Eocene mammals

Finding of sirenian remains in the Lutetian (Middle Eocene) of Santa Brígida (Amer, La Selva, Girona). The finding of fossil bones in the surroundings of the Santa Brígida hermitage (Amer municipality, comarca of La Selva, Girona province, Catalonia) is reported. The fossils are included in hard rock and have not been recovered. They have been identified as belonging to an indeterminate Sirenia. The outcrop is located in the Lower to Middle Lutetian (Middle Eocene) Tavertet Formation (also known as Girona Fm.). Thus, the finding corresponds to
the oldest sirenian remains of Western Europe.
Resum
Es dona a conèixer la troballa d’ossos fòssils als voltants de l’ermita de Santa Brígida (Amer, La Selva, Girona). Els fòssils estan inclosos en roca dura i no han estat extrets. S’han identificat com pertanyents a un Sirenia indeterminat. L’aflorament correspon a la Formació Tavertet (també coneguda com Fm. Girona) del Lutecià inferior a mitjà (Eocè mitjà). Així, la troballa correspon a les restes més antigues de sireni conegudes a Europa Occidental.

In recent years, a series of discoveries of a diverse early Eocene (~54.5 Ma) terrestrial mammal fauna from the
open-cast lignite mines of western India (District Surat, Gujarat) have attracted global attention. The principal early Eocene
mammal-bearing horizon in India occurs in the Cambay Shale, and is sandwiched between the two main hyperthermal
events ETM2 and PETM, making this mammal fauna the oldest known (basal Eocene, at least ~54.5 Ma) assemblage of
Cenozoic mammals in South Asia. The assemblage comprises over 10 placental mammal orders including artiodactyls,
perissodactyls, primates (the APP taxa), creodonts, condylarths, insectivores, apatotherians, proteutherians, rodents, bats,
lagomorphs, tillodonts, together with possible marsupials, preserved in a low-energy, coastal marsh-bay complex of
mangrove swamps and lagoonal muds. Several of these mammalian orders (the APP taxa) are known to make their first
appearance in the Holarctic (northern) continents around the Paleocene-Eocene boundary (56 Ma) during the intense
warming interval that coincided with this boundary. A review of this mammal fauna and its implications for faunal
exchanges between India and Asia and the timing of India-Asia collision is presented here along with a discussion on
taxonomic/nomenclatural issues where necessary. Significantly, data from the Vastan lignite mine supports an Indian origin
(i.e. the Out-of-India hypothesis) for the order Perissodactyla whose antiquity can be traced to the primitive Indian genus
Cambaytherium. A similar scenario is likely for certain other groups such as the adapoid and possibly omomyid primates,
whose early history is traceable to the primitive Indian taxa Marcgodinotius and the poorly known Vastanomys,
respectively.
The presence of early Eocene Holarctic mammals in India, especially those of medium to large size, points to
significant terrestrial faunal exchanges between India and Europe/Asia around the Paleocene-Eocene boundary. Previous
paleobiogeographic interpretations have emphasized the European affinities of the Indian early Eocene terrestrial fauna but
have not highlighted the component with Asian ties. Taxa with Asian affinities assume particular significance when seen in
the context of the timing of India-Asia collision. The discovery of a primitive tapiromorph at Vastan (Cambaylophus) with
close relations to the basal Eocene (Bumbanian) Chinese tapiromorph Orientolophus, and the more recent find of a
nyctitheriid insectivore (Indonyctia cambayensis) with close affinities to the late Paleocene Voltaia from Kazakhstan,
suggest that terrestrial faunal connections between the Indian and Asian landmasses were in place at or slightly before the
Paleocene-Eocene boundary at 56 Ma, providing independent evidence that a subareal contact of some kind was established
between these landmasses in response to the initiation of India-Asia collision. The exact route of India-Asia faunal
exchanges is not yet resolved, but it is likely that faunal migrations between India and Asia were facilitated by the Kohistan-
Ladakh Arc which acted as a filter bridge between the two landmasses.
Apart from the early Eocene mammal fauna from Cambay Shale, significant advances have been made in recent years
in our understanding of early middle Eocene (~48 Ma) anthracobunids and raoellid artiodactyls from the Subathu
Formation of NW Himalaya and correlative sequences in Pakistan, and the late middle Eocene (~42 Ma) cetaceans and
sirenians from Kutch, western India. Anthracobunids have recently been removed from Tethytheria and reinterpreted as
stem perissodactyls along with cambaytheres, whereas, raoellids, particularly Indohyus, are believed to be closely tied to the
origin of whales as a sister taxon to the order Cetacea which includes whales. Combined together, the Himalayan and Kutch
Eocene faunas of India and the coeval faunas of Pakistan have helped to elucidate in great detail the first steps of a major
evolutionary transition from a land mammal to obligate marine predators via a number of intermediate cetacean families
(Pakicetidae, Ambulocetidae, Remingtonocetidae, Protocetidae, and Basilosauridae). The excellent fossil record from
Indian subcontinent has allowed a comprehensive study of the evolution of the various organ systems involved in whale
origins, including locomotion, hearing, balance, feeding and osmoregulation.
Another major group to which fossil finds from India have contributed significantly in recent years is the order Sirenia
that comprises marine herbivorous mammals (dugongs, manatees, and their relatives). Diagnostic cranial material of
sirenians described from the Eocene through Miocene sequences of Kutch, western India has added significantly to our
understanding of the past taxonomic and ecological diversity of this order.

The condylarths, or archaic ungulates, are a paraphyletic mammalian group including a number of fossil taxa whose relationships are unresolved. Included are two genera from the Paleocene and Eocene of North America, Meniscotherium and Phenacodus. Some workers place both genera in the family Phenacodontidae, while others exclude the highly dentally derived Meniscotherium.  In this study, we use growth increments in histological thin sections to examine the timing of crown formation in five molars of Meniscotherium and one each of Phenacodus intermedius and Phenacodus trilobatus. We also use perikymata counts on an additional six molars of Meniscotherium. Although estimated body mass and molar dimensions in Meniscotherium are smaller than in either species of Phenacodus, molar formation times are longer, ranging from 0.71 to 1.44 years. Both Phenacodus
molars take less than a year to form. Crown extension rates, the rate at which the crown grows in height, are as low as 3–15 μm per day in Meniscotherium, but range from 13 to 54 μm per day in Phenacodus. Although striae periodicities and daily enamel secretion rate are similar in both genera, the differences in the crown extension rate and overall timing of crown formation suggest differences in life histories and raise questions about the phylogenetic relationship of the two genera.