Reversals in this taxon make it interesting.
Pholidocercus hassiacus (Fig. 1; von Koenigswald & Storch 1983; HLMD Me 7577; Middle Eocene) is a member of the rabbit/rodent/multituberculate clade Glires, but without the large anterior incisors that are found in most other members. This is a reversal hearkening back to basal placentals.
Three upper molars are present,
as in primates, and basal members of Glires, like Ptilocercus, the tree shrew. Other hedgehogs have only two upper molars.
Four upper premolars are present,
one more than in basal placentals and other hedgehogs.
Other hedgehogs have a stub for a tail.
Yet another reversal, Pholidocercus has a long, tail. It is bony and armored, analogous to that of an armadillo (genus: Dasypus). Sister hedgehogs have just a stub for a tail. The curling of all hedgehogs for defense also recalls the spinal flexion of armadillos for defense. This is a trait basal therian mothers originally used to help guide their newborns from birth canal to teat.
Those sacral neural spines
(Fig. 1) are taller than in sister taxa. Armadillos also have tall sacral spines.
The clade Lipotyphyla, according to Wikipedia
“is a formerly used order of mammals, including the members of the order Eulipotyphla as well as two other families of the former order Insectivora, Chrysochloridae and Tenrecidae. However, molecular studies found the golden moles and tenrecs to be unrelated to the others.”
The clade Eulipotyphyla, according to Wikipedia
“comprises the hedgehogs and gymnures (family Erinaceidae, formerly also the order Erinaceomorpha), solenodons (family Solenodontidae), the desmans, moles, and shrew-like moles (family Talpidae) and true shrews (family Soricidae).”
The clade Erinaceidae, according to Wikipedia
“Erinaceidae contains the well-known hedgehogs (subfamily Erinaceinae) of Eurasia and Africa and the gymnures or moonrats (subfamily Galericinae) of South-east Asia.”
When you come across a taxon like Pholidocercus
first you eyeball it and declare it a… a… well, there are so many reversals here that it is best to avoid pulling a Larry Martin and just add it to a wide gamut phylogenetic analysis to let a large suite of traits decide for themselves based on maximum parsimony. Luckily the LRT had enough taxa to nest Pholidocercus with confidence with the hedgehogs, despite the several distinguishing traits and reversals.
Just added to the LRT:
Echinosorex, the extant moonrat. It also has a long tail and nests with Pholidocercus.
von Koenigswald W and Storch Gh 1983. Pholidocercus hassiacus, ein Amphilermuride aus dem Eozan der “Grube Messel” bei Darmstadt (Mammalia: Lipotyphla). Senchenberg Lethaia 64:447–459.