Second part of the Jurassic geological period, from 174 to 161 million years ago
The
Middle Jurassic
is the second
epoch
of the
Jurassic
Period
. It lasted from about 174.1 to 161.5
million years ago
. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare,
[3]
but
geological formations
containing land animal fossils include the
Forest Marble Formation
in England, the
Kilmaluag Formation
in
Scotland
,
[4]
the
Calcaire de Caen
of
France
,
[5]
the
Daohugou Beds
in China, the
Itat Formation
in
Russia
, the
Tiouraren Formation
of
Niger
,
[6]
and the
Isalo III Formation
of western
Madagascar
.
Paleogeography
[
edit
]
During the Middle Jurassic Epoch,
Pangaea
began to separate into
Laurasia
and
Gondwana
, and the
Atlantic Ocean
formed. Eastern Laurasia was tectonically active as the
Cimmerian plate
continued to collide with Laurasia's southern coast, completely closing the
Paleo-Tethys Ocean
. A
subduction zone
on the coast of western North America continued to create the
Ancestral Rocky Mountains
.Significant subduction zones were active along practically all of the continental edges surrounding Pangea, as well as in southern Tibet, southeastern Europe, and other locations, to allow the formation of fresh seabed in the proto-Atlantic Ocean. Plate tectonic activity in subduction zones caused the construction of north-south mountain ranges such as the Rocky Mountains and the Andes all along the west coast of North, Central, and South America.
[
citation needed
]
Fauna
[
edit
]
The Middle Jurassic is one of the key periods in the
history of life
on Earth. Many groups, including dinosaurs and
mammals
, diversified during this time.
[7]
[8]
Marine life
[
edit
]
During this time,
marine life
(including
ammonites
and
bivalves
) flourished.
Ichthyosaurs
, although common, are reduced in diversity; the top marine predators, the
pliosaurs
, grew to the size of
killer whales
and larger (e.g.
Pliosaurus
,
Liopleurodon
).
Plesiosaurs
became common at this time, and
metriorhynchids
first appeared. In the Jurassic seas, a wide range of animals swam. Cartilaginous and bony fish were plentiful. Large fish and marine reptiles were plentiful.
[
citation needed
]
Terrestrial life
[
edit
]
Many of the major groups of
dinosaurs
emerged during the Middle Jurassic, (including
cetiosaurs
,
brachiosaurs
,
megalosaurs
and primitive
ornithopods
).
[7]
Descendants of the
therapsids
, the
cynodonts
, were still flourishing along with the dinosaurs. These included the
tritylodonts
and mammals. Mammals remained quite small, but were diverse and numerous in faunas from around the world.
[9]
[10]
Tritylodonts were larger, and also had an almost global distribution.
[11]
The first crown-group mammals appeared in the late Early Jurassic. A group of cynodonts, the
trithelodonts
, were becoming rare and eventually became extinct at the end of this epoch.
[
citation needed
]
Flora
[
edit
]
Conifers
were dominant in the Middle Jurassic. Other
plants
, such as
ginkgoes
,
cycads
, and
ferns
were also common.
[
citation needed
]
References
[
edit
]
- ^
"International Chronostratigraphic Chart"
(PDF)
. International Commission on Stratigraphy.
- ^
Cresta, S.; Goy, A.; Arias, C.; Barron, E.; Bernad, J.; Canales, M.; Garcia-Joral, F.; Garcia-Romero, E; Gialanella, P.; Gomez, J.; Gonzalez, J.; Herrero, C.; Martinez2, G.; Osete, M.; Perilli, N.; Villalain, J. (September 2001).
"The Global Boundary Stratotype Section and Point (GSSP) of the Toarcian-Aalenian Boundary (Lower-Middle Jurassic)"
(PDF)
.
Episodes
.
24
(3): 166?175.
doi
:
10.18814/epiiugs/2001/v24i3/003
. Retrieved
13 December
2020
.
{{
cite journal
}}
: CS1 maint: numeric names: authors list (
link
)
- ^
Clark, James (June 2009).
"Evolutionary Transitions Among Dinosaurs: Examples from the Jurassic of China"
.
Evolution: Education and Outreach
.
2
(2): 243?244.
doi
:
10.1007/s12052-009-0137-0
.
- ^
British Geological Survey. 2011.
Stratigraphic framework for the Middle Jurassic strata of Great Britain and the adjoining continental shelf: research report RR/11/06
. British Geological Survey, Keyworth, Nottingham.
- ^
Allain, Ronan (24 August 2010).
"Discovery of megalosaur (Dinosauria, Theropoda) in the middle Bathonian of Normandy (France) and its implications for the phylogeny of basal Tetanurae"
.
Journal of Vertebrate Paleontology
.
22
(3): 548?563.
doi
:
10.1671/0272-4634(2002)022[0548:DOMDTI]2.0.CO;2
.
S2CID
85751613
. Retrieved
10 April
2023
.
- ^
Rauhut; Lopez-Arbarello (15 January 2009).
"Considerations on the age of the Tiouaren Formation (Iullemmeden Basin, Niger, Africa): Implications for Gondwanan Mesozoic terrestrial vertebrate faunas"
.
Palaeogeography, Palaeoclimatology, Palaeoecology
.
271
(3?4): 259?267.
Bibcode
:
2009PPP...271..259R
.
doi
:
10.1016/j.palaeo.2008.10.019
. Retrieved
12 April
2023
.
- ^
a
b
Benson RBJ, Campione NE, Carrano MT, Mannion PD, Sullivan C, Upchurch P, and Evans DC. 2014.
Rates of dinosaur body mass evolution indicate 170 million years of sustained ecological innovation on the avian stem lineage
. PLoS Biology 12, no. 5: e1001853.
- ^
Close, Roger A.; Friedman, Matt; Lloyd, Graeme T.; Benson, Roger B.J. (2015).
"Evidence for a mid-Jurassic adaptive radiation in mammals"
.
Current Biology
.
25
(16): 2137?2142.
doi
:
10.1016/j.cub.2015.06.047
.
PMID
26190074
.
- ^
Kielan-Jaworowska, Z., Cifelli, R.L., and Luo, Z.-X. 2004. Mammals from the age of dinosaurs: origins evolution and structure. 630 pp. Columbia University Press, New York.
- ^
Panciroli, E. 2017.
The First Mammals
Archived
2020-08-03 at the
Wayback Machine
Palaeontology Online.
- ^
Kemp, T 2005.
The Origin and Evolution of Mammals
. Oxford University Press.