1. Introduction

Bees are an important component in the evolutionary history and diversification of the angiosperms. The great majority of bees depend on pollen, nectar, oils, waxes, scents and resins from flowering plants for adult and larval nutrition, sexual attractants and nest construction (Renner 2006; Michener 2007). Various morphological adaptations have evolved in bees to enable them to recognize, collect and utilize these plant products (Danforth et al. 2006). The still unanswered question is what basic morphological modifications apoid wasps underwent to change their habits from carnivory to palynivory. And what, if any, additional morphological and physiological alterations in both adults and developmental stages, accompanied this dietary change.

While the fossil record of bees is quite extensive, the great majority are from Cenozoic deposits and have features very similar to extant generic lineages (Zeuner & Manning 1976; Michener 2007). However, amber fossils from the mid-Cretaceous can tell us what morphological changes certain lineages of apoid wasps underwent as they became palynivores.

While floral elements in the mid-Cretaceous Myanmar amber tropical forest were mostly conifers, ginkgos, ferns, liverworts, lycopods, cycads and horsetails, a diverse selection of small flowered angiosperms, many of which are in extinct families, had made their appearance (Poinar 2018a). In this amber forest, as angiosperms were diversifying, primitive bees were evolving from apoid wasps (Danforth & Poinar 2011).

One such stem lineage bee in Burmese amber was a female, described below as Discoscapa apicula gen. et sp. nov. in the new family Discoscapidae (Hymenoptera: Apoidea). This fossil displays both Antophila bee as well as apoid wasp features. Based on its habitus and morphology, including plumose hairs, a rounded pronotal lobe, middle and hind leg scopae and a pair of spurs on the hind tibia, the female bee falls within the monophyletic group that currently includes all living bees that utilize pollen for development of their brood (Michener 2007). Along with these bee characters are narrow hind basitarsi, low placed antennal sockets and wing venation patterns that are characteristic of apoid wasps.

A unique diagnostic character of the new genus not found in either apoid wasps or bees is a bifurcated antennal scape. Pollen grains in scopae on the femur and tibia of the middle and hind legs and on the claw and tarsus of the middle leg show that Discoscapa had recently visited one or more flowers in anthesis. A list of small flowers described from Burmese amber that Discocapa could have visited is presented (Table 1). Additional evidence that the fossil bee had visited flowers is the presence of twenty-one beetle triungulins in the same piece of amber. It is possible that the large number of triungulins caused the bee to accidently fly into the resin.

2. Material and methods

The single female specimen of Discoscapa apicula is positioned at the edge of a small piece of amber. The specimen was obtained from a mine first excavated in 2001, in the Hukawng Valley, southwest of Maingkhwan in Kachin State (26°20′N, 96°36′E) in Burma (Myanmar). This amber site, known as the Noije Bum 2001 Summit Site, was recently assigned to the mid-Cretaceous on the basis of paleontological evidence, placing the age at 97 to 110 Mya (Cruickshank & KO 2003). A more recent study using U-Pb zircon dating determined the age to be 98.79 ± 0.62 Ma, at the Albian/Cenomanian boundary (SHI et al. 2012). Nuclear magnetic resonance (NMR) spectra and the presence of araucaroid wood fibers in amber samples from the Noije Bum 2001 Summit site indicate an araucarian (possibly Agathis) tree source for the amber (POINAR et al. 2007b). Observations and photographs were made with a Nikon SMZ-10 stereoscopic microscope and Nikon Optiphot optical microscope (with magnifications up to 650x). Helicon Focus Pro X64 was used to stack photos for better overall clarity and depth of field.

3. Systematic palaeontology

Some portions of the specimen are damaged, missing or concealed. Part of the face beneath the ocelli had been scraped away, along with the left antenna. Other than the mandibles, the mouthparts are concealed. The terminal tarsal segments of the right hind leg are missing. Wing venation nomenclature and other terminology used in the present work are based on formats displayed in Michener at al. (1994).

Table 1.

Size of flowers described from Burmese amber

Fig. 1.

Discoscapa apicula gen. et sp. nov. in Burmese amber. A – Right lateral view of body. Arrows show four associated beetle triungulins. Arrowhead shows scopae on hind leg femur. Bar = 0.65 mm. Insert shows rounded pronotal lobe (arrow). Bar = 0.70 mm. B – Left lateral view of body. Arrow shows scutum. Bar = 0.62 mm.

Fig. 2.

Discoscapa apicula gen. et sp. nov. in Burmese amber. A – Bifurcated scape. T= torulus; R = radicle; Sc = scape; P = pedicle; S = spur. Arrowhead shows velum with wax-like deposit lining curvature. Bar =106 µm. B – Foreleg strigilis. S = spur, C = comb with velum. Bar = 60 µm.

Fig. 3.

Discoscapa apicula gen. et sp. nov. in Burmese amber. A – Forewing. Arrow shows tegula. Bar = 440 µm. B – Dorsal view of mesosoma. P = pronotum, Sc = scutum, S = scutellum. Bar = 320 µm.

Fig. 4.

Discoscapa apicula gen. et sp. nov. in Burmese amber. A – Forewing. B = Basal vein, C = costa, P = prestigma, R= radius, S= stigma, V= v vein, 1Tc = first transverse cubital vein, 2Tc = second transverse cubital vein, 3Tc = third transverse cubital vein, 1sm = first submarginal cell, 2sm = second submarginal cell, 3sm = third submarginal cell, 1Rv = first recurrent vein, 2Rv = second recurrent vein, r = r vein. Bar = 520 µm. B – Hind wing. R = radius vein, Rs = radial sector, M = medial vein, Cu = cubitus vein, V = V vein, Jl = jugal lobe, Vl = vannal lobe. Bar = 420 µm. C – Right antenna. T = torulus; R = radicle; Sc = scape; P = pedicle; S = spur; C = area of curvature with wax-like deposit on velum. Bar = 146 µm. D – Mandible. Bar = 125 µm. E – Frontal view of head. Arrows show low positioned antennal sockets. C = clypeus; L = labrum. Bar = 300 µm.

Fig. 5.

Discoscapa apicula gen. et sp. nov. in Burmese amber. A – Lateral view of terminal tarsal segments and claw. Arrows show pollen clusters attached to tarsomeres. Bar = 93 µm. B –Ventral view of claws. Arrowhead shows bi-lobed arolium. Bar = 62 µm. C – Hind leg. Bar = 400 µm.

Fig. 6.

Tibial spurs of Discoscapa apicula gen. et sp. nov. in Burmese amber. A – Single spur on mesotibia. Bar = 108 µm. B – Paired spurs on metatibia. Bar = 84 µm.

Fig. 7.

Discoscapa apicula gen. et sp. nov. in Burmese amber. A – Scopa at femur-tibial junction of midleg. Bar = 130. B – Plumose hairs on midleg femur. Bar = 55 µm. C – Pygidium. Arrow shows sting. Bar = 95 µm.

Fig. 8.

Discoscapa apicula gen. et sp. nov. in Burmese amber. A – Plumose hairs on hind leg basitarsus. Bar = 45 µm. B – Plumose hairs on hind leg tibia. Bar = 35 µm.

Fig. 9.

Pollen catching hairs with surrounding pollen grains on the hind leg femur of Discoscapa apicula gen. et sp. nov. in Burmese amber. Bar = 213 µm. Insert shows branches on hairs. Bar = 50 µm.

Fig. 10.

A – Pollen grains associated with pollen catching hairs of Discoscapa apicula gen. et sp. nov. in Burmese amber. Bar = 38 µm. B – Pollen grains of the Burmese amber angiosperm flower Dispariflora robertae. Bar = 16 µm.

4. Discussion

Discoscapa is the second primitive, stem lineage bee to be described from Burmese amber. The first, Melittosphex burmensis Poinar & Danforth (2006), also retained some wasp features and was placed in a separate family, the Melittosphecidae. The three submarginal cells of Discoscapa, as well as the bifurcated scape and low antennal insertions, distinguish it from Melittosphex (POinar & Danforth 2006; Danforth & Poinar 2011).

Considering the morphological variation present today among the various bee genera, it is likely that many of these lineages evolved independently from different lineages of apoid wasps. Another early bee lineage, based on a phylogenomic analysis of the Apoidea, is thought to have evolved from the apoid wasp lineage, Ammoplanina (Crabronidae), which is considered by Sann et al. (2018) as the closest extant relative of bees.

While it may not fit within the crown group of modern bees, Discoscapa apicula was obviously a pollen-collecting species based on the presence of large amounts of pollen in the plumose hairs on the hind femur (Figs. 9, 10A). Melittosphex and Discoscapa are now the oldest known stem lineages of pollen collecting Apoidea and provide insights into morphological features that existed during the transition from predatory wasps to pollen collecting bees. The small size of Discoscapa and Melittosphex place them among the so-called “minute” bees, which are well adapted for visiting correspondingly small flowers since large bees have difficulty obtaining nectar and pollen from small solitary flowers (Stephen et al. 1969).

Pollen in branched hairs on the hind leg femur (Fig. 9) shows that Discoscapa had recently visited one or more flowers in anthesis. Pollen grains attached to the mid-leg tarsi and claws (Fig. 5A) could have arrived there when the bee was visiting the flowers or was cleaning its dorsum since many bees are known to clean their backs with their middle legs (Jander 1976).

A number of very small angiosperm flowers, ranging in size from 0.8 mm to 5.1 mm, have been described from Burmese amber (Table 1). Pollen grains on the hind femur of Discoscapa (Fig. 9) range from 10 µm to 15 µm in diameter, which fall within the size range of pollen grains from the following Burmese amber flowers: Dispariflora robertae Poinar & Chambers 2019c, Eoepigynia burmensis Poinar et al. 2007a, Antiquifloris latifibris Poinar et al. 2016, Micropetasos burmensis Poinar et al. 2013. and Exalloanthum burmense POinar 2018a. Of the above flowers, the pollen on Discoscapa most closely resembled the size, shape and configuration of pollen grains from Dispariflora robertae Poinar & Chambers 2019c (Fig. 10A, B).

Other evidence that Discoscapa had recently visited flowers before it was captured in resin is the presence of 21 beetle triungulins in the same amber piece, some of which are still in contact with Discoscapa (Fig. 1A). It is well known that active first stage beetle triungulins of the families Meloidae, Rhipiphoridae and Cleridae gather in flowers and attach themselves to visiting bees. The triungulins are then carried back to the nest where they feed on the developing stages of the bees and stored provisions (nectar and pollen) (Clausen 1962, 1976; Falin et al. 2000). These triungulins will be described in a separate study.

In their molecular study on early bee diversification (Danforth et al. 2006), the results suggested an African origin for bees since the earliest branches included predominantly African lineages. This implies that bees may have evolved in Gondwana. Since it has been proposed that Burmese amber originated in Gondwana along the northern part of what is now Australia (Poinar 2018b), it is possible that bees in Burmese amber represent the earliest known lineages of these pollinators. Danforth et al. (2006) also concluded that their results showed that the earliest bee lineages were narrow host-plant specialists, which suggests that Melittosphex and Discoscapa may have specialized on one or two species or genera of the Burmese amber angiosperms listed in Table 1.