Extinct leptictids arise from marsupial bilbies and give rise to placental sengis in the LRT

Leptictids
Leptictis: Leidy 1868.  Anagale: Simpson 1931.

Sengis
Macroscelides: Smith 1829.  Rhynchocyon: Peters 1847

Click the links above and below to see the images.

Extinct Leptictis now nests with extinct Anagale
and extant Rhynchocyon now nests with extant Macroscelides in the large reptile tree (LRT, 2340 taxa) all basal to anteaters, armadillos, aardvarks, tenrecs and odontocetes.

SO placental sengis are derived from fossil leptictids in the LRT –

AND fossil leptictids are derived from marsuspial peramelids (Macrotis + Perameles = bilbies) in the LRT.

This third transition to placental reproduction
(marsupial bilbys to placental sengis via fossil leptictids) involved only terrestrial taxa.

The second placental transition
(marsupial possums to placental tree shrews) involved only arboreal taxa.

The first placental transition
(monodelphids to Carnivora + (Pantodonta + Primates)) involved arboreal taxa of larger size that quickly split into terrestrial taxa.

The first, second and third labels here and in the LRT are arbitrary
and do not reflect the chronological order.

Novacek 1986
studied the skull of lepticitids and their ‘higher-level classification’. He wrote, “It should be emphasized, however, that the brain structure in macroscelidids and tupaiids is notably contrasting. To sum, few osteological specializations link tupaiids, macroscelidids, and leptictids and, contrary to the opinions of many early workers, there seems little evidence in support of a close affinity between these taxa.”

In the LRT the split between these clades goes back at least to the Early Jurassic, so plenty of time for elements to evolve in different directions. Even so, the fact that Novacek considered (then rejected) their relationship is notable.

Novacek explored various leptictid affinities
without software assistance concluding “At least six dental and cranioskeletal characters support the eutherian relationships of leptictids.” Novacek compared leptictids to Insectivorans, Tupaiids, Macroscelidids, “Archonta”, Anagalids, Lagomorphs, Zalambdalestids, Pseudictopids, Kennalestes, Gypsonictops, Procerberus, Cimolestes, Asioryctes, “Emotheres”, “Preptotheres” and “Other Groups” all without software assist.

Novacek wrote:
“The old concept of a menotyphlan grouping for tupaiids and macroscelidids has been abandoned in most current systematic treatments. A few remarks on the Menotyphla in the context of this discussion seem warranted because leptictids have been variously cited as ancestral, collateral, or somewhere intermediate to tree shrews and elephant shrews.” (= sengis)

Menotyphla = elephant shrews + tree shrews, both having a cecum in the large intestine, proposed by Haeckel 1866, now considered obsolete.

Novacek wrote:
“Gregory (1910, p. 262) stated that “Ictops” (a junior synonym for Leptictis) was probably ancestral to menotyphlans due to its possession of such primitive characters as an elongate muzzle, a moderate postorbital constriction, a “rounded” braincase, divided parasagittal crests, a stout, slender zygomatic, and a distinct postglenoid. Obviously, these characters by themselves hardly indicate special relationship with either tupaiids or macroscelidids. Also, they do not account for the subsequently discovered morphological diversity within Leptictidae (for example, parasagittal crests are only present in Oligocene
leptictids).”

“Of special interest in this case is the dual nature of the parasagittal crest in Leptictis as compared to the dual expansion of the parietal to bulbous proportions in Macroscelides.”

Novacek’s comments were made
in the pre-cladistic, pre-computer era (just barely). His ‘dartboard’ approach focusing on shared traits (that could be convergent) was the state of taxonomy and paleontology at the time. Like his contemporaries, Novacek 1986 understood placental genesis occurred only once. Surprisingly, that is still the paradigm in college textbooks in 2026. Novacek wrote, “Eutheria are most effectively defined by the geometry of the reproductive system, the presence of the chorioallantoic placenta, the trophoblast, and aspects of reproductive biology (e.g., prolonged gestation) correlated with these structures.”

Novacek did not cite Wortman 1902, who reasoned the placenta appeared 3x by convergence in Mammalia.

Novacek wrote,
“The central hypothesis of this section is that leptictids should once again be returned to the Insectivora.”

According to Wikipedia – Insectivora “Before the era of widespread DNA sequencing, the grouping was used as a polyphyletic taxon for a variety of small to very small, relatively unspecialised mammals that feed upon insects. Since any primitive-looking fossil group of placental mammals was commonly assigned to this order for convenience, it was held to constitute the basal stock out of which other placental orders had evolved.”

With the help of software in 2026
the LRT precisely nests Leptictis with another genus, Anagale, not an obsolete  suprageneric clade and not under the assumption of a single origin for the placenta.

The LRT is a product of its time. It could not have existed earlier.

Leptictis acutidens
(Leidy 1868, Rose 2006, Novacek 1986, early Oligocene) is an extinct transitional taxon between bilbies and elephant shrews = sengis. The The tiny manus appears to have only three digits, but the medial and lateral digits were tiny and may have been lost during retrieval. Note the large calcaneal heel, a trait usually found in digitigrade running mammals, but also retained in armored Holemesina.

This hypothesis of interrelationships
requires testing with a similar taxon list and whatever set of 200+ characters others may be working with.

References
Gregory WK and Simpson GG 1926a. Cretaceous mammal skulls from Mongolia. American Museum Novitates 225: 1–20.
Gregory WK and Simpson GG 1926b. Cretaceous mammal skulls from Mongolia. Nature 118: 698–699.
Halliday TJD, Upchurch P and Goswami A 2015. Resolving the relationships of Paleocene placental mammals. Biological Reviews. 92 (1): 521–550.
Leidy J 1868. Proceedings of the Academy of Natural Sciences of Philadelphia 20:316.
Matthew WD 1906. The Osteology of Sinopa, a Creodont Mammal of the Middle Eocene.
Proc. U. S. Nat. Mus., Vol. XXX, pp. 203-233, pl. XVI.
McKenna MC 1963. New evidence against Tupaioid affinities of the mammalian family Anagalidae. American Museum Novitates 2158:1-16.
Novacek MJ 1986. The skull of leptictid insectivorans and the higher-level classification of eutherian mammals. Bulletin of the American Museum of Natural History. 183(1):112pp.
Osborn HF 1908. New fossil mammals from the Fayum Oligocene, Egypt. Bulletin of the American Museum of Natural History. 24: 265–272.
Peters WC 1847. Bericht über die zur Bekanntmachung geeigneten Verhandlungen der Königlich Preußischen Akademie der Wissenschaften zu Berlin 12:36.
Rose KD 2006. The postcranial skeleton of early Oligocene Leptictis (Mammalia: Leptictida), with a preliminary comparison to Leptictidium from the middle Eocene of Messel. Palaeontographica Abteilung A, 278(1-6), 37-56.
Shaw G 1800. General zoology or systematic natural history. Vol. I. Part 2. Mammalia. London: Printed for G. Kearsley.
Simpson GG 1931. A new insectivore from the Oligocene, Ulan Gochu horizon, of Mongolia. American Museum Novitates 505:1-22.
Wortman JL 1901-1903. Studies of Eocene Mammalia in the Marsh Collection, Peabody Museum. Am. Jour. Sci. 11:333–348.

wiki/Anagale
wiki/Rhynchocyon
wiki/Macroscelides
wiki/Leptictis

pterosaurheresies.wordpress.com/2024/08/22/wortman-1902-was-first-three-origins-for-placental-mammals-hypothesis/

Source: https://pterosaurheresies.wordpress.com/2026/03/11/extinct-leptictids-arise-from-marsupial-bilbies-and-give-rise-to-placental-sengis-in-the-lrt/