Belongs within: Boreosphenida.
Contains: Adapisoriculidae, Zhelestidae, Asioryctitheria, Procerberinae, Leptictida, Atlantogenata, Boreoeutheria.
The Eutheria are a group of mammals representing the total group of placental mammals (Placentalia), living species of which are distinguished from other mammals by the live birth of offspring after their extended development in the mother, nourished by a well-developed placenta. Because this character is related to soft-body features only, it is unknown where in the eutherian stem the modern placenta evolved; stem eutherians may have resembled crown placentals in their reproductive biology or retained more plesiomorphic features. Eutherians are first definitively known from the early Cretaceous; the late Jurassic Juramaia sinensis, originally described as a eutherian, may have been a stem therian (marsupials+placentals). Phylogenetic analysis suggests that all known fossil Mesozoic eutherians lie outside the placental crown group (Wible et al. 2007). Most early eutherians were generalised, probably insectivorous forms. Examples include Acristatherium yanensis, described from a partial skull from the Early Cretaceous (Aptian) of China. Murtoilestes abramovi is known from two molars from the lower Cretaceous of Transbaikalia (Russia). Montanalestes keebleri is known from a partial lower jaw from the Albian of Montana. The mid-Cretaceous (Cenomanian) of Uzbekistan has provided such species as Sheikhdzheilia rezvyii and Eozhelestes mangit. Eozhelestes may be related to the late Cretaceous Paranyctoides, species of which are known from both Asia and North America.
The North American genus Cimolestes has been recognised from the Cretaceous to the Palaeocene, making it potentially an independent survival from the lineage leading to crown placentals of the end-Cretaceous extinction event.
Life in the Palaeocene: we don’t need no Placentalia?
Published 18 September 2008
Sixty-five million years ago last Tuesday, the mighty dinosaurs went extinct. Well, they didn’t all go extinct, but that’s how it’s usually expressed because “the mighty dinosaurs went extinct except for a number of volant clades that actually continued to do pretty well for themselves, really” somehow just doesn’t have quite the same ring to it. What remains a fact is that something pretty significant happened to the ecosystem at the end of the Cretaceous, leading to a major turnover that’s usually represented as out with the dinosaurs, bring in the mammals. It is true that the mammals showed a significant rise in diversity during the Palaeocene, the time period immediately following the Cretaceous. However, few of the prominent mammalian groups of the time would be recognisable today.
Modern mammals are divided between monotreme, marsupials and placentals. It is the Placentalia (the group we ourselves belong to) that have been the most successful of the three groups overall, a success that has generally been attributed to their reproductive system of nourishing developing foetuses for longer periods and giving birth to more developed young*. When the fossil record is actually taken into account, Placentalia are a subset of a larger group called Eutheria. Eutherians are the total group containing placentals and all fossil mammals more closely related to placentals than marsupials, while placentals are the crown group of the eutherian lineages that have survived to the present.
*Whether this is really the secret of the placentals’ success is more debatable than generally let on. For instance, it has been suggested that in highly unpredictable environments such as the arid centre of modern Australia, marsupials, with their lower nutrient commitment to developing offspring, may actually have the edge reproductive system-wise.
The eutherian and marsupial lineages had separated from each other by the early Cretaceous, but the question of when the modern placentals arose has been a hotly debated topic. While a number of Cretaceous lineages have been suggested to belong to the Cretaceous crown group—Zhelestidae as relatives of the ungulates (hoofed mammals), while Zalambdalestidae were close to rodents and lagomorphs (Archibald et al., 2001)—recent analyses have placed these taxa outside the placental crown, and the fairly comprehensive analysis by Wible et al. (2007) suggested that none of the fossil eutherians known from the Cretaceous are placentals. This stands in fairly stark contrast to molecular dating studies, which are fairly unanimous in suggesting that the modern placental orders diverged from each other during the Cretaceous. Either the molecular dating is all wrong for some reason, or the placentals were around in the Cretaceous and we just haven’t found them yet.
Still, whether it was the ancestors of the placentals or a number of lineages that survived the end of the Cretaceous, the fossil evidence indicates at least four eutherian lineages survived into the Palaeocene. The Cimolestidae and Leptictidae, families present in both the Cretaceous and the Palaeocene, were placed by Wible et al. (2007) outside the placentals, while the Taeniodonta, a eutherian lineage of unknown relationships, was represented in the late Cretaceous by the species Schowalteria clemensi (Fox & Naylor 2003). Whether the various other lineages known from the Palaeocene diverged from these lines after the end of the Cretaceous or also survived from earlier times is a decidedly open question.
As already indicated, few of the Palaeocene eutherians can be related directly to modern placental orders. Instead, the Palaeocene was the time of a number of lineages that are no longer with use—herbivores such as the pantodonts and dinocerates, small insectivores such as apatemyids and leptictids, carnivores such as creodonts and arctocyonids. Martin Jehle’s Paleocene Mammals website has detailed coverage of many such groups. Palaeocene mammals were also quite distinct from modern taxa in the overall range of morphologies—for want of a better way to put it, Palaeocene eutherians tend to look, well, lumpier than modern species. The broad grasslands that currently dominate the terrestrial part of the world were not yet in existence, and the Palaeocene was a time of forests. As a result, the grassland-adapted cursorial morphologies like modern horses and antelope were also absent, and the low-slung waddler was king.
So how did these Palaeocene waddlers relate to the modern taxa evolutionarily? The only answer we can really give at this point is, who knows? The relationships between the Palaeocene and the modern eutherian orders remain almost completely unknown, and those few connections that have been accepted in the past have been profoundly shaken. For instance, many of the Palaeocene families have been included in the ‘condylarths’, a heterogeneous assemblage believed to be related to the modern ungulates. However, it has become well established in recent years that the ungulates represent at least three separate lineages, with the artiodactyls (even-toed hoofed mammals), perissodactyls (horses and rhinoceros) and paenungulates (elephants and hyraxes) all arising from separate ancestors in the placental tree. Which condylarths are related to which modern ungulates? For that matter, are they related to any of them? If the ungulate morphology arose at least three times in lineages that survived to the present, why should we assume that it couldn’t have also appeared independently in extinct lineages? Similar issues surround Palaeocene ‘insectivoran’ families, whose association with possibly polyphyletic modern insectivorans should be regarded as doubtful.
In light of the findings of Wible et al. (2007), we might even doubt whether many of the Palaeocene eutherians even represent placentals. The classification of McKenna and Bell (1997) united many early eutherians such as Cimolestidae, Pantodonta and Taeniodonta (as well as the modern pangolins) into a group called Cimolesta, which was then included in the Ferae with creodonts and Carnivora. While pangolins may indeed be related to carnivorans, Cimolestidae, as referred to above, are not even placentals. What then becomes of the rest of the “Cimolesta”? Are they also stem-eutherians like Cimolestidae, or are they true placentals?
Such questions are not mere curiosities—the answer could have significant effects on our understanding of Palaeocene ecology. At least some stem eutherians such as the Zalambdalestidae possessed epipubes, bones that support the pouch in marsupials but are absent from placentals (Kielan-Jaworowska 1975). Because of the restrictions epipubes place on the expansion of the abdomen, they may be incompatible with a placental reproductive system. As a result, we cannot assume that stem eutherians bore well-developed young like modern placentals do. Did pantodonts walk around with pouches slung from their bellies?
Systematics of Eutheria
<==Eutheria (see below for synonymy)
| i. s.: Otlestes Nesov 1985SM93
| `--O. meimanAAE01
| Acristatherium yanensisLY11, GP11
|--Juramaia Luo, Yuan et al. 2011SK20, LY11
| `--*J. sinensis Luo, Yuan et al. 2011LY11
`--+--Montanalestes Cifelli 1999MH20, FN03
| `--M. keebleriWR07
`--+--+--Eomaia Ji, Luo et al. 2002SK20, JL02
| | `--*E. scansoria Ji, Luo et al. 2002JL02
| |--Murtoilestes Averianov & Skutchas 2001SK20, FN03
| | `--M. abramoviWR07
| `--Prokennalestes Kielan-Jaworowska & Dashzeveg 1989SK20, FN03
| |--P. minorWR07
| `--P. trofimoviWR07
`--+--AdapisoriculidaeGP11
`--+--Sheikhdzheilia rezvyiiHUG17, WR07
`--+--ZhelestidaeHUG17
`--+--+--Eozhelestes mangitWR07
| `--ParanyctoidesHUG17
| |--P. aralensisWR07
| |--P. maleficusWR07
| |--P. megakerosWR07
| |--P. quadransHUG17
| `--P. sternbergiWR07
`--+--AsioryctitheriaHUG17
|--Bobolestes Nesov 1985HUG17, SM93
| `--B. zengeWR07
|--+--Batodon Marsh 1892HUG17, V67
| | `--B. tenuisWR07
| `--Maelestes Wible, Rougier et al. 2007WR07
| `--*M. gobiensis Wible, Rougier et al. 2007WR07
`--+--Cimolestes Marsh 1889HUG17, FN03 [incl. Nyssodon Simpson 1927V67]
| |--*C. incisus Marsh 1889V66
| |--C. cerberoides Lillegraven 1969GI06
| |--C. cuspulus Gheerbrant 1992GI06
| |--C. lucasiD07
| |--C. magnus Clemens & Russell 1965GI06
| |--C. propalaeoryctesWR07
| |--‘*Nyssodon’ punctidens Simpson 1927V66
| `--C. stirtoniWR07
`--+--Betonnia tsosiaHUG17
|--Eoryctes Thewissen & Gingerich 1989HUG17, AM02
| `--E. melanusWR07
|--Puercolestes Reynolds 1936HUG17, V67
| `--*P. simpsoni Reynolds 1936V66 [=Cimolestes simpsoniWR07]
`--+--+--Chacopterygus minutusHUG17
| `--ProcerberinaeHUG17
`--+--+--Purgatorius Van Valen & Sloan 1965HUG17, SM93 [Purgatoriidae, Purgatoriinae]
| | |--P. coracisOB13
| | |--P. janisaeWR07
| | `--P. unioWR07
| `--+--OxyprimusWR07
| | |--O. erikseniWR07
| | `--O. galadrielae Van Valen 1978MC00
| `--Protungulatum Sloan & Van Valen 1965HUG17, D07
| |--P. coombsiOB13
| |--P. donnae Sloan & Van Valen 1965MC00
| |--P. gorgonWR07
| |--P. mckeeveriWR07
| `--P. sloaniD07
`--+--LeptictidaHUG17
`--+--Prodiacodon Matthew 1929HUG17, SFY02 [incl. Palaeolestes Matthew 1918 (preoc.)V67]
| |--*P. puercensis (Matthew 1918)V67
| |--P. concordiarcensis Simpson 1935S35
| |--P. crustulumOB13
| `--P. tauricinereiHUG17
`--Placentalia (see below for synonymy)OB13
| i. s.: Plioceros dehliniMHL03
| Tingamarra Godthelp, Archer et al. 1992LA02
| `--*T. porterorum Godthelp, Archer et al. 1992LA02
| Kharmerungulatum Prasad, Verma et al. 2007PV07
| `--*K. vanvaleni Prasad, Verma et al. 2007PV07
| Parapodemus Scaub 1938P04
| Germanomys Heller 1936P04
| Blackia Mein 1970P04
| Sulimskia Reumer 1984P04
| Stachomys Kowalski 1960P04
| Smithozapus Sulimski 1962P04
| Pliopetes Kretzoi 1959P04
| Kowalskia Fahlbusch 1969P04
| Prospalax Méhely 1908P04
| Paenelimnoecus Baudelot 1972P04
| Alloblarinella Storch 1995P04
| Orientalomys De Bruijn & Van der Meulen 1975P04
| Archaeodesmana Topachevskij & Pashkov 1983P04
| Blancomys Van de Weerd, Adrover et al. 1977P04
| Stephanomys Schaub 1938P04
| Castyllomys Michaux 1969P04
| Occitanomys Michaux 1969P04
| Pseudomeriones Schaub 1934P04
| Spermophilinus De Bruijn & Mein 1968P04
| `--S. bredaiM-SK04
| Keramidomys Hartenberger 1966P04
| Tylodon Gervais 1848V66
| Acamana Simpson et al. 1962 [Amilnedwardsiidae]SM93
|--AtlantogenataHUG17
`--BoreoeutheriaHUG17
Eutheria [Anagalida, Bunotheria, Cimolesta, Cimolestidae, Didelphodonta, Leptictimorpha, Mesodonta, Tupaioidea, Ungulatomorpha]
Placentalia [Altungulata, Anicanodonta, Bruta, Cheiroptères, Dilambdodonta, Edentata, Educabilia, Eparctocyona, Epitheria, Exafroplacentalia, Gyrencephala, Lipotyphla, Menotyphla, Meridiungulata, Pantomesaxonia, Pedimanes, Preptotheria, Soricomorpha, Unguiculata, Ungulata, Volitantia]OB13
*Type species of generic name indicated
References
[AAE01] Archibald, J. D., A. O. Averianov & E. G. Ekdale. 2001. Late Cretaceous relatives of rabbits, rodents, and other extant eutherian mammals. Nature 414: 62–65.
[AM02] Asher, R. J., M. C. McKenna, R. J. Emry, A. R. Tabrum & D. G. Kron. 2002. Morphology and relationships of Apternodus and other extinct, zalambdodont, placental mammals. Bulletin of the American Museum of Natural History 273: 1–117.
[D07] Dixon, D. 2007. The Complete Illustrated Encyclopedia of Dinosaurs & Prehistoric Creatures. Hermes House: London.
[FN03] Fox, R. C., & B. G. Naylor. 2003. A Late Cretaceous taeniodont (Eutheria, Mammalia) from Alberta, Canada. Neues Jahrbuch für Geologie und Paläontologie—Abhandlungen 229 (3): 393–420.
[GI06] Gheerbrant, E., M. Iarochene, M. Amaghzaz & B. Bouya. 2006. Early African hyaenodontid mammals and their bearing on the origin of the Creodonta. Geological Magazine 143 (4): 475–489.
[GP11] Goswami, A., G. V. R. Prasad, P. Upchurch, D. M. Boyer, E. R. Seiffert, O. Verma, E. Gheerbrant & J. J. Flynn. 2011. A radiation of arboreal basal eutherian mammals beginning in the Late Cretaceous of India. Proceedings of the National Academy of Sciences of the USA 108 (39): 16333–16338.
[HUG17] Halliday, T. J. D., P. Upchurch & A. Goswami. 2017. Resolving the relationships of Paleocene placental mammals. Biological Reviews 92 (1): 521–550.
[JL02] Ji, Q., Z.-X. Luo, C.-X. Yuan, J. R. Wible, J.-P. Zhang & J. A. Georg. 2002. The earliest known eutherian mammal. Nature 416: 816–822.
Kielan-Jaworowska, Z. 1975. Possible occurrence of marsupial bones in Cretaceous eutherian mammals. Nature 255: 698–699.
[LA02] Long, J., M. Archer, T. Flannery & S. Hand. 2002. Prehistoric Mammals of Australia and New Guinea: One Hundred Million Years of Evolution. University of New South Wales Press: Sydney.
[LY11] Luo, Z.-X., C.-X. Yuan, Q.-J. Meng & Q. Ji. 2011. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476: 442–445.
[MH20] Mao, F., Y. Hu, C. Li, Y. Wang, M. H. Chase, A. K. Smith & J. Meng. 2020. Integrated hearing and chewing modules decoupled in a Cretaceous stem therian mammal. Science 367: 305–308.
McKenna, M. C., & S. K. Bell. 1997. Classification of Mammals Above the Species Level. Columbia University Press: New York.
[MHL03] Meng, J., Y. Hu & C. Li. 2003. The osteology of Rhombomylus (Mammalia, Glires): implications for phylogeny and evolution of Glires. Bulletin of the American Museum of Natural History 275: 1–247.
[M-SK04] Moyà-Solà, S., M. Köhler, D. M. Alba, I. Casanovas-Vilar & J. Galindo. 2004. Pierolapithecus catalaunicus, a new Middle Miocene great ape from Spain. Science 306: 1339–1344.
[MC00] Muizon, C. de, & R. L. Cifelli. 2000. The “condylarths” (archaic Ungulata, Mammalia) from the early Palaeocene of Tiupampa (Bolivia): Implications on the origin of the South American ungulates. Geodiversitas 22 (1): 47–150.
[OB13] O’Leary, M. A., J. I. Bloch, J. J. Flynn, T. J. Gaudin, A. Giallombardo, N. P. Giannini, S. L. Goldberg, B. P. Kraatz, Z.-X. Luo, J. Meng, X. Ni, M. J. Novacek, F. A. Perini, Z. S. Randall, G. W. Rougier, E. J. Sargis, M. T. Silcox, N. B. Simmons, M. Spaulding, P. M. Velazco, M. Weksler, J. R. Wible & A. L. Cirranello. 2013. The placental mammal ancestor and the post-K–Pg radiation of placentals. Science 339: 662–667.
[P04] Popov, V. V. 2004. Pliocene small mammals (Mammalia, Lipotyphla, Chiroptera, Lagomorpha, Rodentia) from Muselievo (north Bulgaria). Geodiversitas 26 (3): 403–491.
[PV07] Prasad, G. V. R., O. Verma, A. Sahni, V. Parmar & A. Khosla. 2007. A Cretaceous hoofed mammal from India. Science 318: 937.
[SFY02] Scott, C. S., R. C. Fox & G. P. Youzwyshyn. 2002. New earliest Tiffanian (late Paleocene) mammals from Cochrane 2, southwestern Alberta, Canada. Acta Palaeontologica Polonica 47: 691–704.
[S35] Simpson, G. G. 1935. New Paleocene mammals from the Fort Union of Montana. Proceedings of the United States National Museum 83 (2981): 221–244.
[SM93] Stucky, R. K., & M. C. McKenna. 1993. Mammalia. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 739–771. Chapman & Hall: London.
[SK20] Sulej, T., G. Krzesiński, M. Tałanda, A. S. Wolniewicz, B. Błażejowski, N. Bonde, P. Gutowski, M. Sienkiewicz & G. Niedźwiedzki. 2020. The earliest-known mammaliaform fossil from Greenland sheds light on origin of mammals. Proceedings of the National Academy of Sciences of the USA 117 (43): 26861–26867.
[V66] Van Valen, L. 1966. Deltatheridia, a new order of mammals. Bulletin of the American Museum of Natural History 132 (1): 1–126.
[V67] Van Valen, L. 1967. New Paleocene insectivores and insectivore classification. Bulletin of the American Museum of Natural History 135 (5): 217–284.
[WR07] Wible, J. R., G. W. Rougier, M. J. Novacek & R. J. Asher. 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary. Nature 447: 1003–1006.