The Triassic/Jurassic boundary is marked by a faunal hiatus. Among others, most amphibians and ammonoids were extinct. At the beginning of the Lower Jurassic, the "real" ammonites (order ammonitida) evolved from the surviving phyllocerats (genus Monophyllites), with Psiloceras. At the same time, the lytocerats separated from the phyllocerats. In combination with a slight global cooling, the newly formed continental shelf areas allowed a fast radiation and evolution of the new ammonoids, but also belemnites, bivalves and brachiopods. Sponge-microbe reefs formed giant reef systems in the Upper Jurassic. The oceans were populated by the first bony fish.
The land is dominated by reptiles - the warm humid climate delivers a variety of food for them. The largest dinosaurs ever can be found in the Middle Jurassic. Mammals also existed, but are generally small (mouse to rat size) and have to compete with reptiles as undifferentiated omnivores in small niches. Reptiles also rule the oceans, among them mostly species that have already gained a perfect adaption to aquatic life during the Triassic (ichthyosaurs, plesiosaurs). Short-tailed flying reptiles replace the common long-tailed flying reptiles during the Upper Jurassic. The primitive bird Archaeopteryx from the Upper Jurassic of Solnhofen is the famous link between reptiles and birds.
The humid climate led to a spreading of conifers, seed ferns and horsetails, although horsetails never again reached the size of their paleozoic ancestors.
Germany offers some wold-famous sites whose outstanding fossil preservation gives a unique insight into the Jurassic life. The exceptional preservation is not limited to certain locations but certain strata. For example the Posidonia Shale (Lower Toarcian) that is still today mined in the region of Holzmaden-Ohmden and Dotternhausen near Balingen, or the Upper Jurassic plattenkalks (platy limestones) of Nusplingen near Balingen (Upper Kimmeridgian) and Frankenalb (Solnhofen, Eichstätt, Painten - Upper Kimmeridgian to Tithonian). All these areas are today protected by law to preserve special finds for science and the public.
(Ages according to FAUPL 2000)
Lower Hettangian (planorbis zone), Tunstall/North Yorkshire (UK)
Psiloceras planorbis is the first "real" ammonite and therefore ancestor of all jurassic and cretaceous ammonites (except for the older phyllocerats and the lytocerats that have evolved from the phyllocerats in the Lower Jurassic). The smooth shell as well as the general shape strongly resemble to the phyllocerat Monophyllites, the ancestor of the psilocerats.
Seirocrinus subangularis (MILLER)
Pliensbachian, Charmouth (GB)
?Anchisauripus sp.
Lower Jurassic, Southern France
Upper Pliensbachian (margaritatus zone), Zell unter Aichelberg
unidentified
Lower Toarcian (tenuicostatum zone), Ohmden
The Posidonia Shale (Lower Toarcian) that formed under anoxic conditions delivered numerous extremely well preserved fossils - often not only the stomach contents but also skin and even embryos are preserved.
Phragmoteuthis conocauda (QUENSTEDT)
Lower Toarcian (falcifer zone), Ohmden
Preservation of soft body parts in the Posidonia Shale is often so good that muscle fibres and the inc sac of cephalopods (except ammonites) still can be seen.
This fragment of a belemnite-like phragmoteuthid shows a part of the chambered phragmocon, the muscles of the mantle and the inc sac. They are preserved as jet (black) and phosphate (white). The arms with chitinous hooks are not preserved.
Upper Bajocian, Gruibingen
With a length of more than 70 cm Megateuthis is the largest known belemnite.
Scale: 10 cm
Upper Bajocian (niortense zone), Aichelberg
Heteromorphic (not a plain, advolute spiral) ammonites of the Jurassic occur for the first time during the upper Bajocian. Besides Spiroceras or the elongated Hamites, this feature can be found on various later jurassic ammonites but is not this clearly visible and mostly limited to a break of the adult living chamber.
Acrosalenia pustulata FORBES
Bathonian, Landaville (F)
Sea urchins are quite common in the Middle and especially Upper Jurassic, preservation of coronas with adhering spines and Aristotle's Lantern however is extremely rare. Required for this is a sudden covering of the living animals with sediment, for example during storms, to prevent the parts from falling apart.
Brachyphyllum sp.
Upper Kimmeridgian, Painten
Although plant remains like trunks and logs can be found in nearly all jurassic stages, well preserved plant fossils are rare. Some single finds are known from the Posidonia Shale, but only in the kimmeridgian and tithonian plattenkalks that formed in tropical lagoons they are more abundant. They were transported from nearby islands into the lagoon over short distances and therefore are often very well preserved.
Cycadopteris jurensis (KURR)
Upper Kimmeridgian (subeumela subzone), Brunn
Palm ferns are an important element of jurassic and cretaceous floras. Their current occurence in tropical latitudes give a hint on the past climate.
Axosmilia sp.
Lower Tithonian, Gerstetten
From the middle Upper Jurassic onwards, large sponge-microbe reefs grew on most parts of the South West German continental shelf. Due to the slowly falling sea level at the end of the Jurassic environmental conditions for sponges got worse. The reefs died and subsequently were populated by corals. At some locations the corals and other fauna were afterwards silicified, so the fragile fossils can be etched out of the limestone with thinned hydrochloric acid.
Lower Tithonian (hybonotum zone), Langenaltheim
One can see the mark that the empty ammonite shell left when touching the soft sediment. After that it tilted and came to ly directly next to the mark. Preservation of such details is only possible when there's no water movement, for example in the Solnhofen plattenkalks lagoons.
Scale: 5 cm
Lower Tithonian, Mörnsheim
The in situ preservation of the calcified jaw or cover organs - their function is still subject of discussions - of ammonites within the living chamber is not unusual in calm water sediments. The missing shell part on the left is probably a bite mark that injured the ammonite so heavily that it sank down to the lagoon's floor. Obviously the predator was not able to eat it's prey. Why despite such finds no one ever discovered an ammonite with preserved soft body parts is still unknown.
Tharsis dubius (BLAINVILLE)
Lower Tithonian, Mörnsheim
The first true bony fish developed during the Lower Jurassic. Unlike their ancestors, the ganoid fish, they have reduced scales and a bony spinal column. While they are comparably rare in the Lower Jurassic, they make up a good part of the Upper Jurassic plattenkalks fish fauna and even occur there in vast swarms.
Sinosura sp.
Lower Tithonian (moernsheimensis subzone), Hienheim
Entirely preserved brittle stars are always rare. A mass occurence like in the Hienheim plattenkalks lagoon that comprises several beds is therefore even more amazing. The good preservation is due to the brittle stars' burrowing habits - at the moment of their death they already were within the sediment and therefore were protected against decay.