A new species of the bee genus Ctenoplectrella in middle Eocene Baltic amber (Hymenoptera, Megachilidae)

Abstract A new species of the extinct bee genus Ctenoplectrella Cockerell (Megachilinae: Ctenoplectrellini) is described and figured from two females preserved in middle Eocene (Lutetian) Baltic amber. Ctenoplectrella phaeton sp. n. is distinguished from its congeners on the basis of its body proportions, integumental sculpturing, wing venation, and pubescence, and is one of the more distinctive members of the genus. A revised key to the species of Ctenoplectrella is provided.


Introduction
The Eocene was the last epoch harboring a truly disparate bee fauna relative to the composition of forms we are so familiar with in today's ecosystems. Following the Eocene-Oligocene transition the bee fauna began to look relatively modern at least in terms of the general appearance of the tribes, genera, and subgenera comprising the diversity in the latest Paleogene and Neogene periods. However, from the Eocene and earlier epochs we find regularly taxa that harbor unique combinations of traits that render them challenging to fit amongst their modern counterparts even at higher taxonomic levels, alongside otherwise more modern forms (Engel 1998(Engel , 2001(Engel , 2004(Engel , 2008(Engel , 2011Wappler and Engel 2003;Engel and Perkovsky 2006;Ohl and Engel 2007;Patiny et al. 2007;Michez et al. 2007Michez et al. , 2009Wedmann et al. 2009). Those glimpses into the Early Paleogene highlight a diversity of genera and tribes quite distinct, exhibiting not only plesiomorphic features relative to species of today but obviously with unique adaptations and apomorphies which did not persist to the present (e.g., the bizarre facial morphologies of species of Succinapis Engel: vide Engel 2001). Our knowledge of these remarkable bees continues to grow, mostly from European deposits but now also from distant biogeographic regions such as India (Rust et al. 2010;Engel unpubl. data).
Herein we describe a recently recognized new species of the Eocene bee genus Ctenoplectrella Cockerell (1909aCockerell ( , 1909b. The genus was described originally alongside the genus Glyptapis Cockerell in a distinct subfamily, Glyptapinae, which Cockerell (1909b) considered to be "near the stem-form of the Megachilidae" but simultaneously indicated that he believed their closest modern relative to be Ctenoplectra Smith (hence the name he chose for the group of fossil species considered herein). Cockerell's assertion is difficult to understand given that there is actually little morphological similarity between Ctenoplectrella and Ctenoplectra, today recognized to be a distinct lineage of Apidae, and even less so between Glyptapis and the latter. Engel (2001) provided the first revision of the Eocene bee fauna and recognized Ctenoplectrella and Glyptapis as definitive megachilids, and at first placed them within the Osmiini, later elevating them both to tribal status alongside the other megachilines (Engel 2005). All four Glyptapis species, as well as the known five species of Ctenoplectrella, are from Baltic amber (middle Eocene) except for C. zherikhini Engel and Perkovsky, which is known from the Late Eocene amber of the Rovno region of the Ukraine (Table 1).
While Ctenoplectrella and Glyptapis are definite oddities, we have noted a considerable similarity between Ctenoplectrella and the rare living genus Aspidosmia Brauns [presently in Anthidiini (vide Michener 2007), but apparently more basally related among the Megachilinae (Litman et al. in press)], with two species in southern Africa (Brauns 1926;Peters 1972). Both genera share a metatibia with relatively long setae suggesting a scopa and a forewing with a rather elongate prestigma and basal vein strongly arcuate, meeting Cu orthogonally (i.e., at a right angle), and it is likely that they are closely related, with Aspidosmia perhaps representing the sole survivors of the ctenoplectrelline lineage. Naturally, this conclusion requires rigorous cladistic testing but is tantalizing given the number of biogeographic connections between the sub-Saharan or East Asian fauna and extinct taxa from the European Paleogene (e.g., Ander 1942;Petrunkevitch 1958;Larsson 1978;Lourenço and Weitschat 1996;Böhme and Weitschat 1998;Engel 2001;Liu and Engel 2010;Weitschat and Wichard 2010).

Material and methods
Morphological terminology follows that of Engel (2001) and Michener (2007), while the format for the description generally follows that used by Engel (2001Engel ( , 2008 and Engel and Perkovsky (2006). Photomicrographs were prepared using a Nikon D1x digital camera attached to an Infinity K-2 long-distance microscopic lens. Measurements were made with an ocular micrometer attached to an Olympus SZX-12 stereomicroscope and are provided for the holotype, with those of the paratype in parentheses. Paratype. ♀, on curved edge in same amber piece as holotype and with same repository (Figs 1, 4).
Diagnosis. This species resembles Ctenoplectrella cockerelli Engel in the forewing with vein 2rs-m strongly and doubly arcuate, the basal vein confluent with cu-a, the first submarginal cell shorter than the second submarginal cell, and the punctate mesepisternum and terga. However, C. phaeton can be distinguished from C. cockerelli and
Present study C. viridiceps Cockerell, 1909aCockerell 1909a, 1909bEngel 2001 Rovno Amber (Bartonian-Priabonian?) C. zherikhini Engel & Perkovsky, 2006 Engel and Perkovsky 2006 remaining species of the genus by its robust body, punctate metepisternum (impunctate in C. cockerelli), and much shorter and sparser body pubescence. Description. Female: Total body length 5.77 mm (6.15 mm); forewing length 3.85 mm (3.92 mm). Head slightly wider than long; paraocular carina present; pedicel about as long as combined lengths of first and second flagellomeres; interocellar distance 2.5 times median ocellar diameter, 1.5 times longer than ocellocular distance; ocelloccipital distance about 1.6 times median ocellar diameter. Intertegular distance 1.46 mm. Outer surfaces of pro-and mesotibiae apically with small posterior spine. Prestigma relatively short, slightly more than two times longer than broad (prestigma width measured to its margin); basal vein strongly arcuate, confluent with cu-a; second abscissa of Rs basad 1m-cu by about six times vein width; 2rs-m distad 2m-cu by vein width, 2rs-m doubly arcuate; second submarginal cell slightly longer than first submarginal cell; seven distal hamuli, arranged in a single, evenly-spaced series. Sixth metasomal sternum with broadly rounded apical margin.
Integument in general smooth and shiny between punctures, weakly imbricate laterally on terga. Outer surface of mandible with minute punctures separated by a puncture width or less. Frons with small punctures separated by 1-1.5 times a puncture width, punctures becoming denser towards vertex. Pronotum laterally with minute punctures separated by a puncture width or less. Mesoscutum with small punctures separated by 1-2 times a puncture width (Fig. 2); tegula with Metanotum impunctate and smooth. Mesepisternum with faint, scattered, larger punctures than on mesoscutum, nearly impunctate anteriorly to omaulus, punctures denser ventrally. Metepisternum more densely punctate than on mesepisternum, punctures separated by a puncture width or less dorsally, punctures sparse ventrally. Propodeum impunctate basally, lateral and posterior surfaces with minute punctures separated by more than two times a puncture width. Metasomal terga with small punctures separated by 1-2 times a puncture width, without distinct depressed marginal zones; sterna with coarser punctures than on terga, punctures smaller and finer on first sternum.
Color apparently brown, without maculations. Wing membrane hyaline; veins strong and dark brown.
Male: Unknown. Etymology. The specific epithet is taken from Phaeton and treated as a noun in apposition. In Greek mythology Phaeton died when he tried to drive the chariot of the sun across the sky. Phaeton's sisters wept and their tears turned to amber.
Comments. The supraclypeus, clypeus, and mandibles are obscured by dense Schimmel (whitish froth of microscopic bubbles resembling mold) in the holotype and by a fracture in the amber piece in the paratype. However, the strong apical tooth and distinct outer ridge of the mandible is barely visible in the holotype, thus suggesting a similar mandibular shape as in other species of Ctenoplectrella.