A brief note on haramiyidans and the Mesozoic radiation of the mammals

Mammals have a longer history in the “age of the dinosaurs” (Mesozoic) than in the “age of the mammals” (Cenozoic). However, much of the Mesozoic history of the mammals remains obscure, not in small part due to the fact that mammals were mostly small in size and in large part excluded from larger size ecological role in land, water, the air by archosaurs and other reptiles. Even in typically “mammalian” roles in many parts of the world they were excluded or limited by the notosuchid crocodiles. Nevertheless, mammalian history in the Mesozoic is central to understanding a large part of their evolution. Moreover, exclusion from larger-size niches does not translate into lack of diversity among the early mammals. The question of what is a mammal, i.e., what distinguishes them from other basal synapsids or stem-mammals has to be first answered before any discussion on the Mesozoic history of the mammals. In the early days of evolutionary studies there was the tendency to see mammals emerging via a gradual transition from reptiles. These “reptiles” from which mammals arose were called the mammal-like reptiles. However, in reality these were proto-mammals, no more “reptile” than in the sense of retaining ancestral amniote features to different degrees. As our understanding improved, these proto-mammals were seen as basal representatives of the synapsid clade whose crown is the mammal clade. Students of evolution have typically seen synapsids as serially acquiring more mammal-like features on the basic amniote body plan starting from the Carboniferous, through the Permian and the into the Triassic eventually culminating in mammals some point before the late Triassic. The most mammal-like of the basal synapsids form the eucynodont clade within which mammals are nested. The major groups of eucynodonts are: 1) the cynognathids; 2) the diademodontids and traversodontids which comprise the diademodontoid clade; 3) the chiniquodontids and probainognathids; 4) the tritylodontids; 5) the trithelodontids and therioherpetontids; 6) the brasilodontids; 7) the mammals. Even within the

There has been much contention regarding which of the above are the closest sister groups of the mammals. Most recent studies have converged on brasilodontids (e.g. Brasilitherium), followed by tritheledonts (e.g. Pachygenelus) being the closest sister group of mammals, in turn followed by the tritylodonts (e.g. Oligokyphus, Kayentatherium and Bocatherium) and the probainognathids there after. A major clue to this difficulty in determining the closest sister group of the mammals came from the publication of a juvenile specimen of Probainognathus from South America in 1994, which showed several derived features similar to the more derived synapsids or earliest mammals like Morganucodon. However, these derived features shared with more derived synapsids were absent in the adults. This suggested that mammals emerged via paedomorphism in the derived eucynodonts. This paedomorphism appears to have been driven by the selective pressures for smaller body size and could have occurred parallelly in multiple Triassic eucynodont groups resulting in emergence of mammal-like features in several of them. Over the past 25 years several morphological studies have helped define a clear clade mammaliforms forms clearly closer to the crown mammalia, i.e., the common ancestor of monotremes, marsupial and placentals to the exclusion of the other eucynodonts mentioned above. Colloquially this clade will be called mammals here. Mammals thus defined are first seen in the late Triassic emerging even as the explosive radiation of archosaurs and the rise of the dinosaurs among them was well underway. Morphologically the following delineate mammals from the other eucynodonts:
-A notable dentary condyle articulating with a glenoid fossa on squamosal
-Features of the occipital condyles
-Formation of much of the side wall of the braincase by the anterior lamina of the temporal bone
-Features relating to the foramina for the maxillar and mandibular branches of the trigeminal nerve
-Several features of the brain case in the region proximal to the ear

Based on the presence of these shared features several groups of the Mesozoic mammals can be recognized:

A slightly outdated (i.e. lack Juramaia described later by the author) yet useful phylogenetic view of early mammal evolution by Zhe-Xi Luo.

1) Morganucodonts: One of the earliest mammal groups with fossils from the late Triassic itself and surviving till Middle Jurassic.
2) haramiyidans: One of the earliest mammal groups with fossils from the late Triassic itself and surviving till Late Cretaceous, perhaps finally becoming extinct alongside the non-avian dinosaurs.
3) Docodonts: One of the earliest mammal groups as suggested by Tikitherium from late Triassic of India. Underwent a great global Jurassic radiation including aquatic forms such as Haldanodon and the flat-tailed platypus-like Castorocauda. They survived till at least the end of the Early Cretaceous.
4) Sinoconodon: A primitive form the Early Jurassic of uncertain affinities.
5) Australosphenidans: The basal-most clade of mammalia proper. They underwent a great global Jurassic radiation with lineages such as Shuotheriids and Asfaltomylids and in the Cretaceous gave rise to the monotremes which today are only seen in Australia: the platypus and echidna.
6) Fruitafossor: An enigmatic Late Jurassic digging mammal with Aardvark-like dental adaptations; uncertain affinities.
7) Eutriconodonts: Widely radiated in Jurassic and Early Cretaceous giving rise to numeorus insectivorous forms and also the largest mammalian predators of the Mesozoic like Repenomamus
8) Volaticotherium: A gliding mammal from the Middle Jurassic with a morphology similar to a flying squirrel.
9) Tinodonts: First seen in the Late Jurassic and last through the Early Cretaceous.
10) The multituberculates: A great radiation of mammals first seen in the late Jurassic and lasting past the K-Pg boundary into the Cenozoic finally dying out around 35 Mya. They had a come back in the Paleocene just after the great K-Pg extinction.
11) Spalacotheroids: A Cretaceous radiation of mammals of some diversity that became extinct alongside the non-avian dinosaurs.
13) Henkelotherium and Dryolestids: Henkelotherium was late Jurassic arboreal mammal that appears to be related to a much larger group the dryolestids. While the dryolestids appear to have undergone a major radiation in the Northern continents during the Jurassic, it was in the southern continents like that they thrived till the end of the age of the dinosaurs. Included carnivores like Foxraptor and the enigmatic long-canined Cronopio.
14) Stem boreosphenidans: Forms closer to the common ancestor of placentals and marsupials than other mammals.
16) Marsupials and their stem forms like Sinodelphys from the Early Cretaceous.
15) Placentals and their stem forms like Juramaia from the Oxfordian age of the Late Jurassic.

Of these the haramiyidans are of particular significance because they are one of the earlier lineages of mammals in the fossil record. They predate the Jurassic in their first appearance – the Jurassic being the beginning of the great mammal radiation in which all the major mammal lineages including the stem placentals emerged. Thus, in a sense the Jurassic was about as important to mammals as it was for dinosaurs. Hence, the phylogenetic position of the haramiyidans is critical to understand the early evolution of mammals. There have been two competing hypothesis in this regard: 1) Based on the Triassic fossils of Haramiyavia it has been proposed that they are one of the basal-most lineages of mammals. 2) Based on their dental morphology they have been linked to the multituberculates within crown mammalia proper. If this were true then the divergences within crown mammalia will be pulled back to the Triassic. Three new fossils, all from China add a new twist to this story, still leaving things somewhat unclear, but adding a lot of new information on the biology and ecology of the enigmatic haramiyidans.

Continued…