As question I sometimes ask myself when designing fictional mammal species is what can and can not have trunks. A trunk as typically defined with a complex fusion of the upper lip and nostrils, resulting in a muscular appendage. Trunks evolved multiple times in placental mammals, but once you get outside of Placentalia they become frustratingly rare.
Among non-placental eutherians zalambdalestids are usually described as having a proboscis like modern sengi and shrews (Archibald 2011, for example), but if this can be described as a trunk sensu stricto is hard to say, as the upper-lip is usually not involved, muscle systems outside of the nasal area are involved and this proboscis is not as flexible as proper trunks, being more used as a sensory organ. Palorchestes and related marsupials have tapir-like nasal anatomy and so far I haven’t found any studies dismissing this, meaning that Palorchestidae was indeed the sole clade of non-placental mammals to have unambiguous trunks, though some bandicoots, shrew-opossums and possibly a few extinct taxa all have shrew-like muscular proboscises.
It’s possible that brandoniid meridiolestidans had sengi-like snouts (Rougier 2008) and Necrolestes has been suggested to have appendages similar to the star-nosed mole. Dryolestoids and other cladotherians in general had fairly therian-like facial muscle anatomy to hypothetically pull out trunks given the opportunity.
But the further back you go the more uncertain things get. Neither eutriconodonts, symmetrodonts not multituberculates have trunks, not even shrew-like nasal proboscises; all tend to have proportionally small nostrils and almost no instances of expanded nasal cavities. While we do know these animals suckled milk and had muscular lips (Kielan-Jaworowska 2004), the exact nature of their facial anatomy and how it compares to modern mammals leaves a lot to be desired.
The further back we go, things continue to become more bizarre, as we do not know precisely when muscular lips evolved in synapsids or which shape they took. Certainly, the development of a fleshy beak in monotremes suggest that early mammals could experiment in ways therians cannot, and vice versa. However, one particular group of non-mammalian synapsids does offer some very interesting answers: haramiyidans.
As opposed to their historical classification as multituberculate relatives, haramiyidans have consistently been recovered as non-mammalian mammaliaforms as of late (Luo 2015, Meng 2017), with Huttenlocker 2018 being of particular interest in both the discovery of the skull of Cifelliodon wahkarmoosuch as well as the placement of gondwanatheres as haramiyidans rather than as mammals. All these taxa have done several odd things with their snouts: in Cifelliodon wahkarmoosuch, the snout is elongated and flat, much as in sirenians, while in gondwanathere skulls we see rather large, expanded nostrils (Vintana sertichi) and massive flanges similar to those seen in ground sloths and glyptodonts (Vintana sertichi, Groeberia minoprioi). None of this indicates a trunk in these taxa, but it certainly suggests that these animals experimented with a variety of labial tissues and have anatomy that could lead to the evolution of trunks or similar proboscises.
So, trunked haramiyidans, and gondwanatheres in particular, are perfect for any speculative evolution projects you have in mind.
Because haramiyidans share the same palinal mastication mechanism as multituberculates, why they have extensively experimented with their lips while multies did not remains a mystery for now. As noted in the description of Vintanasertichi, its is possible that gondwanatheres at least had some subtle nuances in their chewing mechanism multituberculates did not have (Krause 2014).
So there you have it. Surprise me with your trunked abominations.
J. David Archibald, Extinction and Radiation: How the Fall of Dinosaurs Led to the Rise of Mammals, JHU Press, 15/03/2011
Guillermo W. Rougier, Laura Chornogubsky, Silvio Casadio, Natalia Paéz Arangoa, Andres Giallombardo, Mammals from the Allen Formation, Late Cretaceous, Argentina, Cretaceous Research Volume 30, Issue 1, February 2009, Pages 223–238, doi.org/10.1016/j.cretres.2008.07.006
Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure (New York: Columbia University Press, 2004), 14, 531.
Luo, Zhe-Xi; Gates, Stephen M.; Jenkins Jr., Farish A.; Amaral, William W.; Shubin, Neil H. (16 November 2015). “Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution”. PNAS: 201519387. doi:10.1073/pnas.1519387112. PMC 4697399.
Qing-Jin Meng; David M. Grossnickle; Di Liu; Yu-Guang Zhang; April I. Neander; Qiang Ji; Zhe-Xi Luo (2017). “New gliding mammaliaforms from the Jurassic”. Nature. in press. doi:10.1038/nature23476.
Huttenlocker AD, Grossnickle DM, Kirkland JI, Schultz JA, Luo Z-X. 2018. Late-surviving stem mammal links the lowermost Cretaceous of North America and Gondwana. Nature Letters
Krause, David W.; Hoffmann, Simone; Wible, John R.; Kirk, E. Christopher; Schultz, Julia A.; von Koenigswald, Wighart; Groenke, Joseph R.; Rossie, James B. (2014–11–05). O’Connor, Patrick M., Seiffert, Erik R., Dumont, Elizabeth R., Holloway, Waymon L., Rogers, Raymond R., Rahantarisoa, Lydia J., Kemp, Addison D., Andriamialison, Haingoson. “First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism”. Nature. Nature Publishing Group, a division of Macmillan Publishers Limited. 515: 512–517. Bibcode:2014Natur.515..512K. doi:10.1038/nature13922. ISSN 1476–4687. PMID 25383528.