. 8), whereas Anaxagorea and most Ambavioideae have thicker, irregular ruminations. Both ingroup topology and outgroup comparison indicate that irregular ruminations are ancestral in Annonaceae, as they are shared with Eupomatia, Degeneria and, probably, Galbulimima (Doweld & Shevyryova, 1998).
Given the distribution of lamelliform ruminations in Annonaceae, their presence in the Nigerian fossils might seem to be evidence for the presence of Annonoideae or some derived member of Malmeoideae, but this is ambiguous. With the combined morphological and molecular tree of Doyle et al. (2000), parsimony optimization indicated that the common ancestor of Malmeoideae and Annonoideae could have had irregular, spiniform or lamelliform laminations, so that the spiniform and lamelliform types could have been derived independently from the irregular type or one from the other (Doyle et al., 2004). Doyle et al. (2004) therefore argued that the Nigerian seeds could only be used as evidence that the combined Malmeoideae–Annonoideae clade had diverged from Ambavioideae by this time (68 Mya). More recently, as noted by Couvreur et al. (2008b), this picture has been further blurred by molecular evidence that the African genus Meiocarpidium is the earliest branching taxon in the ambavioid clade, because this genus is reported to have lamelliform ruminations (Van Setten & Koek-Noorman, 1992). If these ruminations are indeed comparable with those in Annonoideae and Malmeoideae, which should be verified, their presence in Meiocarpidium could mean that lamelliform ruminations originated earlier, with a reversal to irregular in Ambavioideae, or that they originated twice and the fossils are relatives of Meiocarpidium rather than the Malmeoideae–Annonoideae clade (Fig. 8). Given that both scenarios are equally parsimonious, the Nigerian seeds only provide a minimum age for the crown node of Annonaceae as a whole.
The Nigerian seeds are of broader significance for the ecological evolution of angiosperms, as they represent some of the oldest large diaspores in the group. Most Cretaceous angiosperm fruits and seeds are much smaller, and one of the most striking changes in the early Tertiary is an increase in diaspore size, which has been variously linked with climatic or faunal changes at the Cretaceous–Tertiary boundary (Upchurch & Wolfe, 1987; Wing & Tiffney, 1987; Eriksson, Friis & Löfgren, 2000). In representing an exception to the rule for Cretaceous diaspore size, the Nigerian seeds are potentially critical in evaluating the causal factors responsible for these changes.
Better evidence for diversification in Annonaceae is provided by perichalazal seeds with ruminate endosperm from the early Eocene London Clay, described by Reid & Chandler (1933) as 13 new species of Anonaspermum O.M.Ball (see also Collinson, 1983). These show all three main types of rumination: lamelliform (e.g. A. commune, A. pulchrum, A. rotundatum), spiniform (A. punctatum) and irregular (A. ovale, A. complanatum, A. corrugatum), plus intermediate types (Fig. 8). Together, setting aside the possibility that the spiniform seeds are related to Cyathocalyx or the lamelliform seeds to Meiocarpidium, these fossils indicate that Malmeoideae and Annonoideae, as well as Anaxagorea and Ambavioideae, had diverged by c. 50 Mya (Doyle et al., 2004).
The discovery of a fossil fruit containing seeds of the A. ovale type led Chandler (1978) to transfer this species to the extant genus Uvaria, based on the large number of seeds (17 preserved) in two alternating rows and the bipartite vs. quadripartite endosperm ruminations. She contrasted this with the torulose fruits containing one row of seeds in such taxa as Desmos Lour. and Dasymaschalon Dalla Torre & Harms, now known to belong to the same liana clade (Uvarieae Hook.f. & Thomson, sensuChatrou et al., 2012). This fossil is substantially older than the molecular age of Uvaria found by Zhou et al. (2012; stem, 32 Mya; crown, 27 Mya). However, although the data of Van Setten & Koek-Noorman (1992) confirm that such fruits with two rows of seeds are indeed typical of Uvaria (and the genera synonymized with it by Zhou et al., 2010), similar fruits also occur in Fissistigma Griff., Mitrella Miq., Dielsiothamnus R.E.Fr. and Toussaintia Boutique (all Uvarieae; the last two with one row of seeds), and in many other clades of Annonaceae. In addition, the thickness and irregularity of the ruminations in the fossil would be anomalous in Uvarieae and more typical of Ambavioideae, as noted above. Van Setten & Koek-Noorman (1992) also noted variation between bipartite and quadripartite ruminations in many genera other than Uvaria (e.g. Dielsiothamnus). These observations suggest that it would be unwarranted to accept this fossil as a record of Uvaria, or even of Uvarieae as a whole.
The oldest evidence for crown Annonaceae is provided by Futabanthus, a fossil flower described by Takahashi, Friis & Crane (2008) from the early Coniacian of Japan (c. 89 Mya), which has a most likely trimerous perianth, numerous stamens and numerous carpels borne on a flattened receptacle (Fig. 9). Like most Annonaceae, it lacks inner staminodes, which occur in both Anaxagorea and the outgroups Degeneria, Galbulimima and Eupomatia (Endress, 1984). The loss of inner staminodes may therefore be a synapomorphy that links Futabanthus with the clade consisting of Annonaceae other than Anaxagorea. The fact that the stamens have an extended connective apex, like the outgroups, Anaxagorea, most Ambavioideae and the genus Greenwayodendron Verdc. near the base of Malmeoideae (Doyle & Le Thomas, 1996; Doyle et al., 2000), rather than the peltate apex of most members of Malmeoideae and Annonoideae, may place it near the base of the Ambavioideae–Malmeoideae–Annonoideae clade. The absence of inner staminodes also tends to exclude Futabanthus from other positions in the clade consisting of Degeneria, Galbulimima, Eupomatia and Annonaceae.
Figure 9. Scanning electron micrographs of flower of Futabanthus from the Coniacian of Japan (Takahashi et al., 2008): A, view of whole flower; scale bar, 1 mm; B, close-up showing stamen morphology; scale bar, 0.5 mm. Images kindly provided by Masamichi Takahashi.
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An alternative hypothesis is that Futabanthus diverged earlier in Magnoliales, near Magnoliaceae, and never had inner staminodes. It differs from Magnoliaceae in lacking an elongate floral receptacle, but, contrary to Takahashi et al. (2008), this does not rule out a relationship, as the elongate receptacle is an autapomorphy of Magnoliaceae that would not be expected in their common ancestor with other Magnoliales (Endress & Doyle, 2009). However, the short length and the close packing of the stamens, as in Annonaceae but not other Magnoliales, are potential synapomorphies that favour a relationship with Annonaceae. Futabanthus therefore provides a minimum age of c. 89 Mya for the crown node of Annonaceae.
Kvaček & Eklund (2003) interpreted a still older flower, Pecinovia annonoides J.Kvaček & H.Eklund from the mid-Cenomanian of the Czech Republic (c. 96 Mya), as possibly related to Annonaceae, based on its probably trimerous perianth and numerous stamens with a globular apical extension of the connective. However, this flower appears to differ from most Annonaceae in being unisexual and having two rather than three whorls of tepals, and the connective extensions are more convex than the peltate apices of most Annonaceae and do not cover the anthers as much. The combination of characters in the pollen, which is small, globose and monosulcate and has finely verrucate sculpture, is also unlike that of any living Annonaceae. Most monosulcate Annonaceae have a smooth, foveolate or reticulate tectum, and the most similar verrucate sculpture is found in Miliuseae, in which the pollen is either inaperturate or disulculate (Doyle & Le Thomas, 2012). An analysis of this fossil in a broader systematic context is needed to evaluate its relationships.