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A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics
Zhe-Xi Luo,1*Alfred W. Crompton,2Ai-Lin Sun3
A fossil from the Early Jurassic (Sinemurian, ~195 million years
ago) represents a new lineage of mammaliaforms, the extinctgroups more
closely related to the living mammals than to nonmammaliaformcynodonts. It has an enlarged cranial cavity, but no postdentarytrough
on the mandible, indicating separation of the middle earbones from the
mandible. This extends the earliest record of thesecrucial mammalian
features by some 45 million years and suggeststhat separation of the
middle ear bones from the mandible andthe expanded brain vault could
be correlated. It shows that severalkey mammalian evolutionary
innovations in the ear region, thetemporomandibular joint, and the
brain vault evolved incrementallythrough mammaliaform evolution and
long before the differentiationof the living mammal groups. With an
estimated body weight ofonly 2 grams, its coexistence with other
larger mammaliaformswith similar "triconodont-like" teeth for
insectivory within thesame fauna suggests a great trophic
diversity within the mammaliaforminsectivore feeding guild, as
inferred from the range of bodysizes.
1 Section of Vertebrate Paleontology, Carnegie
Museum of Natural History, Pittsburgh, PA 15213, USA.
2 Museum of Comparative Zoology, Harvard University,
Cambridge, MA 02138, USA.
3 Institute of Vertebrate
Paleontology and Paleoanthropology, Chinese Academy of Sciences,
Beijing 100044, China.
*
To whom correspondence should be addressed. E-mail:
luoz{at}carnegiemuseums.org
