Poinar and Steeves: Virola dominicana 1
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Virola dominicana sp. nov., (Myristicaceae) from Dominican amber
George Poinar, Jr.
Department of Zoology, Oregon State University, Corvallis, OR 97331.
poinarg@science.oregonstate.edu
Royce Steeves *
Centre for Biodiversity Genomics, Department of Integrative Biology, University of
Guelph, Guelph, Ontario, N1G 2W1. rsteeves@uoguelph.ca
*Corresponding author
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Abstract:
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Magnoliales, however the family is poorly represented by fossil collections. We describe
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Virola dominicana sp. nov. (Myristicaceae), the first record of fossilized Myristicaceae
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flowers, from mid-Tertiary (45-15 mya) Dominican amber. The description is based on
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24 male flowers in 17 pieces of amber, thus providing some indication of intra-specific
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variation, including a 2-tepaled flower. Diagnostic characters of the new species are the
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long simple or few-branched trichomes on the perianth margins, the small pollen grains
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and a short staminal column. There are no endemic members of the Myristicaceae in
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Hispaniola today and it is speculated that V. dominicana disappeared from the region
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during the Pliocene-Pleistocene cooling events. These fossils provide observations of
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possible pollinators of Virola and establishes the presence of Myristicaceae in the
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Western Hemisphere during the mid-Tertiary.
The Myristicaceae is a member of the early diverging angiosperm order
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Keywords: Virola dominicana, Myristicaceae, Dominican fossil Amber, paleobotany
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Introduction
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Amber, the fossilized resin of woody plants, may contain a great wealth of ancient
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organisms and is one of the best media for preserving delicate structures like flowers. Not
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only is the perianth often preserved, but also details of trichomes and even complete
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pollen cells. Flowers up to 100 million years old have been preserved in amber, although
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these cannot be assigned to extant genera (Poinar and Chambers 2005; Chambers et al.
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2010). Flowers found within Dominican amber have been used to identify members of
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the genera Persea (Lauraceae)(Chambers et al. 2011a), Trichilia (Meliaceae) (Chambers
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et al. 2011b), and Trochanthera (Balanophoraceae or Moraceae) (Poinar et al. 2008).
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The Myristicaceae comprises an assemblage of canopy to subcanopy trees of
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some 21 genera and about 500 species distributed in tropical and subtropical regions of
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Central and South America, Asia, Africa and Madagascar (Doyle et al. 2004; Smith
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1937). Despite the family’s phylogenetic placement within the early diverging magnoliid
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grade of angiosperms (Soltis et al. 2011), the Myristicaceae are poorly represented by
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fossil collections. Fossilized wood (Boureau 1950), leaves (Wolfe 1977), pollen
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(Frederiksen 1973; Jan du Chene et al. 1978), fruits and seeds (Berry 1929) have been
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described as belonging to the Myristicaceae, however few can be unambiguously
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assigned to the family.
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Six genera of Myristicaceae are endemic to the Neotropics. One of these is Virola,
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which is represented by some 60 species restricted to lowland and cloud rainforests of
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Central and South America. Virola is differentiated from other Neotropical genera of
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Myristicaceae by possessing profuse red latex (rarely green-yellow) and dendritic or
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stalked-sessile stellate hairs on the surfaces of their leaves, flowers and fruits. The
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flowers of these dioecious trees are very small (1-4mm in diameter) and are composed of
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3-4 tepals (petals) that are often covered in a dense dendritic or stellate pubescence.
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Extant species of Virola can only be identified by numerous vegetative and reproductive
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characters as there exists a great deal of overlap among species for any one character.
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The objectives of this study are to 1) describe the staminate flowers of Virola
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dominicana, the only known fossil flowers of the family Myristicaceae, and 2) quantify
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the floral variation that occurred within Virola in Hispaniola (modern day Haiti and
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Dominican Republic) during the mid-Tertiary.
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Materials and Methods
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Septentrional, between Puerto Plata and Santiago, of the Dominican Republic. Dating of
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Dominican amber is equivocal with the latest proposed age of 20-15 mya based on
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foraminifera (Iturralde-Vinent and MacPhee 1996) and the earliest as 45-30 mya based
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on coccoliths (Cêpek in Schlee 1990). In addition, Dominican amber is secondarily
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deposited in sedimentary rocks, which makes a definite age determination difficult
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(Poinar and Mastalerz 2000). Dominican amber was produced by the leguminous tree,
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Hymenaea protera Poinar (1991) and a re-construction of the Dominican amber forest
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based on amber fossils indicated that the environment was similar to that of a present day
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tropical moist forest (Poinar and Poinar 1999).
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The Dominican amber specimens were obtained from mines in the Cordillera
Observations, drawings, and photographs were made with a Nikon SMZ-10 R
stereoscopic microscope and Nikon Optiphot compound microscope with
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magnifications up to 600X. In some instances, Helicon Focus Pro X64 was used to
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stack photos for better clarity and depth of field.
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Results
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Seventeen pieces of Dominican amber, containing a total of 24 male and no female
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Virola flowers, were examined during the present study. Not all characters were evident
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on every flower since some had been partly polished away during preparation of the
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amber or partially eaten by herbivores or slightly obstructed from view by other
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inclusions. The number of specimens examined for each numerical value is provided.
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Description
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Myristicaceae
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Virola Aublet, 1775
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Type Species: Virola dominicana Poinar and Steeves, sp. nov. (Figs. 1-4)
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Specific Description: Flowers brown to orange-brown; perianth 3 (2) -lobed to middle or
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beyond; lobes thick, rounded to acute at apex, spreading at anthesis; stamens 3,
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apparently opposite tepal lobes where visible in two specimens (supplementary Figs.
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S2+S3), filaments connate in a column; anthers 2-celled, extrose, dehiscing
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longitudinally. Length of the complete flower 3.4 (2.1-5.0) mm (N= 16); length of
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perianth lobes 1.3 (0.7-3.3) mm (N= 24); greatest width of perianth lobes 0.9 (0.5-1.5)
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mm (N= 23); length of fused portion of perianth 0.9 ( 0.5- 1.5) mm (N= 19); pedicel
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length 1.4 (0.7-2.0) mm (N= 17); length of stamenal column 0.5 (0.3-0.8) mm (N= 10);
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length stamen stalk 0.2 (0.1-0.3) mm (N= 6).
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Exterior surface of perianth covered with short, stubby, simple or branched trichomes
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(supplementary Figure S1); lobe margins with long, thick single to multiple- celled
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trichomes ranging from 0.1 to 0.3 mm in length; pollen apparently monosulcate, boat-
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shaped, ranging from 23- 27 µm in diameter (Fig. 2F). Pistillate flower unknown.
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Types: Holotype deposited in the Poinar amber collection (accession # Sd-9-30A) and
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Paratypes (Sd-9-30B through Sd-9-30R) maintained at Oregon State University,
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Corvallis, Oregon.
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Type locality: Amber mine in the northern mountain ranges (Cordillera Septentrional) of
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the Dominican Republic.
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Etymology: The specific name is based on the country of origin of the fossil.
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Discussion
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Despite their early origination in angiosperm evolution and their current
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pantropical distribution, the fossil record of the Myristicaceae is rather meager and no
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fossil flowers have been described prior to this study. Eocene fossil seeds and fruit casts
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from Texas were described as Myristica catahoulensis Berry (Berry 1929; Chesters et al.
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1967), however Doyle et al (2008) postulate that these represent molds of an endocarp of
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Mastixiaceae (Cornales). Leaf fragments from the Eocene of Borneo were described as
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Myristicophyllum minus Geyler (Andrews 1970). Wood described as Myristicoxylon
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princeps E. Boureau from Oligocene-Miocene deposits in the Sahara was also attributed
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to the nutmeg family (Boureau 1950; Collinson et al. 1993). In addition, Miocene (~23-5
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mya) and Eocene (~56-34 mya) fruits from Germany and England were assigned to the
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Myristicaceae and described as Myristicacarpum miocaenicum Gregor and M.
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chandlerae Doyle, Manchester and Sauquet, respectively (Collinson et al. 1993; Doyle et
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al. 2008). Myristicaceous pollen from the Eocene of Nigeria was described as
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Echimonocolpites major du Chene (Collinson et al. 1993).
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A seed described as Virola tertiaria Berry from the Oligocene of Peru, is the only
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previous putative fossil of the genus Virola (Berry, 1929), however a transversely broken
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specimen does not possess tegminal ruminations, which would be indicative of Virola
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(Doyle et al. 2008). A flower of Virola in Dominican amber was previously depicted, but
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not described (Poinar and Poinar, 1999).
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The male flowers described in this study are the first confirmed fossil flowers of
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the family Myristicaceae and represent one of few fossil collections distinctly of
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Myristicaceae origin. Virola dominicana is known only from these 24 flowers entrapped
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in 17 pieces of mid-Tertiary Dominican amber. The spreading petal lobes of V.
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dominicana resemble the modern day species of V. surinamensis, V. multinervia, V.
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duckei and V. flexuosa. However, these extant species have mostly stellate trichomes on
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the lobes (Smith, 1937; Croat 1978), as do other members of the genus (Sabatier, 1997),
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whereas the lobes of V. dominicana appear to possess simple or few-branched trichomes.
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The stamen column (anther length) of V. dominicana (0.3-0.8 mm) is considerably
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shorter than most extant Virola species (0.6-1.9 mm as measured in 8 extant Virola
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species [Smith, 1937; Steeves, 2011]). The shape and size range of the pollen of V.
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dominicana (23-27 µm) falls within the range of members of the genus (22-35
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µm)(Walker and Walker, 1979). The two-petaled (or two-tepaled) flower (Figs. 2A and
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2B) may represent a developmental anomaly as flowers comprised of two petal lobes are
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rarely observed in extant Virola (Steeves, personal observations). We may have found 24
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male Virola flowers and no female flowers as extant species have staminate flowers that
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are easily deciduous owing to their thinner and longer pedicels compared to thick and
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stout pistillate flowers that rarely fall to the ground (Steeves, personal observation).
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A molecular study estimated the crown group of the Myristicaceae to have a
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Miocene origin based on a molecular clock (Doyle et al. 2004), which is not incongruous
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with the estimated age of the present fossils. However, this relatively recent molecular
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age estimate is surprising given the pantropical distribution and relatively poor oceanic
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dispersal potential of the Myristicaceae and the earlier estimates of crown group
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divergence (~100-120 mya) of closely related families such as the Annonaceae (Doyle et
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al. 2004). This relatively young age estimate for the Myristicaceae could be an artifact of
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their poor fossil representation and/or a result of a slower rate of molecular evolution
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compared to closely related families, leading to a mis-calibration in the molecular clock
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estimate. Both the former and latter explanations are plausible given the paucity of fossil
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evidence of the family and the fact that molecular investigations employing some of the
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most variable nuclear and chloroplast DNA markers have found low levels of molecular
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divergence among genera and even species of Myristicaceae (Sauquet et al. 2003; Doyle
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et al. 2004, Steeves 2011). The fossil flowers described in this study place Virola in the
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neotropical region sometime between 45-15 mya depending on age estimates for
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Dominican amber (Iturralde-Vinent and MacPhee, 1996; Schlee 1990).
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Extant members of Virola are of significant ecological and ethnobotanical
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importance in the neotropics. Virola spp. are considered one of the 5-10 most abundant
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fruit producing tree genera in South American (Pitman et al. 2001; Pitman et al. 2002).
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The bright orange-red, fat-rich aril fruits are highly nutritious and are highly sought after
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by birds and spider monkeys (Ateles paniscus).
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Pollinators of Virola have been little studied. Beetles and thrips have been
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observed visiting flowers (Steeves, 2011), however no representatives of these groups
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occurred among the various insects found together with Virola flowers in the examined
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Dominican amber samples. Planthoppers occurred with Virola flowers in several amber
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pieces (Fig. 3) and may have been feeding on the perianth, possibly obtaining glandular
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secretions in the flower and/or feeding on the plants sap. Both worker and alate ants were
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associated with several Virola flowers in the amber samples. One of the identifiable ant
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species present in the amber along with V. dominicana is a worker Cephalotus squamosus
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(Hymenoptera:Formicidae) (Fig. 4). These ants are polyphagous but do include pollen in
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their diet and take pollen back to the nest for larvae and nestmates. Ants visiting male
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flowers could transfer pollen to female flowers, especially if the latter have extrafloral
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nectaries, which Cephalotes ants are known to visit (de Andrade and Baroni Urbani,
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1999). Although extant Virola spp. are not known to possess extrafloral nectaries, ants
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have been observed on the male flowers of extant Virola calophylla (Steeves, 2011) and
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could conceivably transfer the pollen to nearby female trees. The small and rather
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inconspicuous flowers of Virola spp. are unlikely to be predominantly visually attractive
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to pollinators. The flowers do, however, emit pungent odours (similar in smell to lilacs)
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that appear to attract insects even after male flowers have fallen to the ground (Steeves,
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personal observation). It is possible that the insects observed in the inclusions inspected
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for this study represent individuals who were haphazardly trapped in the resin, but their
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close proximity to the flowers (particularly Cephalotes squamosus, Fig. 4) may indicate
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that the attraction of the fallen Virola dominicana flowers is what led to their entrapment.
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The Dominican amber forest was categorized as a tropical moist forest unlike any
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other found in the World today. During the Pliocene-Pleistocene global cooling period,
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many of the stentopic biota of the forest, such as stingless and orchid bees, Mastotermes
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termites and various plants, including Hymenaea protera that produced the amber, were
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trapped by the island’s isolation. Refugia were apparently rare or inadequate for many
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life forms (Poinar and Poinar, 1999). It is likely that V. dominicana disappeared from
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Hispaniola during this cooling period. Although there are no native members of
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Myristicaceae in Hispaniola today (Liogier 1983), Virola surinamensis is native to nearby
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Guadeloupe southward to Grenada in the Lesser Antilles (Smith 1937). The mainland
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distribution of contemporary Virola taxa encompasses lowland and montane rain forest
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habitats (<1500m elevation) from Guatemala to the southern and eastern extent of the
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Amazon Basin and the western coast of Ecuador (Smith 1937).
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This study describes the first fossil flowers of the Myristicaceae, Virola
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dominicana Poinar and Steeves preserved in mid-Tertiary Dominican amber. The 24
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male flowers preserved in amber provide a glimpse into the reproductive biology of a
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genus whose contemporary pollinators are not well established to date. The placement of
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Virola in the Neotropical region during the mid-Tertiary will undoubtedly help future
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studies of the biogeography and evolutionary biology of these ecological and
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ethnobotanically significant trees.
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Acknowledgments
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The authors thank Cesare Baroni Urbani for identifying the species of Cephalotes in Fig.
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4 and Sarah Baldwin for reviewing an earlier version of the manuscript.
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Figures
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Fig. 1. Virola dominicana sp. nov. holotype showing stubby trichomes on the lobe
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blades and long trichomes on the lobe margins. Note also white pollen grains within
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the flower diameter. Bar = 800 µm.
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Fig. 2. Male flowers, indument and pollen of Virola dominicana. (A) Lateral view
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of paratype of Virola dominicana in Dominican amber. Bar = 850 µm. (B) Frontal
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view of two-lobed flower of Virola dominicana in Dominican amber. Bar = 480 µm.
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(C) Long, straight trichomes on the margin of a perianth lobe of the Holotype of
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Virola dominicana in Dominican amber. Bar = 105 µm. (D) Pollen grains of Virola
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dominicana in Dominican amber. Note upper boat-shaped grain. Bar = 23 µm. (E)
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Lateral view of two-lobed flower of Virola dominicana in Dominican amber. Bar =
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28 µm. (F) Detail of pollen grains of the Holotype of Virola dominicana in
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Dominican amber. Bar = 22 µm.
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Fig. 3. A planthopper (Hemiptera: Fulgoroidea) adjacent to a paratype flower of
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Virola dominicana in Dominican amber. Bar = 490 µm.
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Fig. 4. A worker Cephalotes squamosus (Hymenoptera: Formicidae) with its head in
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an open paratype flower of Virola dominicana in Dominican amber. Bar = 1.4 mm.
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