Belongs within: Metatheria.
Contains: Polydolophimorphia, Australidelphia, Didelphidae.
The Marsupialia are the marsupials, the clade of mammals including (among others) opossums, kangaroos and koalas. They are most obviously distinguished from other living mammals by their mode of reproduction: the young are born at an earlier stage of development than in placental mammals, and are subsequently nurtured attached to nipples generally contained in a pouch. Of the basally diverging living marsupial lineages, the Didelphidae, opossums, and the Caenolestidae are found in South America. The remaining marsupials, belonging to the Australidelphia, are mostly Australasian, except for the South American monitos del monte, Dromiciops.
The Caenolestidae, shrew-opossums, are a group of shrew-like carnivorous marsupials, found primarily in dense, humid forest. Members of this family have large procumbent incisors in the lower jaw, used to dispatch prey.
The mysterious name of Queen Lestoros
Published 31 January 2009
Those of you who are familiar with the more encyclopaedically-arranged natural history books will almost certainly have encountered the phenomenon of the Mysterious Name. In the introductory section of the book, where the scope of the text is indicated, there’ll be some sort of taxonomic listing—the phyla of animals, for instance, or the families of birds—with each of the taxa listing being described in a subsequent part of the book. But often, if you’re the sort that will pore over such a listing closely enough, you’ll notice that the listing includes at least one name, one taxon (often more) on which the remainder of the book is silent. It’s there in the beginning, it has its place firmly indicated in the hierarchy—and then silence.
One taxon that throughout my youth remained to me a mysterious name was the Caenolestidae. Caenolestids are small South American marsupials, commonly known as shrew-opossums*. In most lists of marsupial families, they’ll be near the beginning, after the true opossums of the Didelphidae. But all the books I read as a child would skip straight from Didelphidae to Dasyuridae, with nary a hint of anything in between.
*Another sign of their obscurity in the public eye—that they are only given the names of other animals, rather than being thought deserving of a name of their own.
Admittedly, the caenolestids are not a large family. Gardner (2005) lists just six species in three genera, Caenolestes, Lestoros and Rhyncholestes. Four of those species are in Caenolestes, the other two genera are regarded by Gardner as monotypic (though one effect of their understudied status is that no two authors will entirely agree on the caenolestid species list, and some authors may recognise two species in either or both of the smaller genera, while others will recognise only a single genus with as few as three species). Of course, that’s still more species than other mammal families such as Rhinocerotidae or Hominidae that have no trouble claiming page space for themselves, and while caenolestids may be few in number now, they were more abundant in the past. Caenolestids were the most abundant small marsupials in South America during the early Miocene (Marshall 1980).
Living caenolestids are widespread, and probably not particularly uncommon, but specimens are few and far between. This has mainly been blamed on their unobliging choice of habitat—they prefer very dense, humid forest, though they may be concentrated close to open meadows (Nowak 1999). They are shrew-like in appearance (hence the common name), and females lack a pouch (presumably the young just hang directly onto the teats, but females with emerged young seem to have not yet been observed). The front of the lower jaw contains an elongate, procumbent pair of incisors, on which more in a moment.
The most detailed account of their behaviour comes from Kirsch & Waller (1979), who trapped and observed specimens of four caenolestid species. Though stomach contents indicate that the caenolestid diet is mostly invertebrates (Nowak 1999), Kirsch and Waller found that specimens were most attracted to traps baited with meat, and when offered a choice between insects and meat, they would more readily take the latter. A male caenolestid offered newborn rats proved an efficient predator:
The animal would move toward a rat, sniffing vigorously, seize and lift the rat with its forepaws or pin it to the substrate, and bite it several times quickly with its incisors. The caenolestid would then commence eating the rat by biting off a section of the head with its cheek teeth and take successive bites posteriorly.
In fact, the large incisors were used rarely by caenolestids in feeding—almost all biting and chewing was done with the cheek teeth, and the incisors are primarily for dispatching prey. Caenolestids have a distinct flap on either side of the upper lip, and this probably protects the face and whiskers from getting clogged up with blood and dirt while the animal is busy stuffing prey towards the back of its mouth. When offered larger food items such as earthworms, the caenolestids would sit upright on their tails and use their front paws to manipulate their food, similar to the way a mouse does.
Fossil caenolestids (or near-caenolestids, depending on your preferred classification) were ecologically more diverse than modern species, and a number appear to have been herbivorous. One such genus, the Miocene Abderites, had a large sharp and multi-grooved first molar like the teeth of the multituberculates. Marshall (1980), in the last major review of the fossil caenolestids, suggested that the arrival of the caviomorph rodents in South America was what triggered the demise of the caenolestid herbivores, while the more generalised insectivores/carnivores were able to keep sailing on.
Phylogenetically, caenolestids have been difficult. Perhaps the most honest representation of our current state of knowledge of marsupial phylogeny would be a trichotomy between the caenolestids, didelphids and australidelphians (Australian marsupials), with all possible relationships between these three having been suggested in the past. Some earlier authors suggested a relationship between caenolestids and the Australian diprotodont marsupials on the basis of the procumbent incisors, but this hypothesis was pretty firmly flattened when it was established that a different pair of incisors was involved in each of the two groups. Perhaps the most popular option at present is that caenolestids are the sister to australidelphians, to the exclusion of didelphids, as supported by some molecular data (Springer et al., 1998). However, a relationship between didelphids and caenolestids remains a distinct possibility due to the occurence in both of sperm pairing. After leaving the testes, sperm of members of these two families connect up to each other, forming a single moving pair (perhaps enabling them to swim faster through the uterus). However, the homology of this character is debatable, as the sperm connect in a different place in the different families.
Systematics of Marsupialia
Characters (from Nowak 1999): Cranial cavity and brain relatively small; corpus callosum absent. Nasal bones large, expanded posteriorly; zygomatic arches complete, jugal bone extending backward below zygomatic process of squamosal bone as far as glenoid fossa. Palate usually imperfect, with spaces between back molars. Angular process of mandible usually bent inward. Teeth usually numbering 40-50; cheek teeth divided into three premolars and four to five molars. Epipubic bones usually associated with pelvic girdle. Abdominal pouch (marsupium) usually present; mammae usually abdominal, located within pouch. Female reproductive tract bifid; median vagina or birth canal either transitory, forming when offspring are born, or permanent; ureters passing medially between vaginae. Scrotum in males usually in front of penis; baculum absent; vas deferens passing laterally to ureters.
<==Marsupialia |--PaucituberculataSM93 | |--PolydolopimorphiaSM93 | `--CaenolestoideaR06 | |--Palaeothentes Ameghino 1887SM93 [PalaeothentidaeR06] | |--Abderitidae [Abderitinae]R06 | | |--Abderites crispus Ameghino 1902S32 | | |--ParabderitesS32 | | `--Micrabderites Simpson 1932S32 | | `--*M. williamsi Simpson 1932S32 | `--Caenolestidae [Caenolestinae]MJ11 | |--Pseudhalmarhiphus Ameghino 1903SM93 | |--Acdestis Ameghino 1887SM93 | |--Pitheculites minimusS32 | |--Lestoros Oehser 1934 (see below for synonymy)C57 | | `--*L. inca (Thomas 1917) [=Orolestes inca, Cryptolestes inca]C57 | |--Rhyncholestes Osgood 1924C57 | | `--*R. raphanurus Osgood 1924C57 | |--HalmarhipusS32 | | |--H. nanusS32 | | `--H. riggsi Simpson 1932S32 | `--Caenolestes Thomas 1895 [incl. Hyracodon Tomes 1863 non Leidy 1850]C57 | |--*C. fuliginosus (Tomes 1863) [=Hyracodon fuliginosus]C57 | |--C. caniventer Anthony 1921C57 | |--C. condorensisFB15 | |--C. convelatus Anthony 1924C57 | |--C. obscurus Thomas 1895C57 | `--C. tatei Anthony 1923C57 `--+--AustralidelphiaCB-E04 `--Didelphimorphia [Didelphiformes, Didelphoidea]OB13 | i. s.: GaylordiaR06 [EobrasilinaeCGW04] |--DidelphidaeMJ11 |--Derorhynchus [Derorhynchidae]CGW04 |--Protodidelphidae [Protodidelphinae]CGW04 | |--Zeusdelphys Marshall 1987MC00, SM93 | |--Guggenheimia Paula Couto 1952SM93 | |--Procaroloameghinia Marshall 1982SM93 | |--Protodelphis Paula Couto 1952SM93 | |--Robertbutleria Marshall 1987SM93 | `--Bobbschaefferia Paula Couto 1970SM93 | `--B. flumensisRV-R12 `--HerpetotheriidaeCGW04 |--SwaindelphysCGW04 |--Nortedelphys Case, Goin & Woodburne 2004CGW04 | |--*N. magnus Case, Goin, & Woodburne 2004CGW04 | |--N. intermedius Case, Goin & Woodburne 2004CGW04 | `--N. minutus Case, Goin & Woodburne 2004CGW04 `--Peratherium [incl. Herpetotherium Cope 1873]M60 |--P. chesteri Gazin 1952G52 |--P. comstocki Cope 1884G52 |--P. edwardi Gazin 1952G52 |--‘*Herpetotherium’ fugax Cope 1873 [=H. fuzax]M60 |--‘Herpetotherium’ huntii Cope 1873M60 |--P. innominatum Simpson 1928W96 [=Herpetotherium innominatumS96] |--P. knighti McGrew 1959W96 [=Herpetotherium knightiS96] |--P. marsupium (Troxell 1923)G52 [=Herpetotherium marsupiumS96] |--‘Herpetotherium’ stevensonii Cope 1873M60 |--‘Herpetotherium’ tricuspis Cope 1873M60 `--‘Herpetotherium’ valens (Lambe 1908)S96 Marsupialia incertae sedis: Eodelphis cutleriG03 ThylatheridiumD02 Homunculites pristinusS32 Palaepanorthus primusS32 Diarcodon parvus Stephenson 1963F71 Leptosiagon gracilis Owen 1874F71 Mylodon australis Krefft 1870F71 Pachysiagon otuel Owen 1874F71 Palaeopetaurus elegans Broom 1895F71 Phascolagus altus Owen 1874F71 SceparnodonF71 |--S. ramsayi Owen 1883F71 `--S. stephensii Ramsay 1880F71 Thylacopardus australis Owen 1888F71 StilotheriumMJ11 MinoperadectesMJ11 Phalangista vulpinaDV87 Yingabalanara Archer, Every et al. 1990LA02 [YingabalanaridaeBA12] `--*Y. richardsoni Archer, Every et al. 1990LA02 Hypsiprymnus gemardiT99
Lestoros Oehser 1934 [incl. Cryptolestes Tate 1934 non Ganglbauer 1899, Orolestes Thomas 1917 non MacLachlan 1895]C57
*Type species of generic name indicated
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