Dinosaur extinctions related to the Jenkyns Event (early Toarcian, Jurassic)

Abstract

The early Toarcian Jenkyns Event (~183 Ma) was characterized by a perturbation of the global carbon cycle, global warming, which at continental areas led to intensified chemical weathering, enhanced soils erosion, and intensified wildfires. Warming and acid rain affected diversity and composition of land plant assemblages, caused a loss of forests and thereby impacted on trophic webs. The Jenkyns Event, triggered by volcanic activity of the Karoo-Ferrar Large Igneous Province, changed terrestrial ecosystems, and also affected the dinosaurs. Fossil macroplant assemblages and palynological data reveal reductions in the diversity and richness of plant communities. A substantial loss of land plant biomass and a shift to forests dominated by Cheiropelidiaceae conifers occurred as a consequence of seasonally dry and warm conditions. Major changes occurred to hervivore dinosaurs, with extinction of diverse basal families of Sauropodomorpha (‘prosauropods’) as well as some basal sauropods. Ornithischian dinosaurs show patchy records; some heterodontosaurids disappeared and the scelidosaurids (Thyreophora) went extinct during the Jenkyns Event. The dominant carnivorous dinosaurs, the Coelophysoidea (Theropoda), died out during the Jenkyns Event. We interpret the Jenkyns Event as a terrestrial crisis for ecosystems, marked especially by floral changes and the extinction of some dinosaur clades, both hervivores and carnivores.

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Spanish Journal of Palaeontology, 2022
https://doi.org/10.7203/sjp.25683
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Dinosaur extinctions related to the Jenkyns Event (early Toarcian, Jurassic)
Extinciones de dinosaurios relacionadas con el Evento Jenkyns (Toarciense inferior, Jurásico)
Matías REOLID , Wolfgang RUEBSAM & Michael J. BENTON
Abstract: The early Toarcian Jenkyns Event (~183 Ma) was characterized by a perturbation
of the global carbon cycle, global warming, which at continental areas led to intensi ed
chemical weathering, enhanced soils erosion, and intensi ed wild res. Warming and acid
rain a󰀨 ected diversity and composition of land plant assemblages, caused a loss of forests
and thereby impacted on trophic webs. The Jenkyns Event, triggered by volcanic activity
of the Karoo-Ferrar Large Igneous Province, changed terrestrial ecosystems, and also
a󰀨 ected the dinosaurs. Fossil macroplant assemblages and palynological data reveal
reductions in the diversity and richness of plant communities. A substantial loss of land
plant biomass and a shift to forests dominated by Cheiropelidiaceae conifers occurred as a
consequence of seasonally dry and warm conditions. Major changes occurred to hervivore
dinosaurs, with extinction of diverse basal families of Sauropodomorpha (‘prosauropods’)
as well as some basal sauropods. Ornithischian dinosaurs show patchy records; some
heterodontosaurids disappeared and the scelidosaurids (Thyreophora) went extinct during
the Jenkyns Event. The dominant carnivorous dinosaurs, the Coelophysoidea (Theropoda),
died out during the Jenkyns Event. We interpret the Jenkyns Event as a terrestrial crisis
for ecosystems, marked especially by  oral changes and the extinction of some dinosaur
clades, both hervivores and carnivores.
Resumen: El Evento Jenkyns del Toarciense inferior (~183 Ma) se caracterizó en ambientes
continentales por una perturbación del ciclo del carbono, un calentamiento global, un
aumento de la meteorización, la pérdida de suelos y la proliferación de incendios. El
calentamiento y la lluvia ácida afectaron a la diversidad y composición de las asociaciones
vegetales, causó la pérdida de masas forestales y tuvo un fuerte impacto en las redes
tró cas. El Evento Jenkyns, cuyo detonante fue la intensa actividad volcánica de la Provincia
Ígnea de Karoo-Ferrar, cambió los ecosistemas continentales, afectando entre otros a
los dinosaurios. Los datos palinológicos y de las asociaciones fósiles de macroplantas
muestran una reducción de la diversidad y la riqueza de las comunidades vegetales,
especialmente una pérdida de biomasa y la dominancia de coníferas cheirolepidiáceas
en los bosques, en un contexto de condiciones cálidas estacionalmente áridas. Pueden
observarse cambios importantes entre los dinosaurios herbívoros con la extinción de varias
familias basales de sauropodomorfos (“prosaurópodos”) y algunos saurópodos basales.
Los dinosaurios ornitisquios, pese a su registro más incompleto, muestran la desaparición
de algunas especies de heterodontosáuridos y la extinción de la familia Scelidosauridae
(Thyreophora) en relación con el Evento Jenkyns. Los dinosaurios carnívoros de la
superfamilia Coelophysoidea (Theropoda) también se extinguieron durante el Evento
Jenkyns. Por lo tanto, se interpreta que el Evento Jenkyns contituyó una crisis biótica en los
ecosistemas continentales de gran importancia, marcada especialmente por cambios en
la  ora y la extinción de algunos grupos de dinosaurios tanto herbívoros como carnívoros.
Received: 9 November 2022
Accepted: 7 December 2022
Published online: 22 December 2022
Corresponding author:
Matías Reolid
mreolid@ujaen.es
Keywords:
Hyperthermal event
Dinosaur biotic crisis
Sauropodomorphs
Theropods
Ornithischians
Palabras-clave:
Evento hipertermal
Crisis biótica
Dinosaurios
Sauropodomorfos
Terópodos
Ornitisquios
INTRODUCTION
The early Toarcian was characterized by an important
environmental change called the Jenkyns Event (e.g.,
Müller et al., 2017; Reolid et al., 2020, 2021a; Erba et al.,
2022), one of the most signi cant hyperthermal events
of the Mesozoic (e.g., García Joral et al., 2011; Korte &
Hesselbo, 2011; Suan et al., 2011; Danise et al., 2013;
Baghli et al., 2020; Storm et al., 2020; Ruebsam et
al., 2020a, 2020b). TEX86 palaeothermometry proxies
applied to NW Tethys sediments suggest a warming
of 5ºC during the Pliensbachian–Toarcian transition
and a peak of 10ºC warming during the Jenkyns Event
(Ruebsam et al., 2020b). Other processes documented
during the Jenkyns Event include: (1) a perturbation
of the carbon cycle evidenced as a negative carbon

2Reolid, M. et al. - Dinosaur extinctions related to the Jenkyns Event - Spanish Journal of Palaeontology, 2022
isotopic excursion (CIE; e.g., Jenkyns & Clayton, 1986;
Kemp et al., 2005; Hesselbo et al., 2007; Ruebsam et
al., 2019, 2020a); (2) oxygen depleted conditions in
some marine basins, the Toarcian Oceanic Anoxic Event
(T-OAE, Gill et al., 2011; Fonseca et al., 2018; Izumi et
al., 2018; Ruebsam et al., 2018; Suan et al., 2018); (3)
a sea-level rise (e.g., Hallam, 1981; Pittet et al., 2014;
Haq, 2018; Krencker et al., 2019); and (4) a crisis of
marine carbonate productivity (Bucefalo-Palliani et al.,
2002; Mattioli et al., 2004) and acidication (Müller et
al., 2020; Ettinger et al., 2021).
In the context of environmental change, the early
Toarcian is also characterized by a second-order
extinction that a󰀨ected marine ecosystems, including
dynoagellate cysts, foraminifera, ostracods, brachio-
pods, corals, bivalves, and cephalopods (e.g., Hallam,
1987; Little & Benton, 1995; Harries & Little, 1999;
Aberhan & Fürsich, 2000; Macchioni & Cecca, 2002;
Vörös, 2002; Arias, 2009, 2013; Dera et al., 2010;
García Joral et al., 2011; Caruthers et al., 2014; Baeza-
Carratalá et al., 2017; Reolid et al., 2019; Vasseur
et al., 2021; Reolid & Ainsworth, 2022). In addition
to the extinction, the biotic crisis is also expressed
in a decrease of diversity, mainly a󰀨ecting benthic
communities, as well reductions in body size of some
taxa (Harries & Little, 1999; Piazza et al., 2020).
Key triggers of the Jenkyns Event include the emission
of volcanic CO2 and thermogenic CH4 associated with
the emplacement of the Karoo-Ferrar Large Igneous
Province (LIP) and probably the Chon Aike LIP
volcanism (Pankhurst & Rapela, 1995), that broadly
coincide with the negative CIE (Hesselbo et al., 2007;
Moulin et al., 2017; Fantasia et al., 2019; Font et al.,
2022; Fig. 1). In addition, the destabilization of marine
methane hydrates (Hesselbo et al., 2000; Kemp et
al., 2005), intensied wetland methanogenesis (Them
et al., 2017a) and deterioration of climate-sensitive
reservoir permafrost areas during the global warming
(Ruebsam et al., 2019) may have contributed to the
increase of greenhouse gases into the atmosphere. We
might assume heating and acid rain on land, leading to
a loss of forests and soil wash into the sea, together with
aridity and sporadic heavy rainfall conditions on land,
all features of the standard hyperthermal extinction
model (Benton & Newell, 2014; Benton, 2018).
Even though the Jenkyns Event has been mainly
studied in marine sediments, it also impacted
terrestrial settings, and many authors have identied
a displacement of climatic belts with the spread of
aridity (Rodrigues et al., 2019; Lu et al., 2020; Font et
al., 2022) and enhanced weathering (e.g., Brazier et
al., 2015; Montero-Serrano et al., 2015; Percival et al.,
2016; Them et al., 2017b). During the Jenkyns Event,
global warming also a󰀨ected continental ecosystems.
A decrease of 13C fractionation during photosynthesis
in C3 plants, conrms a turn to arid climate for emerged
areas of Western Tethys during the Jenkyns Event
(Rodrigues et al., 2019, 2021; Ruebsam et al., 2020a).
Other changes in terrestrial ecosystems a󰀨ected
diversity and composition of land plant assemblages
(Slater et al., 2019; Jin et al., 2020) and an increase of
wildres in some areas (Baker et al., 2017).
Dinosaur faunas underwent remarkable evolutionary
changes during the Triassic–Jurassic transition
(Benton, 1993; Brusatte et al., 2008a, 2008b; Allen
et al., 2019; Klausen et al., 2020; Novas et al.,
2021; Langer & Godoy, 2022). Nevertheless, the
discontinuous terrestrial fossil record and the lack of
consistent age constraints have made it di󰀩cult to
correlate the fossil records to environmental changes
or to major events in oral evolution (Barrett, 2014). In
any case, the most studied event that had an impact on
dinosaurs is the End Triassic Mass Extinction (ETME;
Benton, 1993; Brusatte et al., 2010; Allen et al., 2019;
Singh et al., 2021) whereas the early Toarcian global
change remains little studied (Rauhut et al., 2016; Pol
et al., 2020; Reolid et al., 2022).
The purpose of this work is to identify the impact of
the Jenkyns Event on terrestrial ecosystems, mainly
focused on vegetational changes and dinosaur
assemblages. This work is a review of the Early
Jurassic record of Sauropodomorpha, Theropoda and
Ornithischia with the appearance and last occurrence
of some lineages, and we discuss the climatic
transformations in continental environments related to
the early Toarcian Jenkyns Event.
MATERIALS AND METHODS
Climatic trends (global warming/cooling) were
reconstructed from oxygen isotope data measured on
belemnite and brachiopod calcitic shells. Boxplots with
a step size of 2.5 Myr were considered for the δ18O
data to assess secular climate trends and variability
within a 2.5 Myr interval (Supplementary Table 1). The
2.5 Myr step size approximates the temporal resolution
and dating inaccuracy of the palaeontological data.
Boxplots and smoothing splines were calculated using
PAST software (Hammer et al., 2001).
The fossil record of continental vertebrates, is patchy,
with large temporal gaps between stratigraphic
formations (Benton, 1998; Lloyd et al., 2008; Benson
& Butler, 2011; Benton et al., 2011, 2013). Further,
much of what we know comes from particular horizons
or formations, some of them Fossil-Lagerstätten.
In addition, the dating of continental sedimentary
formations is often less certain than for marine
formations.
Nonetheless, despite suggestions that the amount of
error in the data might make large-scale interpretation
risky or awed, we argue instead that the broad story of
the history of life as documented in the fossil record is
roughly correct (Sepkoski et al., 1981; Benton, 1998).
We base this assumption on three lines of evidence,
(1) comparisons of cladograms with the fossil record
show good correspondence in most cases (Norell
& Novacek, 1992; Benton et al., 2000), (2) study of
collector curves shows that the application of intense

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Reolid, M. et al. - Dinosaur extinctions related to the Jenkyns Event - Spanish Journal of Palaeontology, 2022
searching by palaeontologists and even the opening up
of new territories such as China, does not materially
a󰀨ect the large-scale knowledge of stratigraphic
ranges of major clades (Benton, 2008; Benton et al.,
2013), and (3) Lagerstätten do not necessarily distort
the records (Walker et al., 2019).
For this study, we include a total of 83 species (17
Ornithischia, 44 Sauropodomorpha and 22 Theropoda)
(see Supplementary Table 2) after a detailed review
of the taxonomic assignments, updated ages of the
lithostratigraphic formations where fossil remains were
recovered, and geographic areas.
RESULTS
Early Jurassic climates
The Early Jurassic climate was predominantly humid
and warm (Frakes et al., 1992), but characterized by
high variability, which involved colder and warmer
periods (e.g., Dera et al., 2011; Korte & Hesselbo,
2011; Korte et al., 2015; Ruebsam et al., 2019, 2020b).
The Early Jurassic interval between the Triassic/
Jurassic boundary and the early Toarcian Jenkyns
Event is characterized by several smaller carbon-
cycle perturbations identied as CIEs (Cramer &
Jarvis, 2020; Schoepfer et al., 2022). The Sinemurian–
Pliensbachian boundary is marked by a negative CIE
(Franceschi et al., 2019) and the Ibex Zone of the lower
Pliensbachian is characterized by a positive CIE and
warming (Armendáriz et al., 2012).
Moreover, di󰀨ering latitudinal climatic belts developed
during the Early Jurassic (e.g., Rees et al., 2000; Dera
& Donnadieu, 2012; Boucot et al., 2013; Philippe et al.,
2017). High latitudinal areas experienced temperate
climate conditions with recurring excursions to warm-
temperate, cold-temperate and cold climates (e.g.,
Zakharov et al., 2006; Devyatov et al., 2011; Ruebsam
& Schwark, 2021). Low-latitude areas along the Tethys
margins were located in the tropical to subtropical
climate belt with high precipitation rates. Semi-arid
to arid conditions prevailed in the interior regions of
Laurentia and southern Gondwana (Parrish et al.,
1982; Rees et al., 2000).
During the Sinemurian and earliest Pliensbachian
global temperatures continued high with equatorial
sea surface temperatures in the range 30–33°C as
calculated from the TEX86 proxy (Robinson et al., 2017).
Based on δ18O from belemnite ndings from the
UK (Bailey et al., 2003), Spain (Rosales et al.,
2004; Gómez et al., 2008) and France (Harazim et al.,
2013), a cooling and regression has been proposed for
the late Pliensbachian. In addition, Tchoumatchenco
et al. (2008) identied sediments of potentially glacial
origin deposited during the times of the Emaciatum
Zone (upper Pliensbachian) and Polymorphum Zone
(lower Toarcian) from the Mediterranean Province.
During the late Pliensbachian, cooling may have
promoted the formation of high-latitudinal glaciation
with wider incidence during the latest Pliensbachian
to earliest Toarcian (Dera et al., 2011; Ruebsam et
al., 2019; Ruebsam & Schwark, 2021; Fleischmann
et al., 2022). The end Pliensbachian is characterized
by a sea-level drop with subsequent ooding and a
negative CIE related to the Pliensbachian–Toarcian
boundary event (Bodin et al., 2016; Al-Suwaidi et al.,
2022; Fleischmann et al., 2022). The negative CIE of
the Pliensbachian–Toarcian boundary and the related
palaeoenvironmental changes have been attributed
to an early phase of Karoo-Ferrar LIP magmatism
(Percival et al., 2015, 2016; Fig. 1).
According to Ruebsam et al. (2020b), latest Pliensba-
chian to early Toarcian equatorial sea surface tem-
peratures varied between 22 and 32°C, attesting to
extremely variable and contrasting climatic conditions.
Subsequently, the sea water temperatures at low lati-
tudes increased by about 10°C during the Jenkyns
Event related to the carbon cycle perturbation (lower
Serpentinum–Levisoni ammonite Zone; Ruebsam et
al., 2020b; Fernández et al., 2021). Studies from oxy-
gen isotopes indicate that temperatures continued high
during the middle and late Toarcian (Dera et al., 2011;
Korte et al., 2015).
Vegetation record
Palynological analyses indicate that the Triassic–
Jurassic fern spike was followed by dominance of the
Cheirolepidiaceae conifers during the Hettangian and
early Sinemurian in China, Australia, New Zealand and
Europe (Akikuni et al., 2010; Bonis et al., 2010; Diéguez
et al., 2010; de Jersey & McKellar, 2013; Gravendyck
et al., 2020; Li et al., 2020). In the Sichuan Basin
(China) the lowermost Jurassic is characterized by
high abundance of fern palynomorphs, and progressive
increase of conifers, mainly Cheirolepidiaceae and
Pinaceae (Li et al., 2020). The palynological record of
the lowest Jurassic in North Germany is characterized
by dominant conifer forests composed of Pinaceae,
Podocarpaceae and Cheirolepidiaceae with abundant
Selaginellales (Gravendyck et al., 2020).
The Pliensbachian in the European record is
characterized by palynological assemblages indicative
of high-diversity forests, dominated by a mixture of
bisaccate pollen-producing conifers and seed-ferns
accompanied by spore-producing mosses (bryophytes)
and club mosses (lycophytes) (Slater et al., 2019;
Danise et al., 2021). Increased proportions of spore-
producing mosses (bryophytes) and clubmosses
(lycophytes) indicate relatively wetter conditions on
land during the Pliensbachian (Slater et al., 2019). In
South America, the record of the pre-Toarcian plant
assemblages of the Cañadón Asfalto Basin (Argentina)
consists of diverse sphenophytes, dipteridacean ferns,
conifers, cycads, seed-ferns, and bennetitaleans,
collectively suggesting humid climates (Escapa et al.,
2008; Choo et al., 2016). The beginning of the Toarcian
is characterized by a decrease in diversity and richness

4Reolid, M. et al. - Dinosaur extinctions related to the Jenkyns Event - Spanish Journal of Palaeontology, 2022
Figure 1. Di󰀨erent proxies of environmental conditions during the latest Pliensbachian to middle Toarcian in the British
sedimentary basins (Cardigan Bay Basin and the Cleveland Basin). Comparison of the δ13C from organic matter of the Mochras
borehole, Cardigan Bay Basin (Xu et al., 2018a; Storm et al., 2020), percentage of Classopolis spp. (Cheirolepidiacea conifer),
richness and diversity (H-index) of land plants from Yorkshire, Cleveland Basin (Slater et al., 2019), Hg/TOC values from the
Mochras borehole as a proxy of global vulcanism (Percival et al., 2016) and 187Os/188Os ratio from the Mochras borehole as
a proxy of continental weathering (Percival et al., 2016). Enhanced global vulcanism is relatively early with respect to the
Jenkyns Event (grey band) with maximum values coinciding with the onset of the negative CIE and subsequent changes in
the land plant community and continental weathering. Note that the Pliensbachian–Toarcian boundary coincides with a volcanic
event and changes in the land plants.
of palynological assemblages with an increase in
abundance of cycads (Chasmatosporites spp.) and a
drop in bisaccate pollen-producing conifers and seed-
ferns (Pieńkowski et al., 2016; Slater et al., 2019).
At the onset of the Jenkyns Event identied by the
negative CIE, land plants su󰀨ered a major drop in
diversity and richness (Slater et al., 2019; Danise et al.,
2021; Fig. 1). Poorly diversied forests dominated by
Cheirolepidiaceae (represented by Classopollis spp.)
and cycads replaced the previous forests of bisaccate
pollen-producing conifers and seed ferns.
Land plant communities during the middle–late
Toarcian, after the Jenkyns Event, presented low
diversity in Europe and South America, with dominance
of the conifer families Cheirolepidiaceae, Cupressaceae
and Araucariacea (Escapa et al., 2008; Olivera et al.,
2015; Choo et al., 2016; Deng et al., 2018; Slater et
al., 2019), together indicating seasonally dry and warm
conditions.
Therefore, the increase of warm -and drought- adapted
plants through the Jenkyns Event, suggests a warmer
and drier climate than in the Pliensbachian with strong
seasonality favouring drought-adapted plants. These
conditions persisted during the middle and late Toarcian
with the abundance of xerophytic and thermophilic
plant groups in both the southern hemisphere (Escapa
et al., 2008; Olivera et al., 2015; Choo et al., 2016; Pol
et al., 2020) and northern hemisphere (Deng et al.,
2018; Slater et al., 2019).
Dinosaur assemblages
Sauropodomorpha. After the extinction of most of the
basal Sauropodomorpha, members of Plateosauridae
and Melanorosauridae, and some genera of basal
sauropods (Brusatte et al., 2010; McPhee et al., 2017;
Novas et al., 2021; Reolid et al., 2022) at the end of the
Triassic, the beginning of the Jurassic is characterized
by the rst occurrence of many taxa. During the earliest
Jurassic new basal sauropodomorphs (‘prosauropods’)
such as Massospondylidae and Anchisauria, radiated
from Hettangian to Pliensbachian (Fig. 2) with
sizes ranging from 1.5 m in Ignavusaurus to 9 m in
Lufengosaurus in the case of Massospondylidae
(Barrett et al., 2005; Knoll, 2010) and from 2.4 m in
Anchisaurus to 10 m in Jingshanosaurus in the case of
Anchisauria (Galton & Upchurch, 2004; Yates, 2004).
Basal Sauropoda diversied at the beginning of the
Jurassic (e.g., Ammosaurus, Antetonitrus, Ledumahadi,
Pulanesaura, Yizhousaurus) with sizes ranging
from 2.5 m in Ammosaurus to 12 m in Ledumahadi
(Brusatte et al., 2010; McPhee et al., 2017, 2018).

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Reolid, M. et al. - Dinosaur extinctions related to the Jenkyns Event - Spanish Journal of Palaeontology, 2022
Moreover, Tonganosaurus, the rst eusauropod (Family
Mamenchisauridae) is recorded in the Hettangian of
the Yimen Formation (China; Li et al., 2010). However,
Eusauropoda diversied during the Middle Jurassic (Pol
et al., 2020; Reolid et al., 2022).
The earliest Jurassic was a time for geographic
dispersion of sauropodomorphs that extended north
of the palaeoequator and are recorded from North
America (Ammosaurus, Anchisaurus, Sarahsaurus,
Seitaad) and Asia (Irisosaurus, Jingshanosaurus,
Lufengosaurus, Xingxiulong, Xixiposaurus,
Yimenosaurus, Yizhousaurus, Yunnanosaurus) (e.g.,
Bai et al., 1990; Yates, 2010; Rowe et al., 2011; Wang
et al., 2017a; Peyre de Fabrègues et al., 2020).
The beginning of the Toarcian constitutes the extinction
boundary for the last basal sauropodomorphs
(‘prosauropods’), including Anchisauria and
Massospondylidae, even though they were diverse
during the Sinemurian and Pliensbachian (Fig. 2).
Basal Sauropoda were also a󰀨ected but some of
them survived the Jenkyns Event in Europe with
Ohmdenosaurus (Wild, 1978), in Africa with Vulcanodon
(Cooper, 1984) and Tazoudasaurus (Allain et al., 2004),
in India with Barapasaurus (Jain et al., 1975), and in
Asia with Isanosaurus, Gongxianosaurus, Sanpasaurus
and Zizhongosaurus (Hou et al., 1976; Bu󰀨etaut et al.,
2000; Yaonan & Wang, 2000; McPhee et al., 2016)
(Fig. 2). However, basal Sauropoda disappeared
before the Aalenian (Middle Jurassic; Figs. 2 and 3),
with the exception of Archaeodontosaurus, a possible
basal sauropod from the Bathonian of Madagascar
(Bu󰀨etaut, 2005).
After the Jenkyns Event, Eusauropoda diversied (Fig.
3) with records of the genera Bagualia, Patagosaurus,
Volkheimeria from South America, Spinophorosaurus
from Africa, and Nebulasaurus from Asia (Remes et al.,
2009; Xing et al., 2015; Pol et al., 2020; Reolid et al.,
2022).
Ornithischia. The other group of herbivorous dinosaurs,
the ornithischians, diversied after the End Triassic
Mass Extinction, at the beginning of the Jurassic in
South Gondwana with the appearance of new genera
corresponding to the Family Heterodontosauridae
(Abrictosaurus, Heterodontosaurus, and Pegomastax;
Thulborn, 1974; Sereno, 2012), but also with the origin
of the Family Fabrosauridae (Africa; Fabrosaurus
and Lesothosaurus; Butler, 2005; Butler et al., 2008)
composed of small bipedal forms (Figs. 2 and 3) generally
< 2 m long. The clade Thyreophora, characterized by
parallel rows of keeled dermal armour scutes or bony
plates (osteoderms) on the dorsal surface of the body
debuted in the Early Jurassic (Fig. 3). They were
represented by the Family Scelidosauridae, which
originated and diversied during the Early Jurassic
in Laurasia (‘Lusitanosaurus’, Scelidosaurus, and
Emausaurus from Europe, Laquintasaura from Central
America, Scutellosaurus from North America, and
Yuxisaurus and ‘Bienosaurus’ from Asia) (Lapparent
& Zbyszewski, 1957; Colbert, 1981; Haubold, 1990;
Dong, 2001; Baron et al., 2016). Scelidosaurids were
quadrupedal and had heavily built bodies (in some
cases reaching 4 m long), but probably Scutellosaurus
and Emausaurus were bipedal forms (Riguetti et al.,
2022). However, in the phylogenetic analyses of
Maidment (2010), Raven and Maidment (2017) and
Norman (2021), Scutellosaurus, Emausaurus and
Scelidosaurus are regarded as successive basal
thyreophorans (see also Thompson et al., 2012), so
Family Scelidosauridae would then be a paraphyletic
group.
The rst genus of Neornithischia, Stormbergia,
appeared during the Hettangian–Sinemurian in South
Gondwana (South Africa and Lesotho; Butler, 2005).
Theropoda. As for the sauropodomorphs, theropods
extended and diversied during the Early Jurassic.
The dominant early Jurassic theropods were Coelo-
physoidea with the families Coelophysidae (Coelophy-
sis, Megapnosaurus, Panguraptor, and Sarcosaurus)
and Dilophosauridae (Dilophosaurus, Dracovenator,
and Shuangbaisaurus) (Figs. 2 and 3). Both families
colonized Africa (Megapnosaurus, Dracovenator)
and Asia (Panguraptor, Shuangbaisaurus) during the
Hettangian and Sinemurian (Yates, 2006; You et al.,
2014; Wang et al., 2017b). The dilophosaurids occu-
pied the top of the trophic chain, with Dracovenator and
Dilophosaurus (5–6.5 m and 270–390 kg; Therrien &
Henderson, 2007; Reolid et al., 2021b) being among
the largest theropods in the Early Jurassic. However,
Marsh and Rowe (2020) proposed that Dilophosaurus
is a stem-averostran theropod rather than a member of
Coelophysoidea.
In addition to coelophysoids, new theropod groups
appeared during the earliest Jurassic, including stem-
averostran Tachiraptor (Langer et al., 2014), basal
forms of Ceratosauria (Saltriovenator; Dal Sasso et
al., 2018) and Tetanurae (Dracoraptor, Sinosaurus,
Kayentavenator, and Cryolophosaurus) (Fig. 2). They
were robuster forms than coelophysoids (Saltriovenator
zanellai and Cryolophosaurus ellioti reached more than
6.5 m; Smith et al., 2007; Dal Sasso et al., 2018).
The beginning of the Toarcian represented a major
break in the evolution of theropods. The coelophysoids
disappeared during the Toarcian (Figs. 2 and 3).
However, new basal Ceratosauria are recorded in
the Toarcian such as Dandakosaurus from India
(Yadagiri, 1982) and Berberosaurus from Morocco
(Allain et al., 2007). After the Jenkyns Event, during
the late Toarcian, the rst allosauroids (Asfaltovenator;
Metriacanthosauridae) and the rst megalosauroids
(Condorraptor and Piatnitzkysaurus; Piatnitzkysauridae)
are recorded in South America (Rauhut, 2005; Carrano
et al., 2012; Rauhut & Pol, 2019) (Fig. 2).

6Reolid, M. et al. - Dinosaur extinctions related to the Jenkyns Event - Spanish Journal of Palaeontology, 2022
Figure 2. Distribution of genera of sauropodomorphs, ornithischians and theropods from Hettangian to Aalenian. The early
Toarcian biotic crisis associated with the Jenkyns Event, is indicated with a dark grey bar. Note that many dinosaur taxa
disappear during the Jenkyns Event or after that but before the Aalenian stage.

7
Reolid, M. et al. - Dinosaur extinctions related to the Jenkyns Event - Spanish Journal of Palaeontology, 2022
DISCUSSION
Early Jurassic diversication of dinosaurs
The Triassic/Jurassic transition is characterized by
severe environmental stress in the ocean and on land,
as well as enhanced atmospheric CO2 concentrations
(McElwain et al., 1999; Cohen & Coe, 2007; Michalík et
al., 2007; Whiteside et al., 2010). The volcanic activity
of the Central Atlantic Magmatic Province (CAMP)
caused a massive input of greenhouse gases into the
atmosphere that triggered global warming (4 to 5ºC;
McElwain et al., 1999; Schoepfer et al., 2022) and mass
extinction both on land and sea (Whiteside et al., 2010).
Acid rain was generated by the CAMP eruptions, as
well as increased wildres and the resulting extensive
deforestation probably caused by warmer and drier
climates (Blackburn et al., 2013; Thibodeau et al., 2016;
Percival et al., 2017; Pole et al., 2018; Alipour et al.,
2021; Zhang et al., 2022).
At the end of the Rhaetian, crurotarsan archosaurs
su󰀨ered substantial extinction and only the
crocodylomorph lineage persisted (Brusatte et al.,
2008a; Benton et al., 2014). Herbivores such as
dicynodonts (Therapsida) and aetosaurs (Crurotarsi)
and carnivores such as poposauroids (Crurotarsi)
and phytosaurs disappeared at the end of the
Triassic (Benton et al., 2014). Among dinosaurs, both
sauropodomorphs and theropods were a󰀨ected by the
extinction (Benton, 1993; Brusatte et al., 2010; Singh et
al., 2021; Reolid et al., 2022).
After the ETME, an evolutionary radiation of dinosaurs
occurred and dinosaurs became the most successful
group of terrestrial vertebrates during the Jurassic and
Cretaceous, occupying many di󰀨erent ecological niches
and exhibiting a wide range of adaptations (Lloyd et al.,
2008; Brusatte et al., 2010; Langer et al., 2010; Benton
et al., 2014). However, Brusatte et al. (2008b, 2010)
and Benton et al. (2014) indicated that the extinction of
competitor groups such as crurotarsan archosaurs at
the end of the Triassic was not met with an explosion of
dinosaurs in terms of morphological disparity.
The Early Jurassic is characterized by a progressive
increase of diversity and presence of plants indicative of
more humid conditions and the development of a more
pronounced latitudinal climate gradient (Rees et al.,
2000; Escapa et al., 2008; Choo et al., 2016; Philippe et
al., 2017; Slater et al., 2019; Gravendyck et al., 2020).
Early Jurassic sauropodomorphs prospered according
to the recovery of primary producers. Most of their key
adaptations such as huge size, quadrupedal locomotion
and graviportal specializations, continued into the Early
Jurassic. The basal sauropods were the same size as
the largest basal sauropodomorphs and shared the
same habitats (Apaldetti et al., 2018). Basal sauropods
and basal sauropodomorphs (‘prosauropods’) probably
avoided competition for trophic resources through
specializations in masticatory apparatus such as
U-shaped jaws, spatulate tooth crowns with small
denticles and the presence of a lateral plate on the
dentary (Barrett & Upchurch, 2007; Yates et al., 2010).
Herbivorous ornithischians during the earliest Jurassic
were globally distributed and relatively diverse and
abundant. The heterodontosaurids diversied and
other families originated such as fabrosaurids and
scelidosaurids (Thyreophora). The diversication
of ornithischians may be related to the end-Triassic
extinction of several herbivore groups that left empty
ecological niches to occupy (Olsen et al., 2002; Butler
et al., 2007; Brusatte et al., 2008b).
In the case of carnivorous dinosaurs, the Early Jurassic
theropods were much more diverse and showed an
increased variability of morphologies compared to the
Late Triassic. The proliferation of theropods drove the
colonization of all continents and increases in size in
many new taxa. The coelophysoids (Dilophosauridae
and Coelophysidae) dominated during the earliest
Jurassic (Brusatte et al., 2010; Fig. 3), and basal forms
of Ceratosauria and Tetanurae appeared (Smith et al.,
2007; Dal Sasso et al., 2018), being robust carnivores
characterized by larger body sizes and more disparate
morphology.
The extinction of many carnivores (rauisuchians,
phytosaurs and ornithosuchids) at the end of the
Triassic surely favoured the evolutionary radiation of
theropods and the colonization of new territories during
the Early Jurassic (Olsen et al., 2002; Brusatte et al.,
2008b, 2010; Benton et al., 2014). Reolid et al. (2022)
point out that the diversication of theropods (taxonomic
and morphological) during the Early Jurassic indicates
specializations for di󰀨erent prey and ecological niches
related to the diversication of sauropodomophs and
ornithischians.
The early Toarcian global warming
Even though the record of Toarcian continental
vertebrates is poor, it is clear that some clades of
herbivores and carnivores disappeared during the
Toarcian biotic crisis (Figs. 2 and 3). During the
Pliensbachian–Toarcian transition and during the
Jenkyns Event, 83% of genera of sauropodomorphs
disappeared (10 of 12 genera), accounting for all
families of ‘prosauropods’ (Massospondylidae and
Anchisauria). Some basal Sauropoda survived into the
early Toarcian (e.g., Barapasaurus, Gongxianosaurus,
Isanosaurus, Ohmdenosaurus, Sanpasaurus,
Tazoudasaurus, Vulcanodon, and Zizhongosaurus;
Jain et al., 1975; Hou et al., 1976; Wild, 1978; Cooper,
1984; Bu󰀨etaut et al., 2000; Yaonan & Wang, 2000;
Allain et al., 2004; McPhee et al., 2016). But these taxa
disappeared before the Aalenian (Fig. 2).
Among the ornithischians, Pliensbachian species
disappear, but new genera of Fabrosauridae and
Heterodontosauridae are recorded during the Middle
Jurassic. In the case of the thyreophorans of the
Family Scelidosauridae, the early Toarcian marks their
extinction (Fig. 3).

8Reolid, M. et al. - Dinosaur extinctions related to the Jenkyns Event - Spanish Journal of Palaeontology, 2022
Changes in vegetation, the primary producers in
trophic chains in terrestrial ecosystems, triggered the
extinction of herbivores and the consequent extinction
of some carnivores. Land plants su󰀨ered a major loss
in richness and diversity at the onset of the negative
CIE that characterized the Jenkyns Event (Deng
et al., 2018; Slater et al., 2019; Danise et al., 2021;
Fig. 1). Enhanced weathering, lost of soils and loss
of richness of vegetation advanced in parallel with
environmental deterioration (Fig. 1). Analyses of spore-
pollen assemblages from both hemispheres show that
vegetation shifted from a Pliensbachian high-diversity
assemblage with conifers, seed ferns and lycophytes, to
an early Toarcian low-diversity assemblage dominated
by Cheirolepidiacea conifers, and minority cycads,
Araucariaceae and Cupressaceae (Escapa et al., 2008;
Olivera et al., 2015; Choo et al., 2016; Slater et al., 2019;
Danise et al., 2021). Cheirolepidiaceae are interpreted
as xerophytic and thermophilous trees of subtropical-
tropical climates, and some cheirolepidaceans were
coastal shrubs that tolerated seasonally dry and warm
conditions (Alvin, 1982; Tosolini et al., 2015), having
characteristics typical of disaster taxa that dominate
disturbed ecosystems after crises.
As for the carnivores, Pliensbachian theropods
were severely a󰀨ected with the extinction of many
taxa during the early Toarcian (Coelophysis,
Cryolophosaurus, Dilophosaurus, ‘Podokesaurus’,
Segisaurus). Coelophysoids disappeared in the early
Toarcian, probably a󰀨ected by the extinction of many
of their potential prey (Figs. 2 and 3). Only two basal
ceratosaurians (Berberosaurus and Dandakosaurus)
have been recorded in the lower Toarcian (Fig. 2).
The e󰀨ects of volcanogenic global warming were
hostile for both marine and continental ecosystems of
the Early Jurassic (e.g., Little & Benton, 1995; Harries
& Little, 1999; Piazza et al., 2020; Pol et al., 2020;
Reolid et al., 2022). The early stages of Karoo-Ferrar
LIP volcanism occurred at the Pliensbachian–Toarcian
boundary (Them et al., 2018; Xu et al., 2018b) and
coincided with the initial shift in land plant communities
(Slater et al., 2019) and a temperature uctuation about
5ºC warming followed by 8ºC cooling in the middle
Polymorphum (Tenuicostatum) ammonite Zone. The
main impact on land plant communities recorded at the
start of the negative CIE contemporaneous with the
main activity phase of the Karoo-Ferrar LIP (Sell et al.,
2014; Burgess et al., 2015; Moulin et al., 2017; Them
et al., 2018; Font et al., 2022; Ruhl et al., 2022; Fig.
1) and Chon Aike LIP volcanism (Pankhurst & Rapela,
1995) was accompanied by about 10ºC warming
(Ruebsam et al., 2020b) and enhanced continental
weathering (Percival et al., 2016; Them et al., 2017b;
Fig. 1). Consequently, during the early Toarcian, drastic
climate uctuations profoundly impacted on continental
ecosystems causing a biotic crisis with shifts in ora
and fauna.
Figure 3. Comparison of the distribution of the main clades of dinosaurs from Hettangian to Aalenian with temperature
uctuations as inferred from δ18O values (box plots show oxygen isotope data in a 2.5 Myr window) and the composite global
δ13C curve from carbonate by Schoepfer et al. (2022) for the Hettangian–Toarcian and by Bartolini et al. (1999) for the Aalenian.
The width of the bars is proportional to the number of genera. The Jenkyns Event constituted an important biotic crisis with
a major role in dinosaur evolution with extinctions and subsequent radiations of new taxa.

9
Reolid, M. et al. - Dinosaur extinctions related to the Jenkyns Event - Spanish Journal of Palaeontology, 2022
CONCLUSIONS
The early Toarcian Jenkyns Event was characterized
in terrestrial environments by global warming,
perturbation of the carbon cycle, enhanced weathering
and wildres. This event profoundly a󰀨ected the course
of dinosaurian evolution. We might expect that heating,
arid conditions, and potential acid rain on land should
lead to a loss of diversity and plant biomass and would
a󰀨ect the rest of the trophic web.
Early Jurassic (pre-Toarcian) plant assemblages were
dominated by conifers (mainly Cheirolepidiaceae and
Pinaceae) and ferns with large fronds. Cycadophytes,
ginkgophytes, and bennetitaleans and seed-ferns also
were common, whereas bryophytes and lycophytes
were more abundant in high latitude regions. The plant
distribution indicates well-established climatic belts. At
this time, sauropodomorphs dominated the herbivore
guild and diversied. The basal sauropodomorphs
(‘prosauropods’) Massospondylidae and Anchisauria
radiated from Hettangian to Pliensbachian. Basal
Sauropoda also diversied in the Early Jurassic.
Ornithischians, the other main group of hervivorous
dinosaurs, diversied in the Early Jurassic in Gondwana
with the radiation of Heterodontosauridae and the origin
of Fabrosauridae. The new Suborder Thyreophora,
represented by the Family Scelidosauridae, diversied
in Laurasia.
Among theropods, Coelophysoidea (families
Coelophysidae and Dilophosauridae) dominated
during the Hettangian and Sinemurian and extended
from North America and Europe to Asia and Africa.
In addition, some basal forms of Ceratosauria and
Tetanurae appeared in the Early Jurassic.
The beginning of the Toarcian is characterized
by decreased diversity and richness of land-plant
assemblages. Low-diversity forests were dominated
by xerophytic, thermophilic conifers such as
Cheirolepidiaceae, indicating seasonally dry and warm
conditions. Climatic and vegetation changes drove the
extinction of all basal sauropodomorph ‘prosauropods’,
at the base of the Toarcian and the basal Sauropoda
surviving the Jenkyns Event disappeared during the
middle and late Toarcian. The ornithischian armored
dinosaurs, Scelidosauridae, died out during the
Toarcian. Carnivorous theropods Coelophysidae and
Dilophosauridae, which were dominant in Jurassic pre-
Toarcian, disappeared with the Jenkyns Event.
Consequently, we recognize the Jenkyns Event as
an important biotic crisis for terrestrial ecosystems,
a󰀨ecting plants and dinosaurs. More studies on
dinosaurs, but also in other continental tetrapods and
in arthropods will help to understand the impact of the
Jenkyns Event in the Mesozoic life history.
Supplementary information. The article has two tables
as supplementary data available at the Spanish Journal
of Palaeontology web-site (https://sepaleontologia.es/
spanish-journal-palaeontology/) linked to the corresponding
contribution. The information provided by the author has not
being copy edited or substantially formatted.
Supplementary Table 1. Data base of dinosaur species
from Hettangian (Early Jurassic) to early Aalenian (Middle
Jurassic) with taxonomic assignments, updated ages of
the lithostratigraphic formations where fossil remains were
recovered and geographic areas.
Supplementary Table 2. Data base of δ18O from shells for the
Hettangian to Toarcian (Early Jurassic) including average,
variance and standard deviation.
Author contributions. MR, WR and MJB conceived the
study, analysed the bibliography and data, and wrote the
manuscript.
Competing interests. The authors declare that they do not
have any competing interests.
Funding. This research was funded by the projects PY20_00111,
UJA-1380715, and RNM-200 Research Group (Junta de
Andalucía, Spain) and PID2019-104625RB-100 (Spanish
Government).
Author details. Matías Reolid1, Wolfgang Ruebsam2 &
Michael J. Benton3. 1Departamento de Geología and
CEACTEMA, Campus Las Lagunillas sn, Universidad de
Jaén, 23071 Jaén, Spain; mreolid@ujaen.es; 2Department of
Organic and Isotope Geochemistry, Institute of Geoscience,
University of Kiel, Ludewig-Meyn Str. 10, 24118 Kiel,
Germany; wolfgang.ruebsam@googlemail.com; 3School of
Earth Sciences, University of Bristol, Wills Memorial Building,
Queens Road, Bristol BS8 1RJ, United Kingdom; Mike.
Benton@bristol.ac.uk.
Acknowledgements. We thank the Editor Carlos Martínez-
Pérez for inviting us to submit this work to the Spanish Journal
of Palaeontology. We thank the Editor Carlos Martínez-Pérez,
Associated Editors Samuel Zamora and Sonia Ros, and the
reviewers Xabier Pereda Suberbiola and Francisco Ortega
for their comments and suggestions.
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