Mammals
first appear in the Triassic. These small nocturnal, highly active,
large brained organisms evolved into many
forms over the next 150 million years. Mesozoic
mammals represent two-thirds of mammalian evolution and yet over
this entire time they remained small (Kemp,
2005, p. 137). Most mammals were shrew size, with a few reaching
the size of
a beaver.
Why they remained small is a mystery, but one thing is clear,
important mammalian characteristics evolved over this span
of time that would allow some of their descendents to evolve
into a diversity of small, medium and large forms during
the Cenozoic. Knowledge of Mesozoic mammals over the past
decade has increased at a tremendous rate and this field
of study finds itself in a constant state of change (Rose,
2006, p. 48). In this article we briefly examine some of
the major groups and key individual finds. Basal Mammals
Adelobasileus
Adelobasileus from the Late Triassic of Texas, at 225 Ma,
represents the earliest mammalian candidate. Ironically,
Adelobasileus is not defined as a mammal by its jaw or
teeth. Known only from a partial skull, it possesses a
number of mammalian braincase features (Benton, 2005, pp.
298-300).
Haramiyidans Haramiyidans (order Haramiyida) represent the next earliest
known mammals and range from the Late Triassic to the Mid
Jurassic. Haramiyidans first appear in the Late Triassic
of Germany (Kemp, 2005, p. 140). Haramiyavia represents one
of the most complete haramiyidan fossils from the Late Triassic
of Greenland. Haramiyavia possesses a postcranial skeleton
similar to morganucodonts; however, the teeth are similar
to mulituberculates (Rose, 2006, p. 51). The similarity in
dentition has led many to argue that haramiyidans are basal
relatives to the multituberculates. Haramiyidans teeth indicate
they were small herbivores or at least omnivores (Kemp, 2005,
p. 140). At present, Haramiyidans are the earliest known
herbivorous mammals.
Morganucodonts Morganucodonts (order Morganucodonta) is a well-known group
of early Mesozoic mammals. Morganucodonts were shrew-sized
nocturnal insectivores. Morganucodon is a weasel like morganucodon
of the Late Triassic and Early Jurassic of England and China.
Morganucodon possesses a mammalian dentary/squamosal jaw
joint; however, it also possesses a reduced, but functional,
quadrate/articular jaw joint (Rose, 2006, p. 52). Thus, Morganucodon had a double jaw articulation. This implies that the malleus
and incus were not yet a part of the middle ear. The differentiated
teeth of Morganucodon are diphyodont, meaning they have two
sets of teeth during life. Remember this is in contrast with
early amniotes and reptiles that have undifferentiated teeth
that are constantly replaced throughout life (polyphydont).
However, its teeth did not show precise occlusion. Morganucodon is known only from skulls, but the skeleton of its close
relative Megazostrodon of South Africa is known. The skeleton
of Megazostrodon exhibits a mosaic of mammal and primitive
synapsid characteristics. Megazostrodon had its ribcage restricted
to the thorasic vertebrae. Megazostrodon had a pelvic girdle
that was more mammal-like; however, their pectoral girdle
was more like early synapsids.
Sinoconodon Sinoconodon from the Late Triassic of China, at (208 Ma),
may also occupy a basal position in mammalian phylogeny.
Sinoconodon is an intermediate form in that it possessed
both mammal-like and early amniote characteristics. Sinoconodon has braincase features like Adelobasileus and possesses a
fully developed dentary/squamosal jaw joint. Sinoconodon also possesses some non-mammalian features, which include
serial tooth replacement (polyphydont) and sustained growth
throughout life (Kemp, 2005, p. 142).
Keuhneotherium Kuehneotherium from the Lower Jurassic of South Wales is
known mostly from isolated teeth. Kuehneotheriids possess
teeth with a triangular cusp pattern that may represent a
precursor to the tribosphenic molars that help to define
modern mammals. Kuehneotherium also retained some primitive
traits such as a double jaw articulation (Rose, 2006, p.
54).
Hadrocodium Hadrocodium is known from a single shrew-sized skull from
the Lower Jurassic of China (195Ma). Hadrocodium is one of
the smallest known mammals and is estimated to have weighed
only 2 g (Rose, 2006, p. 55). Hadrocodium possesses primitive
teeth similar to morganucodonts. However, it has a single
jaw articulation between the dentary and squamosal bones,
which suggests that the quadrate and articular bones had
become ear ossicles (Kemp, 2005, p. 149).
Docodonts Docodonts (order Docodonta) are known only from Jurassic
deposits. Some of the first docodonts were found in the Morrison
formation of Wyoming and Colorado along with the bones of
large sauropod dinosaurs (Rose, 2006, p. 55). Docodonts have
broad, rectangular cheek teeth that show precise occlusion
that may indicate an omnivorous diet (Kemp, 2005, p. 147).
Haldanodon is known from a complete skeleton found in the
Late Jurassic Guimarota lignites of Portugal. The skeletal
features of this organism suggest that it may have been adapted
for burrowing. The fact that it is found in lignites (swamp
deposits), indicate a semi-aquatic lifestyle. Castorocauda,
from the Mid Jurassic of China, is the largest docodont at
half a meter in length. Castorocauda had a skeleton adapted
for burrowing and swimming. Castorocauda is also important
because it is the first mammal fossil preserving evidence
of fur (Rose, 2006, p. 56).
Multituberculates
Multituberculates
(order Multituberculata) are an extinct group of rodent-like
organisms that have the longest evolutionary
history of any mammalian lineage. Multituberculates get their
name from their large grinding molars that have rows of cusps
or tubercles. Multituberculates first appear in the Mid
Jurassic and evolved into many forms, which ranged from mice
to beaver sized organisms. Many of these organisms had blade-like
teeth that may have been used to eat hard seeds. Multituberculate
hip structure suggests that they gave birth to undeveloped
young like marsupials. Multituberculates had a single dentary/squamosal
jaw joint and true inner ear ossicles. Kermackodon and Hahnotherium from
the Mid Jurassic of England are the oldest known multituberculates
(Rose, 2006, p. 56). Ptilodus was
a squirrel-like organism that lived in trees. It had a prehensile
tail and
an ankle structure designed to allow moving down trees headfirst.
Lambdopsalis was a beaver-sized organism that lived
in burrows like prairie dogs do today. Remarkable carnivore
coprolites
from China contain the bones and fur impressions of Lambdopsalis (Kemp,
2005, p. 160). Multituberculates possess many rodent-like
features, which represent convergent evolution as they predate
the first rodents by 180 million years. Multituberculates
were abundant in many Mesozoic and Cenozoic communities
in the northern continents, but went extinct in the Oligocene,
perhaps even displaced by true rodents that had evolved and
diversified by this time.
Triconodonts The order Eutriconodonta is a taxon that represents a diverse
group of extinct mammals that span from the Mid Jurassic
to Late Cretaceous. Triconodonts were rat to cat-sized mammals
that lie at the core of this group. Triconodonts had the
dentary/squamosal jaw joint and the three inner ear ossicles.
Eutriconodonts are named for their teeth, which have three
linear cusps on their molars. The lower molars were interlocked
by a unique tongue-in-groove articulation. Eutriconodonts
had the derived mammalian pectoral girdle (limbs tucked underneath
the body), but retained the ancestoral pelvic girdle (sprawling
hind limbs). Jeholodens is a Cretaceous-aged triconodont
mammal known from the Liaoning Province of China. A single
complete skeleton represents Jeholodens. Skeletal evidence
indicates that this small primitive mammal, like many Mesozoic
mammals, was a nocturnal insectivore. Repenomamus is the
largest mammal known from the Cretaceous of China. Repenomamus was a carnivore up to 1 meter long. One specimen of Repenomamus had the partial skeletal remains of a juvinelle Psittocosaurus preserved within the stomach region (Rose, 2006, p. 62).
Psittocosaurus was a ceratopsian dinosaur. This incredible
fossil is the first evidence of a mammal preying on a dinosaur.
Symmetrodonts
& Dryolestids Several closely related groups of Mesozoic mammals exhibit
molar teeth with a triangular cusp pattern. The symmetrodonts
and eupantotheres (Dryolestoidea and Peramura) represent
mammals that are closely related to the therians (marsupials
and placentals). We will briefly discuss two of these groups,
the dryolestids (Order Dryoletida) and symmetrodonts (Order
Symmetrodonta).
Symmetrodonts were shrew to mouse sized and are known from
the Early Jurassic to Late Cretaceous. Symmetrodonts are
believed to be at the base of the therian radiation because
of the triangular cusp pattern on their molars. Zhangheotherium is one of the few symmetrodonts known from almost a complete
skeleton. Zhangheotherium lived in China during the Early
Cretaceous and possessed skeletal characteristics intermediate
between monotremes and therians.
Dryolestids, the most diverse eupantotheres, range from
the Late Jurassic to the Late Cretaceous. Dryolestids have
a more advanced triangular cusp pattern on their molar teeth
than the symmetrodonts and possessed three inner ear bones.
It is believed by many that the ancestors to modern therians
can be found among the dryolestids.
Tribosphenic
Molar & Modern Mammals
Mammal
groups examined thus far tend to have cheek teeth with
cusps oriented either in a linear fashion or a primitive
triangular fashion. When linear, the cusps on upper molars
fit between the cusps on lower molars. When triangular, the
cusps on upper molars fit into V-shaped valleys between the
tricuspid patterns on the lower molars. The evolution of
the tricuspid pattern is important because it represents
an innovation in processing food.
Sometime in the early Cretaceous the advanced triangular
cusp pattern that defines modern mammals, the tribosphenic
tooth, appears. The linear and primitive triangular tricuspid
patterns represent cheek teeth that are good for cutting
and tearing, but not crushing. The more advanced tribosphenic
tooth has a triangular cusp pattern that creates occlusion
surfaces good for crushing or grinding, like a pestle and
mortar. The tribosphenic tooth is defined by the presence
of a large cusp on the upper molars called the protocone.
The protocone of the upper molar works against a basined
area named the talonid on the corresponding lower molar.
The protocone thus acts as a pestle, while the talonid
acts as the mortar. The tribosphenic molar provided a basic
form that would later be modified into the wide variety
of dentitions exhibited by therian mammals (marsupials
and placentals). This dental structure allowed mammals
to expand into a wide variety of specialized dietary niches
(Rose, 2006, p. 67).
During
the entire reign of the dinosaurs mammals remained small
never getting
much bigger than a cat. The evolutionary
biologist Stephen J. Gould (1988) commented, “Mammals
were relatively rare…They lived under the foot of dinosaurs,
in the nooks and crannies of a dinosaur world.” However,
as we have seen, a succession of characteristics that define
modern mammals evolved among the Mesozoic mammals. Fossil
representatives of monotremes, marsupials and placental mammals
are known from the Cretaceous. The extinction event at the
end of the Cretaceous opened up a multitude of niches. Mammals
had evolved many characteristics during the Mesozoic that
would allow their descendents to evolve into a diversity
of forms that could occupy many of these empty niches.
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