#metatheria

[alphynix]

Towards the end of the Cretaceous, about 69 million years ago, the most diverse and numerous mammals in the northern hemisphere were the metatherians, close relatives of modern marsupials.

And Unnuakomys hutchisoni was the most northern-living of all these metatherians.

About the size of a modern mouse, around 10-15cm long (4-6"), and with teeth that suggest it was a shrew-like insectivore, this little metatherian lived in northern Alaska in what's known as the Paaŋaqtat Province – a region with a distinctive population of endemic polar animals. At the time this area was located at an even higher latitude than it is today, around 80-85ºN, but due to a greenhouse climate it was also warmer, with no permanent ice and the average temperatures staying above freezing.

Unnuakomys was by far the most common mammal species in the Paaŋaqtat Province, represented by numerous fossil teeth and a few jaw fragments, and it also seems to have been the only metatherian living in the whole region. This may just be a preservation bias in the fossil record, but it might also indicate that Unnuakomys was uniquely specialized to endure the several months of continuous darkness each winter in its polar woodland environment, while other North American metatherians were restricted to more southerly latitudes.

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Nix Illustration | Tumblr | Twitter | Patreon

[obscurefossils]

Sulestes

Sulestes was a genus of deltatheroidean mammal from the Cretaceous Period. Its type species is S. karakshi. The known specimens were found in the Bissekty Formation, Kyzyl Kum desert, Uzbekistan. Sulestes was the first and currently only identified metatherian taxon from the Bissekty Formation.

Its autapomorphies include the first premolar's oblique orientation relative to the dentary, an asymmetrical third molar, the presence of a fourth non-vestigial molar, and a double-rooted first premolar.

Sulestes is known from its holotype, a left maxilla fragment, as well as referred specimens. These specimens were originally labelled under "Deltatherus" and "Marsasia" and consist of a right petrosal, right maxilla, two left maxilla fragments, left coronoid process, several right and left dentary fragments, and a few isolated teeth. Sulestes is recovered as the sister taxon to Deltatheroides and Deltatheridium within Deltatheroida. 

Citations: The original description paper, Nessov 1983, is not available online. Nessov and Russell 1994 (description); Averianov, Archibald, and Ekdale 2010 (referred material).

Wikipedia article: here

[robbie-roo]

Marsupials (overview)

Hello, desktop Tumblr it is time to talk about mammals again >:)

I can feel my roommate watching me over my shoulder...

also wow using grammerly on desktop is really annoying! but I can use titles???? woo!

I think I regret choosing desktop for this.... oh well! lets get into it

Ok so we left off talking about monotremes and I will have a post about their bones at some point I pinky swear I just need to figure out what I want to talk about. ok anyways today let's see what should we talk about hmm.... drumroll please.....

Metatheria!

So metatheria are the marsupials of the animal kingdom you've heard of a lot of them and I'm sure you know their calling card- pouches. but we are getting INTO it tonight guys heheheh

ok so some Latin facts- we know that protheria (monotremes) means 'first beast' but metatheria means "in-between beasts"

the first/oldest protheria we know about is the sinodelphys they are found mostly in Australia and South America due to a lack of placental mammals during the whole continental drift business.

ok so what makes a marsupial a marsupial? well let's get into a few relatively universal traits amongst them (remember folks nature doesn't read biology books so some of these are special or gone in certain groups)

so bones (BONES!!!!)

so the pelvic and jaw bones in a metathere are different than in eutheres (we will talk about them next teehee) they do not have a rounded angular process and it's instead spiky (here's a picture of a possum jaw for reference)

There are also extra openings in the back of the pallet and a smaller brain case their teeth are funky too! They have an extra set of incisors.

so I mentioned their funky pelvic bones but what I mean by that is their reproduction is wacko. so like monotremes they have a cloaca and not a vaginal and anal opening. this is why they're "in-between beasts" they have traits that are similar to both ends of the spectrum.

speaking of reproduction their cloaca is subdivided and they will hold eggs internally with a specialized membrane. this membrane surrounds the fetus more than an egg would with a floating placenta surrounding the fetus unlike in us this placenta isn't attached to a uterine horn. Their reproductive structure is also loopy... I'm not kidding look at this shit- this is a kangaroo reproductive structure

WEIRD! They have a pseudovaginal canal which is only there for birth and is part of the cloaca also keep an eye on that second uterus ill get into that more with kangaroos specifically

and it's only in females male reproductive structure is normal

their young are always born underdeveloped. they have an extended lactation period because of this with that special changing milk we mammals have that will deliver the perfect amount of nutrients needed.

so the babies are born super early but those ungrateful little turds have the immediate responsibility to get THEMSELVES into their mother's pouch. Mamma can't help you do everything little Joey.

I need to expand on this because it's fucking INSANE ok so like I said they are underdeveloped and I mean like VERY underdeveloped as in they do not have a full heart or brain yet!!!!! their lungs are just sacks and they don't have a jaw but they DO have a tongue and arms that they use to pull along their mother's hair to get to the milk. again- mama ain't helping you.

Ok i'm ending this here only because this is just general overview stuff and then I'm gonna get into specific species and families next staring with kangaroos :)

[extinctworld-ua]

Chunia

Chunia — примітивний ектоподонтид. Це була особлива група кайнозойських австралійських кускусових, які, можливо, були спеціалізованими насіннєїдами. Ектоподонтиди, яких спочатку вважали однопрохідними, мали коротку морду, великі, спрямовані вперед очі і найбільш незвичайні та складні зуби серед усіх сумчастих. Chunia, найпримітивніший з ектоподонтидів, мав корінні зуби, які були простішими, ніж у інших ектоподонтидів,…

Повний текст на сайті "Вимерлий світ":

https://extinctworld.in.ua/chunia/

[margieargie]

I feel sorry for marsupials; the placental mammals like us get to be Eutheria (good beasts), while they’re apparently stuck being Metatheria (mid beasts).

[animalids]

Spotted-tail quoll (Dasyurus maculatus)

Photo by Caleb McElrea

[synapsidgirl]

UK just went into a lockdown today, I have a lot of not very well defined thoughts about it, so naturally, I'm doing some Thylacosmilus studies. Here's the first one done. He's either just yawning, or maybe it's a threat display. Yes, his ears aren't flattened against his head, but I based this on thylacines, tasmanian devils, and american opossums who didn't/don't seem to be flattening their ears as a part of their threat displays either. At least that's what it looks like to me in pictures and videos. Of course, thylacines, devils, and opossums aren't related to Thylacosmilus, but since the jaws-agape-ears-not-folded threat display seems to be quite common among modern metatherians across different genera, I've decided to go with this, rather than ears-folded-lips-in-a-snarl placental thing. And yes, something bit his ear. I don't know what it was, but it doesn't seem to bother him much. I feel like I could have added more loose tissue around the lower jaw, since the lips would be so substantial, but I quite like how this turned out anyway. Also, I used yawning bloodhounds and basset hounds as reference for the lips situation, since they're very gifted in the jowls departament.

[huxley-paleozoo]

Thylacosmilus

Scientific name: Thylacosmilus atrox Diet: Herbivorous mammals, with some supplemental fruit Projected natural lifespan: 20 years Length: 1.2 meters (4 feet) Weight: 80 kg (176 lbs) Locality: Northern Argentina; 9-3 Ma (Miocene-Pliocene) Exhibit: South America

About Thylacosmilus is one of the largest and last of the sparassodonts, a group of South American predators closely related to marsupials. It has two large saber-like teeth in the upper jaw - but unlike those of saber-toothed cats, they’re rooted far back in the skull. The lower jaw has a pair of flanges at the front that shield the saber teeth. Thylacosmilus has a relatively low bite force, instead using its powerful arms and neck to force prey into submission.

At Huxley Thylacosmilus can be found in the Cenozoic South America section. One of our younger Thylacosmilus is an ambassador animal - you can visit him at the Anning Paleontological Museum on weekends!

Notable Behavior We have three adult Thylacosmilus that we rotate through the exhibit areas. Although we gave them the opportunity to interact with each other, they did so rarely. A lot of times, when they see each other, they do what we think is a territorial display: they will open the jaws wide - up to almost 110 degrees! - and show off the saber teeth. They all use the same areas of their enclosures as a latrine, including where they mark territory.

Like the incisor teeth of rodents, the saber teeth of Thylacosmilus never stop growing through their life. In order to keep the teeth at a manageable size, we give them lots and lots of things to chew on, from durable rubber toys to rawhide and bones. They’re very good at destroying things.

Keeper Notes Have you ever been licked by a Thylacosmilus? They have huge tongues and they drool. A lot.

[alphynix]

Gliding has convergently evolved multiple times within mammals, from the Jurassic-aged haramiyids and volaticotheres to numerous species of modern marsupials, rodents, and colugos.

And yet despite the huge diversity of gliding mammals, and their particular prevalence in tropical forests, there's an entire continent famous for its rainforests that's somehow completely lacking any modern examples: South America.

It's not clear why the gliding lifestyle never took off in South America, but the continent is surprisingly devoid of any other gliding vertebrates, too. The only exceptions are a few species of flying frogs in the northwestern tropical forests around Colombia.

But back in the early Eocene, about 53-50 million years ago, there was at least one South American gliding mammal. Some fossil limb bones found in the Itaboraí Formation in southeastern Brazil look very much like those of a gliding mammal – long and thin, with a locking elbow joint, knees adapted for jumping, and flexible ankles typical of tree-climbers.

These remains haven't been given a new scientific name, however, because there's a good chance they belong to an already-described species. Fossils from Itaboraí are found disarticulated, broken, and with bones of multiple different species jumbled together, so most fossil mammals named from the site have been based on their more easily distinguishable teeth and jaw fragments.

The problem is matching those teeth with these bones.

Currently the best identity guess based on size is Gaylordia macrocynodonta. This mammal would have been around 30cm long (1'), about the size of a modern rat, and had distinctive large canine teeth. It used to be classified as a marsupial related to opossums, but more recent studies have found it to have actually been a marsupialiform metatherian instead, much more closely related to Pucadelphys and sparassodonts than to any modern true marsupials.

Gaylordia's crushing molars suggest it was carnivorous, able to crunch through bones or hard-shelled invertebrate prey. This would be a very unusual diet for a gliding mammal, since most other mammalian gliders are herbivores or omnivores – the only other known predatory examples were the volaticotheres over 110 million years earlier.

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Nix Illustration | Tumblr | Twitter | Patreon

[animalids]

Photo by Rob McLean

[extinctworld-ua]

Anachlysictis

Anachlysictis — рід вимерлих хижих ссавців з групи Sparassodonta, представник метатеріїв (група, що включає сумчастих та їхніх близьких родичів), котрі населяли Південну Америку протягом кайнозою. Типовий вид — Anachlysictis gracilis.

Повний текст на сайті "Вимерлий світ":

https://extinctworld.in.ua/anachlysictis/

[a-dinosaur-a-day]

Kelenken guillermoi

By Scott Reid 

Etymology: For Kélenken, the Bird of Prey Spirit of the Tehuelche Tribe

First Described By: Bertelli et al., 2007

Classification: Dinosauromorpha, Dinosauriformes, Dracohors, Dinosauria, Saurischia, Eusaurischia, Theropoda, Neotheropoda, Averostra, Tetanurae, Orionides, Avetheropoda, Coelurosauria, Tyrannoraptora, Maniraptoromorpha, Maniraptoriformes, Maniraptora, Pennaraptora, Paraves, Eumaniraptora, Averaptora, Avialae, Euavialae, Avebrevicauda, Pygostaylia, Ornithothoraces, Euornithes, Ornithuromorpha, Ornithurae, Neornithes, Neognathae, Neoaves, Inopinaves, Telluraves, Australaves, Cariamiformes, Phorusrhacoidea, Phorusrhacidae, Phorusrhacinae

Status: Extinct

Time and Place: Between 15.5 and 13.8 million years ago, in the Langhian age of the Miocene of the Neogene

Kelenken is known from the Collón Curá Formation of Rio Negro Province, Argentina

Physical Description: Kelenken was a Terror Bird, a type of large flightless terrestrial dinosaur from the late Cenozoic era, mostly in South America though a few reached North America when the continents collided. These were some of the top predators of their environments, built for chasing down food over long distances and then using their monstrous beaks to tear into flesh. Kelenken was one of the biggest members of the group, about two meters tall, and it had one of the largest known skulls of any bird, with fused bones to give it significant strength in its head and strong bire force. Its limbs were very robust, probably allowing for powerful movements of the legs. It had very small wings, as other Terror Birds did, and as such they probably weren’t used very much (think of the arms of Carnotaurus but the bird version). It had a long, winding neck, and its head was mostly beak. The beak was hooked at one end. It had a short tail, though we don’t have any feather impressions to know if it had any sort of feather decoration at the end of it. It would have had a huge bite force, and it possibly was a faster runner than other Terror Birds.

Diet: Kelenken primarily fed on large animals, especially faster moving ones such as hoofed mammals

Behavior: Kelenken probably spent most of its time chasing down its prey, though it’s uncertain how. It’s possible that it would chase down prey, catch it, and shatter its bones with its beak by repeatedly pecking at it (if… pecking is an accurate word to use with such a huge beak doing the pecking). It’s also possible that it would chase down smaller prey, pick it up, and shake it vigorously to break the prey’s back. It also possibly scavenged off of prey. Honestly, it seems likely that Kelenken would utilize all these strategies, and those not even come up with, to get any prey it could in an opportunistic fashion. 

By Michael B. H. 

Kelenken was a fairly fast animal, and probably able to run for longer distances. As its environment was transitioning to an open grassland rather than a forest, this ability to run moreso than its earlier relatives was probably a direct adaptation for that open habitat. As such, Kelenken would have been extremely alert and on the lookout, both for food and for potential dangers, as it was tall enough to poke out above the grass and be visible to anything looking for it. Kelenken could fight with others of its species and against other animals both with its beak and with its feet, as it had very robust legs that would have been good for kicking, possibly very rapid and sharp kicks.

As only a few specimens of Kelenken are known, it’s difficult to determine its social behavior, especially since its closest living relatives are the Seriemas - a much smaller sort of dinosaur. Still, it’s possible to at least glean some behavioral patterns from seriemas. It’s possible that, like living seriemas, Kelenken only spent its time in pairs and small groups, which may have worked together cooperatively to take down larger mammalian sources of prey. It’s also possible that it may have been territorial over its nests, with breeding pairs avoiding other Kelenken and fighting with those they come across. This is all speculative, but would make sense given the environment and size of the animals. Kelenken, being a bird, almost decidedly took care of its young, though we cannot know if both parents took care of the young as in modern Seriemas, or if only one parent did the job at a time as in modern flightless birds. More fossils might be able to clear up this picture, though of course, it’s possible we’ll never know. 

By Ripley Cook 

Ecosystem: Kelenken lived in the part of the Miocene that was the hottest, called the Middle Miocene Climatic Optimum. This was immediately followed by drastic cooling. Thus, Kelenken lived in a uniquely warm period of the time in which Terror Birds dwelled. The formation was surrounded by extensive rivers and lakes, as basins formed in South America due to tectonic plates separating. This environment was a growing grassland, with the pampa overtaking the old deciduous forest that had once been there.  

Kelenken is the only dinosaur known from the formation, but there were many different types of mammals present. There were the heavy-bodied hoofed mammals Protypotherium, Epipatriarchus, and Interatherium; the smaller hoofed mammals Hegetotherium and Pachyrukhos; the weird fast hoofed mammal related to Macrauchenia, Theosodon; armadillo relatives such as Proeutataus, Paraeucinepeltus, Peltephilus, Prozaedyus, and Stenotatus; the anteater relative Neotamandua; the rodents Galileomys, Guiomys, Maruchito, Microcardiodon, Neosteiromys, Protacaremys, Acarechimys, Alloiomys, Megastus, Neoreomys, Prolagostomus, and Stichomys; the new-world monkey Proteropithecia; the sabre-toothed marsupial Patagosmilus; and the extinct shrew opossum relative Abderites. There were also reptiles such as boa relatives like Waincophis, lizards, and turtles like Chelonoidis. There was also the horned frog Wawelia.

Kelenken probably would have hunted a variety of these things, especially the hoofed mammals, though some would have been too big for it to eat. It definitely would have been in direct competition with Patagosmilus, which probably would have specialized on different prey than Kelenken.

Other: Kelenken is most closely related to phorusrhacids such as Phorusrhacos and Titanis, two of the other more famous genera in this group.

~ By Meig Dickson

Sources under the Cut 

Abello, María Alejandra, and David Rubilar Rogers. 2012. Revisión del género Abderites Ameghino, 1887 (Marsupialia, Paucituberculata). Ameghiniana 49. 164–184. Accessed 2019-02-15.

Albino, A.M. 1996. Snakes from the Miocene of Patagonia (Argentina) Part I: The Booidea. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 199. 417–434. Accessed 2019-02-27.

Angolin, F. L., P. Chafrat. 2015. New fossil bird remains from the Chinchinales Formation (Early Miocene) of Northern Patagonia, Argentina. Annales de Paleontologie 101: 87 - 94.

Baez, A. M., S. Peri. 1990. Revision de Wawelia gerholdi, un anuro del Mioceno de Patagonia. Ameghiniana 27(3-4):379-386

Bertelli, S., L. M. Chiappe, C. Tambussi. 2007. A new phorusrhacid (Aves: Cariamae) from the middle Miocene of Patagonia, Argentina. Journal of Vertebrate Paleontology 27(2): 409 - 419.

Bondesio, P.; J. Rabassa; R. Pascual; M.G. Vucetich, and G.J. Scillato Yané. 1980. La formación Collón-Curá de Pilcaniyeú Viejo y sus alrededores (Río Negro, República Argentina) su antiguedad y las condiciones ambientales según su distribución su litogenesis y sus vertebrados. Actas del Segundo Congreso Argentino de Paleontología y Bioestratigrafía y Primer Congreso Latinoamericano de Paleontología 3. 85–99.

De Broin, F., and M. De la Fuente. 1993. Les tortues fossiles d'Argentine: synthese (The fossil turtles of Argentina: synthesis). Annales de Paléontologie (Invertebrés-Vertebrés) 79. 169–232. Accessed 2019-02-27.

Forasiepi, A.M., and A.A. Carlini. 2010. A new thylacosmilid (Mammalia, Metatheria, Sparassodonta) from the Miocene of Patagonia, Argentina. Zootaxa 2552. 55–68. Accessed 2019-02-27.

Gonzaga, L. P., A. Bonan. 2017. Seriemas (Cariamidae). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona.

González Ruiz, Laureano Raúl; Flavio Góis; Martín Ricardo Ciancio, and Gustavo Juan Scillato Yané. 2013. Los Peltephilidae (Mammalia, Xenarthra) de la Formación Collón Curá (Colloncurense, Mioceno Medio), Argentina. Revista Brasileira de Paleontologia 16. 319–330. Accessed 2019-02-27.

González Ruiz, Laureano Raúl; Alfredo Eduardo Zurita; Gustavo Juan Scillato Yané; Martín Zamorano, and Marcelo Fabián Tejedor. 2011. Un nuevo Glyptodontidae (Mammalia, Xenarthra, Cingulata) del Mioceno de Patagonia (Argentina) y comentarios acerca de la sistemática de los gliptodontes "friasenses". Revista Mexicana de Ciencias Geológicas 28. 566–579. Accessed 2019-02-27.

Kay, R.F.; D. Johnson, and J. Meldrum. 1998. A new Pitheciin Primate from the Middle Miocene. American Journal of Primatology 45. 317–336. Accessed 2019-02-27.

Kramarz, Alejandro; Alberto Garrido; Analía Forasiepi; Mariano Bond, and Claudia Tambussi. 2005. Estratigrafia y vertebrados (Aves y Mammalia) de la Formación Cerro Bandera, Mioceno Temprano de la Provincia del Neuquén, Argentina. Revista Geológica de Chile 32. 273–291. Accessed 2017-10-20.

McDonald, H.G.; S.F. Vizcaíno, and M.S. Bargo. 2008. Skeletal anatomy and the fossil history of the Vermilingua, 64–78. S. F. Vizcaíno, W. J. Loughry (eds.), The Biology of the Xenarthra. Accessed 2019-02-27.

Náñez, Carolina, and Norberto Malumián. 2019. Foraminíferos miocenos en la cuenca Neuquina, Argentina: implicancias estratigráficas y paleoambientales. Andean Geology 46. 183–210. Accessed 2019-02-27.

Oriozabala, C.; J. Sterli, and L. González Ruiz. 2018. Morphology of the mid-sized tortoises (Testudines: Testudinidae) from the Middle Miocene of Northwestern Chubut. Ameghiniana 55. 30–54. Accessed 2019-02-27.

Pardiñas, U.F.J. 1991. Primer registro de primates y otros vertebrados para la Formacion Collón Curá (Mioceno medio) del Neuquén, Argentina (First record of primates and other vertebrates from the Collón Curá Formation (middle Miocene) of Neuquén, Argentina). Ameghiniana 28. 197–199. Accessed 2019-02-27.

Pearson, Paul N., and Martin R. Palmer. 2000. Atmospheric carbon dioxide concentrations over the past 60 million years. Nature 406. 695–699. Accessed 2019-02-27.

Pérez, M.E., and M.G. Vucetich. 2011. A new Extinct Genus of Cavioidea (Rodentia, Hystricognathi) from the Miocene of Patagonia (Argentina) and the Evolution of Cavoid Mandibular Morphology. Journal of Mammalian Evolution 18. 163–183. Accessed 2019-02-27.

Pérez, M.E.. 2010. A new rodent (Cavioidea, Hystricognathi) from the middle Miocene of Patagonia, mandibular homologies, and the origin of the crown group Cavioidea sensu stricto. Journal of Vertebrate Paleontology 30. 1848–1859. Accessed 2019-02-27.

Rehr, D. 2007. Prehistoric Predators - Terror Bird. 5:40 - 11:40. National Geographic.

Silvestro, Daniele; Marcelo F. Tejedor; Martha L. Serrano Serrano; Oriane Loiseau; Victor Rossier; Jonathan Rolland; Alexander Zizka; Alexandre Antonelli, and Nicolas Salamin. 2017. Evolutionary history of New World monkeys revealed by molecular and fossil data. BioRxiv _. 1–32. Accessed 2017-09-24.

Tambussi, C. P. 2011. Palaeoenvironmental and faunal inferences based on the avian fossil record of Patagonia and Pampa: what works and what does not. Biological Journal of the Linnean Society 103: 458 - 474.

Vera Nardoni, Bárbara; Marcelo Reguero, and Laureano González Ruiz. 2017. The Interatheriinae notoungulates from the middle Miocene Collón Curá Formation in Argentina. Acta Palaeontologica Polonica 62. 845–863. Accessed 2019-02-27.

Vucetich, M.G., and A.G. Kramarz. 2003. New Miocene rodents from Patagonia (Argentina) and their bearing on the early radiation of the Octodontoids (Hystricognathi). Journal of Vertebrate Paleontology 23. 435–444. Accessed 2019-02-27.

Vucetich, M.G.; M.M. Mazzoni, and U.F.J. Pardiñas. 1993. Los roedores de la Formación Collón Curá (Mioceno Medio), y la ignimbrita Pilcaniyeú, Cañadón del Tordillo, Neuquén. Ameghiniana 30. 361–381. Accessed 2019-02-27.

[eridanromantic]

Woah didn’t know dragonflies have been around that long!! Wasn’t there one that was like 3 feet long or something? The Metatheria, the Clade that marsupials descended from dates back to around the late Jurassic, so the lil possum’s grandpaw was running around with dinosaurs

Yup! The only real change in evolution for the dragonflies have been its size. They were around before mammals, I think. Or was it marsupials...? Idk, but they have been around for a LONG time.

[synapsid-taxonomy]

Sorry if this sounds dumb, but how exactly did the non extant groups of mammals (i.e. those outside of eutheria, metatheria, and monotremata) bring forth their offspring?

Outside of Holotheria, the answer would have probably been eggs. For pretty much the entire expanse between where monotremes branched off, marsupials, and eutherians, however, nobody really knows (although I’d presume it probably wasn’t extended pregnancies as in eutherians).

[kinka-juice]

Mammal is anything what makes milk with a boob, is an endotherm (warm-blooded) and typically has hair. Mammary - > Mammal.

Of extant (still living) mammals the earliest split is the egg-laying monotremes. There are roughly five species of monotremes depending on your lumping and splitting: platypus, short beaked echidna, and three species of long beaked echidna.

The next split is "Metatheria", aka Marsupials, both from Australasia and the Americas. They do not form a complex placenta between mom and fetus, and they leave the womb early and move to a pouch, where they develop by drinking milk instead of using an umbilical cord. They are born at an extremely undeveloped state compared to the third group.

That would be the Eutherians. Every living mammal that is not a monotreme or marsupial is a Eutherian. Also known as placental mammals. Us, mice, bats, whales, aardvarks, etc. We have a placenta in the uterus and do most of our initial growing there fueled by our mom through the umbilical cord, not in an egg or in a pouch.

What all groups have in common, and no mammal has ever lost, is Milk. Milk does a mammal good. Yes, we also all are endotherms and have hair (well, some have lost hair, mostly for aquatic or fossorial purposes), but the big thing is milk.

If it drinks milk secreted from a mammary gland as a baby (Don't @ me about pigeons, penguins, flamingos or cockroaches, Crop milk and roach milk are different - and coconuts aren't mammals either) it's a mammal.

Here's about the best cladogram I can find, because there is no good illustrated cladogram on this topic for reasons I don't know. (Please don't ask where Turtles fit into amniotes, this is the worst question in tetrapod taxonomy, I was taught anapsida was real and that was a convenient lie by my Comparative Vertebrate Anatomy Prof to avoid the Worst Question).

But because we often define groups by relatedness, not by traits, you can also define mammal as something like "any amniote more closely related to a lemur than a lizard". At least for extant species.

Fun milk-related mammal tangent: go read about the dayak fruit bat.

My bio info could be off- but marsupials aren't mammals, right? Please tell me that if they aren't, they also share hair/fur with mammals...

marsupials are a group within the larger group of mammals, the same way that bats are! they're both just specific weird types of mammal.