| Dendromaia | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Family: | †Varanopidae |
| Genus: | †Dendromaia Maddin, Mann, & Hebert, 2019 |
| Type species | |
| †Dendromaia unamakiensis Maddin, Mann, & Hebert, 2020 | |
Dendromaia is an extinctgenus ofvaranopid from theCarboniferous ofNova Scotia. It contains a singlespecies,Dendromaia unamakiensis.[1]Dendromaia is the oldest known varanopid and the only one found in Nova Scotia. Known from a large partial skeleton preserved with its tail wrapped around a much smaller partial skeleton,Dendromaia may also represent the oldest known occurrence ofparental care in the fossil record. While the larger skeleton possessed certain mycterosaurine-like features, the smaller skeleton resembled basal varanopids such asArchaeovenator andPyozia, creating uncertainty over whether characteristics at the base of Varanopidae have legitimatephylogenetic significance or instead reflect the immaturity of basal varanopid specimens.[1]
Dendromaia unamakiensis is known from a slab and counterslab containing two skeletons. The specimen,NSM017GF020.001, was discovered in a petrifiedlycopod stump atPoint Aconi onCape Breton Island inNova Scotia. This site is part of theSydney Mines Formation, which is dated to the lateMoscovian stage of theCarboniferous period, 309-306 million years ago.[2]"Dendromaia" roughly translates to "the mother in the tree",[3] according to its discovery in a stump and proposedparental care. Thespecific name referencesUnama'kik, theMi'kmaq name for Cape Breton Island. The genus and species were described byHillary Maddin,Arjan Mann, andBrian Hebert in 2019.[1] Hebert had discovered the specimen in 2017.[3][4]
The two skeletons had different sizes and preserved different areas, with the larger skeleton (designated theholotype) incorporating a large portion of the rear half of the body and the smaller skeleton (designated theparatype) including a skull and only fragments of the postcranium, 1/4th the size of the larger one. These skeletons were inferred to represent two individuals of a single newtaxon based on their similar overlapping anatomy, apparentvaranopid ancestry, close association, and the fact that there are no other varanopids known from Nova Scotia.[1]
The 6 preserveddorsal vertebrae of the larger skeleton were among the most characteristic bones in the animal. They shared several traits withMycterosaurus, such as tall rectangular neural spines with slight depressions at their base. However, the rib facets at the tip of the transverse processes were not vertical, but instead oriented diagonally. Like other non-varanodontinevaranopids, the lower edge of each centrum had a rounded keel. 10 incomplete caudal vertebrae preserved on the counterpart were elongated and had low neural spines. Holocephalous (single-headed) ribs andHeleosaurus-likegastralia were also present, thoughosteoderms were seemingly absent. The plate-like bones composing thepelvis were not fused to each other, andDendromaia had a large pubic foramen like that ofHeleosaurus. Thefemur was lightly-built and twisted, similar tomycterosaurines. The rest of the leg and foot was present but incomplete.[1] The larger specimen may have been 20 to 30 centimeters (7.9-12 inches) long from the tip of the snout to the base of the tail, and its full tail length is unknown but likely elongated.[4][5]
The small skeleton's poorly-preserved skull was triangular and pointed like those ofArchaeovenator,Pyozia, andHeleosaurus. It had thin, curved teeth on themaxilla (without a canine region) and smaller teeth on thepalate, which also possessed a varanopid-likepterygoid and cultriform process. Isolatedhyoid bones were also identifiable. Like the larger skeleton, the small skeleton's vertebrae had a rounded keel on their underside. It also possessed several limb bone fragments, including a twistedhumerus.[1]
Dendromaia is the oldest member of a family ofamniotes known asvaranopids. Varanopids are traditionally considered earlysynapsids (part of the lineage of amniotes leading to mammals), although some studies alternatively propose that they arediapsid reptiles related toOrovenator.[6][7] The describers ofDendromaia prefer a position for varanopids among synapsids, citing both biological characteristics (parental care) and arguing that traits which link varanopids to diapsids may be based on oversampling of juvenile specimens at the base of Varanopidae.[1] Recent studies from the early 2020s also support their traditional placement as synapsids on the basis of high degree of bone labyrinth ossification, maxillary canal morphology and phylogenetic analyses.[8][9][10]
To investigate the position ofDendromaia among other varanopids, the describers adapted aphylogenetic analysis matrix previously utilized by Brocklehurst & Fröbisch (2018).[11] The resulting strict consensus tree (average result of most parsimonious trees) of theparsimony analysis placedDendromaia in apolytomy near the base of Varanopidae, along withPyozia and aclade which forks into Varanodontinae and Mycterosaurinae. TheBayesian analysis recovers a similar result, albeit with the polytomy resolved, placingDendromaia as thesister taxon toPyozia.[1] The followingcladogram is based on the results of the Bayesian analysis in Maddin, Mann, & Hebert (2019):[1]
| Varanopidae |
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The delicate preservation of the skeletons indicated that they likely died and were quickly buried at the same place and time. They were positioned with the smaller skeleton encircled by the tail of the larger skeleton. Thesetaphonomic qualities led the paleontologists who described them to propose that the two skeletons weredenning together under the roots of alycopod tree, with the smaller skeleton likely representing the offspring of the larger skeleton. This may be the oldest fossil evidence ofparental care, predating the previously oldest evidence, an aggregation of varanopids from thePermian ofSouth Africa which may represent their own species (Microvaranops parentis)[12] or specimens ofHeleosaurus scholtzi.[1] On the other hand, it remains a possibility that the two skeletons were not close relatives, and instead simply sheltered from a storm in the same stump.[3] The anatomy of the juvenileDendromaia skeleton is remarkably similar to that ofbasal varanopids such asArchaeovenator andPyozia, indicating that these genera may be based on juvenile specimens of larger varanopids. Likewise, certain characteristics (such as reduced dentition and limb development) found to link basal varanopids with diapsids may be a consequence of juvenile specimen sampling, rather than validphylogenetic signals.[1]
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