History of Paleomammology: Cuvier's opossum

"Every life form is to be considered as a unit, a unified and closed system, in which single parts correspond each to other and work together: A specific action invokes a specific reaction. No part may change if not other parts also change. Hence the result, that if we observe a single part, we can deduct the other ones."

On the first of February 1796 the French naturalist Cuvier held a presentation about known modern and fossil elephants. He studied many bones found in Europe, Siberia, America, Africa and India, and as an important result he finally described a new fossil species, the first elephant - Elephas primigenius.
Cuvier is convinced that the bones came from an extinct creature of distant past, not, as some other naturalists claim, represent the remains of the war-elephants of Hannibal in 218 B.C.

Cuvier is today considered the founder of comparative anatomy and palaeontology; with only some parts of the skeleton it is possible to reconstruct the whole animal and its behaviour.
"If the guts of an animal are specialized to digest meat, the jaw and teeth must be suitable to swallow the prey, and the claws have to catch and tear apart the flesh. Its system of locomotion must be suitable to hunt prey, and its senses must register prey from the distance. Nature must given it a brain and the instinct to entrap the victim."

He dedicated many publications to this approach: His researches on modern mammals comprise the osteology of the Indian Rhinoceros (Rhinoceros indicus = R. unicornis), of the tapir and Hippopotamus, sloth, manatee and many others.
He examined the remains of the fossil giant sloth, Megalonyx of Megatherium, the cave hyena, the Palaeotherium, as well as studying and describing various extinct species of rhinoceros, hippopotamus, elephant, manatee, seal and cave bear.
He examined intensively the remains of mammals found in the layers of Montmartre, an area with quarries in the periphery of Paris, describing the fossil marsupial-like Didelphys gypsorum - denominated for its resemblance to the American opossum as Opossum of Montmartre.

Fig.1. Didelphys gypsorum as published in "Recherches sur les fossiles de ossements quadrupedes" (1812).

"The imprint is weak, it should be observed very closely to recognize something. But its posses an immense value, this are the footprints of an animal, from which we would otherwise have no trace."
The fossil is well preserved, and observing the characters of the jaw and teeth, Cuvier recognizes that the fossil is related to the modern opossum. Following his own rules, he predicts that along with the skeleton there must be found two small bones, that in modern marsupials sustain the poach. It was a triumph for Cuvier's method when the bones were finally discovered under the scrutiny of invited colleagues.*

The summary results of the geological and paleontological studies of Cuvier were finally made public in 1812 with two separate works, the first is the celebrated "Recherches sur les fossiles de ossements quadrupedes", and the second is the "Discours sur les revolutions de la surface du globe."

Fig.2. Stratigraphic profile published by Cuvier and Brongniart in their work "Essai minéraligique sur les environs de Paris" (1st edition 1808), figure from here, for the stratotype see here.

Cuvier, as previously Buffon, claims that the fossils show an alternation of different faunas during the geological history of the earth, various "revolutions" have changed the face of the earth and species have become extinct and been replaced by new ones.
To support this hypothesis he begins to study and map the stratigraphic successions of the basin of Paris. In collaboration with the young geologist Alexandre Brongniart, after four years of work, in 1808 they publish the "Essai minéraligique sur les environs de Paris" (1st edition 1808), complete with a geological map and a stratigraphic column with seven formations, some of which contain the fossils vertebrates studied by Cuvier (especially the Formation of chalk colored in blue).

The method of comparative anatomy promoted by Cuvier marks the beginning of another aspect in palaeontology - the reconstruction of life appearance of extinct animals by scientists and artists. Using the principles he formulated, Cuvier commissioned the first scientific accurate reconstruction of an extinct mammal. In his book "Recherches sur les fossiles…[]" (1812) were published the reconstruction drawings of two paleomammals of the French Tertiary, Palaeotherium and Anoplotherium.
This simple act is an attempt to create life starting from the skeletons of ancient animals, today such art seems natural, but it was a sensation at that time, for the first time anyone beside the experts could see and admire the creatures of the past. Although Cuvier's drawings are simple sketches that show the animals in outline, they have been widely reproduced over the years.

Fig.3. The reconstructions of Palaeotherium and Anoplotherium as drawn by the artist C.L. Laurillard in Cuvier's " Recherches sur les fossiles…[]" (1812).

*Gideon A. Mantell notes that in some marsupials these bones are only small appendages of cartilage, as an example he cites the case of the opossum with the dog head or Tasmanian hyena - the thylacine. If we would only be in possession of the single bones, and no living specimens - although at the brink of extinction [sic], the animal could not be attributed with certainty to the marsupials (1854, "The Creation of medals" pag.804).

References:

BENTON, M.J.; COOK, E. & HOOKER, J.J. (2005): Mesozoic and Tertiary Fossil Mammals and Birds of Great Britain. Geological Conservation Review Series 32: 215
CUVIER, G. (1825): Discours sur les Révolutions de la surface du Globe, et sur les changemens qu'elles ont produits dans le règne animal. Dufour et d'Ocagne, Paris.
CUVIER, G. (1812): Recherches sur les ossemens fossiles ou l'on rétablit les caractères de plusieurs animaux dont les révolutions du globe ont détruit les espèces. Dufour et d'Ocagne, 4 Vol.. Paris.
CUVIER, G. (1851): Die Erdumwälzungen. deutsch bearbeitet und mit erläuternden Bemerkungen über die neuesten Entdeckungen in der Geologie und Paläontologie versehen. Verlag von Ambr: Abel.