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” (Forrester and McHugh 2010). Sphindidae are represented in the Recent fauna by only nine genera and approximately 66 valid extant species (J.V. McHugh, personal communication), though much of their biodiversity remains undocumented. For example, Forrester and McHugh (2007) mentioned that they have identified more than a hundred undescribed species in a single sphindid genus, Aspidiphorus Latreille. As their common name suggests, sphindid beetles feed exclusively on slime molds (myxomycetes) in both larval and adult stages, while most other aspects of their ecology remain elusive (Lawrence and Newton 1980; Burakowski and Ślipiński 1987). Sphindidae appears to be closely related to Protocucujidae, which is supported by multiple lines of morphological and molecular evidence (e.g., Leschen et al. 2005; Robertson et al. 2015; McKenna et al. 2019). Four extant subfamilies have been proposed within Sphindidae based on a morphological phylogenetic analysis, i.e., Protosphindinae, Odontosphindinae, Sphindiphorinae, and Sphindinae (McHugh 1993).

The fossil record of Sphindidae is very sparse. All putative pre-Quaternary sphindid fossils were reported from amber deposits. Kirejtshuk et al. (2015) described five species of Libanopsis Kirejtshuk from the Early Cretaceous Lebanese amber, and assigned them to a new subfamily, Libanopsinae. Later, Kirejtshuk et al. (2019) described the genus Burmops Kirejtshuk from the mid-Cretaceous Burmese amber, and moved the previous described genus Pleuroceratos Poinar & Kirejtshuk from Silvanidae to Sphindidae, treating both as members of the extant subfamily Protosphindinae. However, Pleuroceratos has externally open procoxal cavities (Liu et al. 2019), which is discordant with a placement in Protosphindinae. Tihelka et al. (2020) further found that the morphology of Pleuroceratos is actually characteristic of Phloeostichidae, and confirmed its position in Phloeostichidae with a formal phylogenetic analysis.

Here, we report a new sphindid genus and species from mid-Cretaceous Burmese amber, Trematosphindus newtonigen. et sp. nov., adding to our knowledge on the Mesozoic diversity of this family.

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 Bouchard et al. 2011)

The parsimony analysis under implied weights yielded two most parsimonious trees, where the placement of Notosphindus McHugh & Wheel differed (Fig. 7). The results are consistent with the parsimony analysis under equal weights by McHugh (1993). The Bayesian inference recovered the position of Notosphindus as sister to (Carinisphindus McHugh + Sphindus Chevrolat), though with only a low posterior probability (Fig. 7). In both analyses, the fossil Trematosphindus newtoni, was recovered in a relatively basal position within Sphindidae. In the implied-weighted parsimony analysis, Trematosphindus was recovered as sister to all extant sphindids except Protosphindus, while in the Bayesian inference it appeared to be sister to all extant sphindids except Protosphindus and Odontosphindus LeConte.

Kirejtshuk et al. (2015) described Libanopsis, a genus of putative sphindids, from Lebanese amber. Libanopsis possesses a series of characters different from extant sphindids, including the lack of a clear frontoclypeal suture and (sub)contiguous metacoxae. Thus, Libanopsis may be isolated from other sphindid subfamilies, and possibly represent a stem-group of Sphindidae (Kirejtshuk et al. 2015). Unlike Libanopsis, Trematosphindus represents a crown-group sphindid. Our phylogenetic analysis placed it as the sister taxon of all other extant sphindids except Protosphindus under parsimony, and as sister to all other sphindids excluding Protosphindus and Odontosphindus in the Bayesian analysis. Trematosphindus possesses a combination of apomorphic and plesiomorphic characters. It shares some plesiomorphic characters with Protosphindus, the earliest branching lineage in crown-group Sphindidae, including the absence of supraocular antennal grooves, the non-emarginate lateral margin of the clypeus, and the 11-segmented antenna (also shared with Odontosphindus and Sphindiphorus Sen Gupta & Crowson). However, Trematosphindus differs from Protosphindus in abdominal ventrites 2–4 with a distinct basal band of depressions (Fig. 2E), and the absence of raised carinae on elytra. It differs from Odontosphindus and Sphindiphorus, two other basal sphindid genera, as well as Protosphindus in having a pygidium with a longitudinal median groove (Fig. 3E). This well-defined median groove on pygidium was previously known in Aspidiphorus and Sphindiphorus (Forrester 2003). In other sphindids, the pygidium either entirely lacks any depressions or grooves, or has only an indistinctly defined median depression. The mandibles of Trematosphindus are probably flattened at apex, while in Protosphindus and Odontosphindus the mandibles are broad at apex (McHugh 1993).

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 2F, 3C). External exoskeletal cavities have been widely reported in Coleoptera, and some of them have been suggested as a storage place for fungal transport (Grebennikov and Leschen 2010). Sphindidae are known to feed on myxomycetes (slime molds). Though these large pronotal cavities are absent in extant sphindids, the surface punctures of sphindids have been associated with slime mold spores, and therefore likely play a role in transporting slime molds (McHugh 1990, 1993). As such, the large pronotal cavities in Trematosphindus may have fulfilled a similar function. The presence of surface punctures is a plesiomorphic for the family. Trematosphindus possibly evolved large lateral pronotal cavities later to further increase the efficiency of fungal transport. Alternatively, those cavities may be interpreted as glandular pores on callosity. A somewhat similar glandular callosity at anterior pronotal angles can be found in some Boganiidae (Crowson 1990; Escalona et al. 2015; Cai and Huang 2019) and some Cryptophagidae (Bousquet 1989; Otero and Johnson 2013; Otero and Pereira 2019). Historically, Sphindidae, Boganiidae and Cryptophagidae are all placed in the superfamily Cucujoidea. However, recent phylogenetic analyses recovered that this Cucujoideasensu lato is paraphyletic and contains three separate clades (McKenna et al. 2019; Cai et al., 2021). Each of the three families mentioned above is placed in a different clade, and are therefore only distantly related to each other. Therefore, the openings at the anterior pronotal angles probably evolved independently in these families.

Slime molds are ubiquitous in humid substrates such as most wood, soil, and dung worldwide (Stephenson et al. 2008). While the spores and spore-bearing structures of slime molds provide food for a diverse range of beetles, sphindids stand out as the only family in which all species appear to be obligately associated with this food source (Lawrence and Newton 1980). The fossil record of slime molds is exceedingly scare, owing to the fragile nature of their fruiting bodies. Nonetheless, recent discoveries of exceptionally preserved slime molds in Burmese amber (Poinar and Vega 2019; Rikkinen et al. 2019) suggest that the group was diverse in saproxylic habitats by the mid-Cretaceous. The growing fossil record of Cretaceous sphindids provides corroborating evidence that slime molds were important players in terrestrial ecosystems in the Mesozoic. Future discoveries of fossils sphindids can shed further light on the co-evolution between slime molds and beetles. Because some sphindids display a degree of host specificity (Lawrence and Newton 1980), the fossil record of cryptic slime mold beetles can provide valuable indirect calibration points for inferring the timescale of the slime mold tree of life.

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.