in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
Palaeontological Implications in Conodonts that not Permit the Group
Overpass the Triassic-Jurassic Extinction Event
P. Plasencia * , A. Márquez-Aliaga
Departmento de Geología and ICBiBE, Universidad de Valencia, Dr. Moliner, 50 Burjassot, 46100 Valencia, Spain
(E-mail: Pablo.plasencia@uv.es; Ana.Marquez@uv.es)
Conodonts are one of the most significant groups
that disappeared during the Triassic-Jurassic extinction
event. After a biodiversity and abundance maximum in
the Ordovician, in the Silurian the biodiversity de-
creased, but recovered during the Devonian. The
decadence of the group started in early-middle Carboni-
ferous, with a long-term diversity reduction that
continued along the Permian. But after the Upper
Permian mass extinction the conodonts recovered
during the Early and Middle Triassic, with a
biodiversity comparable to the early Carboniferous one.
A series of extinction and recovery episodes, taking
place in the Upper Triassic (Carnian), produced the
total disappearance of the group, with only two species,
Zieglericonus rhaeticus and Misikella ultima, reaching
the late Rhaetian (Rigo et al., 2005).
Interestingly, the conodonts exceed the largest
extinction event known, at the end of the Permian, but
were extinguished at the end of the Triassic, not
exceeding the limit of the Triassic-Jurassic (TJB). In
this work we are going to compare both events and
discuss about some paleobiological aspects of the
extinction of conodonts. Our study is based on a data-
base of species of Triassic conodonts from a revision of
the literature and the study of scientific collections.
With this data, we have calculated the origination, ex-
tinction and radiation rates of conodonts in the interval
from the Changhsingian (Upper Permian) to Rhaetian
(De Renzi et al., 1996; Plasencia and Márquez- Aliaga,
2005; Plasencia, 2009). In summary, this work analyzes
the conodont diversity between the Permian/ Triassic
and Triassic/Jurassic extinction events.
In reference to the Permian-Triassic Boundary
(PTB) we considered the last Permian stage, the
Changhsingian (2.8 m.y.). Only 9 species of 5 genera
and 3 families overpass the Wuchiapingian limit.
Despite the crisis, the conodonts evolved in 26 different
taxa during this period, with an origination rate of 1.03
and an extinction rate of 0.75. The PTB is overpassed
by 13 species of 3 genera and 3 families (Gondolellidae,
Ellisonidae and Spathognathodontidae). The origina-
tion rate during the Induan (1.5 m.y.) is 1.69 and the
extinction rate of 1.89, with a radiation rate of 0.28.
Two of the families (Ellisonidae and Spathogna-
thodontidae) disappeared during early Triassic.
Despite of the total extinction in the Rhaetian, the
really extinction of the conodonts happened during
Carnian. The forms that dominated the Middle Triassic
(Neogondolella, Paragondolella or Gladigondolella)
and other that evolved during Carnian (i.e. Carnie-
pigondolella) disappeared during the Carnian-Norian
boundary. Norian was a period of recovery, but the
boundary with the Rhaetian marks the beginning of a
diversity reduction. Despite of some gondolellids forms,
like Mockina, the Rhaetian conodonts were forms with
reduced size and complexity. This can be observed in
the platform-less Misikella or in the P1 element of
Zieglericonus, which was a coniform element, similar
to the primitive conodonts and very rare in the post-
Ordovician ones.
For the TJB, we studied the Rhaetian (4 m.y.). The
species that overpass the Norian-Rhaetian boundary
were 7, with 3 genera and only 1 family (Gondolel-
lidae). This number is quite similar to the Changh-
singian, and even superior to the species that surpassed
other Triassic stage boundaries, like the Olenekian-
Anisian or Carnian-Norian. The main difference with
the PTB is that we have only 5 species during the
Rhaetian that means an origination rate of 0.17 and an
extinction rate of 0.42, which is not particularly high,
but represents more than double than the first one, so
the radiation rate is -0.25, the lowest of all the studied
period. Thus, the extinction of conodonts can be
interpreted, instead of a sudden mass extinction, as a
progressive one due to the reduction of diversity during
the whole Upper Triassic that concluded with the total
extinction of the group just before the beginning of the
Jurassic (Hallam, 2002).
To explain the biological crisis of the TJB have
been pointed out several environmental causes that
include sea-level changes, long-term climate changes
and others (i.e. Tanner et al., 2004). However, about the
conodont extinction we point out a biological evolutive
answer of this Palaeozoic group, with less capable
forms to get adaptation in both the stress environmental
conditions and to the new Mesozoic biota competition
that produced the definitive extinction of the group.
Key words: Evolutionary Rates; Conodonta;
Extinction; Triassic-Jurassic boundary
References:
De Renzi M., Budurov K., Sudar M. The extinction of
conodonts in terms of discrete elements at the
Triassic-Jurassic boundary. Cuadernos de Geolo-
gía Ibérica, 1996, 20: 347-364.
Hallam A. How catastrophic was the end-Triassic mass
extinction? Lethaia, 2002, 35(2): 147-157.
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