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Corresponding author: Chen‑Yang Cai ( cycai@nigpas.ac.cn ) Academic editor: Ricardo Pérez-de la Fuente
© 2021 Yan-Da Li, Erik Tihelka, Zhen-Hua Liu, Di-Ying Huang, Chen‑Yang Cai.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract
The cryptic slime mold beetles, Sphindidae, are a moderately diverse cucujoid beetle family, whose members are obligately tied to slime molds throughout their life. The fossil record of sphindid beetles is sparse; stem-sphindids and crown-group members of uncertain systematic placement have been reported from Cretaceous ambers. Here we review the Mesozoic fossil record of Sphindidae and report a new sphindid genus and species, Trematosphindus newtoni gen. et sp. nov., from Albian/Cenomanian amber from northern Myanmar (ca. 99 Ma). Trematosphindus is set apart from all other sphindids by the presence of distinct lateral cavities on the anterior pronotal angles. Our phylogenetic analysis identifies Trematosphindus as an early-diverging genus within Sphindidae, sister to the remainder of the family except Protosphindus, or Protosphindus and Odontosphindus. The new fossils provide evidence that basal crown slime mold beetles begun to diversify by the mid-Cretaceous, providing a valuable calibration point for understanding timescale of sphindid co-evolution with slime molds.
Sphindidae, Trematosphindus, myxomycetes, Cretaceous, Burmese amber
The family Sphindidae, cryptic slime mold beetles, is a group of widespread beetles belonging to the diverse and, as currently conceived, paraphyletic polyphagan superfamily “Cucujoidea” (
The fossil record of Sphindidae is very sparse. All putative pre-Quaternary sphindid fossils were reported from amber deposits.
Here, we report a new sphindid genus and species from mid-Cretaceous Burmese amber, Trematosphindus newtoni gen. et sp. nov., adding to our knowledge on the Mesozoic diversity of this family.
The Burmese amber specimens studied herein (Figs
Photographs under incident light were taken with a Zeiss Discovery V20 stereo microscope. Widefield fluorescence images were captured with a Zeiss Axio Imager 2 light microscope combined with a fluorescence imaging system. Confocal images were obtained with a Zeiss LSM710 confocal laser scanning microscope, using the 488 nm Argon laser excitation line. Images under incident light and widefield fluorescence were stacked in Helicon Focus 7.0.2 or Zerene Stacker 1.04. Confocal images were stacked with colour coding for depth in ZEN 2.3 (Blue Edition), or without colour coding in Helicon Focus 7.0.2. Microtomographic data were obtained with a Zeiss Xradia 520 Versa 3D X-ray microscope at the micro-CT laboratory of
To evaluate the systematic placement of the new species, a morphological phylogenetic analysis was performed using both parsimony and Bayesian inference. The data matrix was mainly derived from a previously published dataset (
Parsimony analysis was performed under implied weights using the program TNT 1.5 (
A Bayesian inference for morphological traits was conducted using MrBayes 3.2.6 (
Trees were drawn with the online tool iTOL 5.7 (
The following abbreviation of institution is used:
Order Coleoptera Linnaeus, 1758
Suborder Polyphaga Emery, 1886
Superfamily Cucujoidea Latreille, 1802
Family Sphindidae Jacquelin du Val, 1860
(For references of high-rank taxon names, see
Trematosphindus newtoni sp. nov.
The generic name is composed of the Greek “trema”, hole, and generic name Sphindus, in reference to the cavity at each anterior pronotal angle. The name is masculine in gender.
Head without any distinct grooves. Antennae 11-segmented. Pronotum with large cavities at anterior pronotal angles. Pronotal lateral edges dentate. Procoxal cavities closed externally. Elytra without raised carinae. Pygidium with a distinct median longitudinal groove.
Trematosphindus is somewhat similar to several families in (or formerly in) the broadly defined Cucujoidea (e.g., Biphyllidae, Cryptophagidae, Boganiidae, and Protocucujidae) in the general habitus, shape of antennal club, or the presence of cavities/glandular pores. Boganiidae and Cryptophagidae can be easily ruled out as potential relatives of Trematosphindus, based on their externally open procoxal cavities (procoxal cavities externally closed in Trematosphindus and Sphindidae). Biphyllidae (now in Cleroidea) and also Cryptophagidae can be distingushed from Trematosphindus by their laterally closed mesocoxal cavities (mesocoxal cavities laterally open in Trematosphindus and Sphindidae). As the sister taxon of Sphindidae, the monogeneric family Protocucujidae shares many features with Trematosphindus (and other Sphindidae,
The species is named after Dr. Alfred F. Newton, an authority on coleopteran systematics.
Holotype, NIGP175114, female. Paratype, NIGP175115, sex unknown.
Amber mine located near Noije Bum Village, Tanai Township, Myitkyina District, Kachin State, Myanmar; unnamed horizon, mid-Cretaceous, Upper Albian to Lower Cenomanian.
As for the genus.
Body narrowly oval, convex. Surface with moderately large, rounded punctures and hair- to scale-like setae. — Head (Fig.
NIGP175114: BL 2.40 mm, BW 1.12 mm, HL 0.67 mm, HW 0.77 mm, PL 0.71 mm, PW 1.05 mm, EL 1.53 mm. NIGP175115: BL 1.90 mm, BW 0.75 mm, HL 0.50 mm, HW 0.55 mm, PL 0.54 mm, PW 0.73 mm, EL 1.16 mm.
Details of Trematosphindus newtoni gen. et sp. nov., holotype, NIGP175114, under confocal microscopy. A: Head, anteroventral view. B: Prothorax, ventral view. C: Fore legs. D: Posterior portion of metathorax and anterior portion of abdomen, ventral view. E: Posterior portion of abdomen, ventral view. F: Head, dorsolateral view, with arrowhead showing the cavity at anterior pronotal angle. G: Prothorax, dorsal view. H: Elytral base, dorsal view. I: Elytral apex and pygidium, posterodorsal view, with arrowhead showing the distinct median groove on pygidium. Abbreviations: an, antenna; cl, clypeus; el, elytron; fr, frons; lbp, labial palp; md, mandible; mtf, metafemur; mttb, metatibia; mtts, metatarsus; mtv, metaventrite; mxp, maxillary palp; pc, procoxa; pf, profemur; pn, pronotum; ps, prosternum; ptb, protibia; sc, scutellum; v1–5, ventrites 1–5. Scale bars: 200 μm.
Details of Trematosphindus newtoni gen. et sp. nov., paratype, NIGP175115, under confocal microscopy, with depth color-coding. A: Head and prothorax, dorsal view. B: Prothorax, dorsal view, with arrowhead showing the cavity at anterior pronotal angle. C: Elytral base, dorsal view. D: Elytral apex, dorsal view. E: Head and prothorax, ventral view. F: Fore legs. Abbreviations: an, antenna; el, elytron; lbp, labial palp; md, mandible; msf, mesofemur; mxp, maxillary palp; pf, profemur; pn, pronotum; ptb, protibia; pts, protarsus; sc, scutellum. Scale bars: 200 μm.
The parsimony analysis under implied weights yielded two most parsimonious trees, where the placement of Notosphindus McHugh & Wheel differed (Fig.
A notable character distinguishing Trematosphindus from all extant and fossil members of Sphindidae is the presence of a large oval cavity at each anterior pronotal angle (Figs
Slime molds are ubiquitous in humid substrates such as most wood, soil, and dung worldwide (
The original confocal and micro-CT data are available in Zenodo repository (http://doi.org/10.5281/zenodo.5579977).
The authors have declared that no competing interests exist.
We are grateful to Richard A. B. Leschen and Steven L. Stephenson for the help discussion, and Joseph V. McHugh and one anonymous reviewer for the detailed comments on the earlier version of this paper. We also thank Su-Ping Wu for technical help in micro-CT reconstruction, and Yan Fang for technical help in confocal imaging. Financial support was provided by the Second Tibetan Plateau Scientific Expedition and Research project (2019QZKK0706), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB26000000 and XDB18000000), and the National Natural Science Foundation of China (41688103).
Character list
Data type: .rtf
Explanation note: List of characters used in the phylogenetic analyses (adapted from
Morphological dataset
Data type: .nex
Explanation note: Morphological dataset used for the analyses.