Primitive or Advanced?
For a long time, trilobites were considered the most primitive
of arthropods, since they were among the first Cambrian arthropods
discovered, but this turns out to have been mistaken; an artifact of
preservation. The first recorded Cambrian outcrops only preserved species
with hard, shelly parts, such as trilobites, brachiopods, mollusks, and echinoderms.
When the Burgess Shale,
Chengjiang, and other
similar Cambrian Konservat-Lagerstätten (remarkably
preserved fossil deposits) were discovered, large numbers of more delicate
Cambrian arthropod species (that typically do not preserve at all) were revealed
as contemporaries of trilobites, rather than
their descendants (see diorama below). Trilobites are now considered relatively advanced
among the Paleozoic arthropods, and the search for the first arthropods
and the ancestors of trilobites is
pushed back into the Precambrian.
Status of "Trilobitomorpha"
In the 1959
Trilobite Treatise, many of the Burgess Shale arthropods were included
in the arthropod Subphylum Trilobitomorpha. Today that subphylum
is no longer considered valid -- it served as a convenient unit for various
paleozoic taxa with similarities in limb structure, but has fallen aside for
more rigorous analyses. One prevalent recent classification recognizes the
Trilobita as a class of arthropods sitting comfortably within
the Superclass Arachnomorpha (an expanded concept based on the Chelicerata),
alongside the Superclass Crustaceomorpha (based on an expansion of
the Crustacea), together comprising the Subphylum Schizoramia (Arthropods
bearing biramous limbs) contrasting with the Subphylum Atelocerata
(insects, myriapods, and allies). This classification diagram is shown
below.
TRILOBITA AMONG THE MAJOR CLADES OF PALEOZOIC
ARTHROPODA
While there is certainly a great deal of diversity of form among
the arthropods and near-arthropods (such as the large, predatory protarthropodan
Anomalocaris), it seems reasonable that shared common ancestry
in the Pre-Cambrian was the basis for the radiations of the early Cambrian,
such as seen at the Burgess Shale (Canada), Chengjiang (China) and Sirius
Passet (Greenland). This shared ancestry and close relationship despite
seemingly great divergence of form is seen in the very similar molecular
biology of modern crustaceans and insects, and suggests
to some workers that the Subphylum and Superclass designations for the Arthropoda
may be superfluous (especially that dividing the Atelocerata from the Schizoramia).
Arachnomorpha and Crustaceomorpha
Among the diversity of Paleozoic arthropods, two large groups
have emerged in recent phylogenetic analyses: a group of crustacean-like
arthropods referred to as Crustaceomorpha, and a group referred
to as Arachnomorpha. The diversity within the Arachnomorpha is dominated
by Trilobita and Chelicerata, each forming major clades. Arachnomorpha
(Størmer 1944) is equivalent to a grouping called Arachnata (Lauterbach
1983), which is defined as an inclusive grouping of non-crustacean arthropods:
a clade stemming from the ancestor of Trilobita and Chelicerata. The
Arachnomorph cladogram and its two major clades (sensu Cotton and Braddy
2004) is shown below.
“Chelicerate Clade” and “Trilobite
Clade”
Arachnomorph diversity allows attempts to elucidate
the relationships between trilobites and other Paleozoic arachnomorphs (see,
for example, Edgecombe & Ramskøld 1999, Cotton & Braddy 2004).
It is relatively easy to exclude a large subset of arachnomorphs that fall
into a “Chelicerate Clade,” which can be generally characterized
as arachnomorphs bearing a post-anal telson/tail, and a pair of limbs per
segment (among other synapomorphies). The Burgess Shale arthropods Sidneyia
and Yohoia are two good example members of this Chelicerate Clade
(see above).
In contrast, the “Trilobite Clade” includes arachnomorphs lacking
a telson, and with dorsal segments that may cover more than one pair of
legs. The pygidium of trilobites is a good example of a fused single dorsal
tergite covering several pairs of legs. Taken to extreme, the single-piece
carapace of Tegopelte, and the two-piece carapace of Naraoia
also represent this trend of decoupling of dorsal tergites with limb pairs
(see below). We owe much of our recent understanding of the trilobite clade
to the exceptionally well preserved assemblages of arthropods from Chengjiang,
which greatly complements the assemblage from the Burgess Shale.
Features of the “Trilobite Clade”
Most of the arachnomorph trilobite clade share features in common with
trilobites, such as a hypostome, 3-4 pairs of post-antennular legs under
the cephalon, similar limb structure, etc. Such similarities have led to
proposals to include some trilobite-like arachnomorphs in the Class Trilobita
(e.g., the Naraoiidae). However,
some recent cladistic analyses of arachnomorph taxa have elevated Helmetiidae
as a clade closely aligned with the trilobites (see figure above, and discussion
below), which would create a paraphyletic situation if the naraoiids are
also considered trilobites. It seems more reasonable to recognize the similarities
of members of the “Trilobite Clade” of the Arachnomorpha, while also recognizing
their distinctiveness.
Below, each of the members of the "Trilobite Clade" are discussed,
and the constituent species are pictured. Several different sources were
used to compile the reconstructions pictured here, and I am pleased to note
that this site is the first publication (web or otherwise) to picture all
of the pertinent genera and species of the "Trilobite Clade" in one place,
and in a consistent style.
Helmetiidae
Helmetia expansa Burgess Shale
Kuamaia lata Chengjiang
Kuamaia muricata Chengjiang
Skioldia aldna Chengjiang
Rhombicalvaria acantha Chengjiang
When the Burgess Shale animal Helmetia expansa was described
(above, left), it was known from only a single, incompletely preserved
specimen that displayed neither complete limbs nor antennae. The remarkable
Chengjiang biota includes four species of helmetiids, clearly similar to
Helmetia, and demonstrating antennae, trilobite-like limbs,
stalked ventral eyes, and an anterior lobe. Most of the species show a distinct
head shield, thorax, and tail shield, very similar to trilobites. The exception,
Skioldia aldna, displays features intermediate between typical
helmetiids and tegopeltids such as Saperion (described below). In
several recent cladistical analyses, helmetiids emerge as the arachnomorph
clade most similar to trilobites, displacing the Naraoiidae as the clade
nearest to trilobites, and suggesting that the idea of naraoiids as a "ninth
order" of trilobites should probably be set aside.
Tegopeltidae
Tegopelte gigas Burgess Shale
Saperion glumaceum Chengjiang
The large, enigmatic arthropod Tegopeltegigas from the
Burgess Shale was initially described as a segmented "soft-bodied trilobite"
akin to Naraoia. More recent reexamination of the specimen indicated
the four purported tergite boundaries of Tegopelte were asymmetrical
and probably compression fold artifacts. Saperion glumaceum, a similar
large, ovoid arthropod with multiple somites covered by a single dorsal
shield, was documented from the Chengjiang fossil beds, and was placed in
the family Tegopeltidae in recognition of its similarity to Tegopelte.
The ventral details of Tegopelte are poorly preserved, while specimens
of Saperion clearly revealed stalked ventral eyes, and an anterior
lobe similar to that found in helmetiids. My reconstruction of Tegopelte
presumes that it also bears similar structures, although they are not clearly
visible in the Burgess type specimen. Compelling similarities between Saperion
and the helmetiid Skioldia (see above), particularly the splayed
tergal boundaries that fail to reach the lateral margins, indicate the close
relationship between helmetiids and tegopeltids. Indeed, the only diagnostic
feature of tegopeltids is that the margin is entire, smooth, and unadorned
by spines or serrations. It is clear that Helmetiidae and Tegopeltidae form
a single monophyletic clade.
Naraoiidae
Naraoia compacta Burgess Shale
Naraoia spinifera Burgess Shale
Naraoia spinosa Chengjiang
Naraoia
bertiensis Canada
Misszhouia
longicaudata Chengjiang
The genera Naraoia and Misszhouia are very closely related
and together comprise the family Naraoiidae. Misszhouia was originally
classified as a member of the genus Naraoia, but was recently split
out on the basis of its dissimilar antennal orientation (see above, right).
When Naraoia compacta was described, its similarity to an early
meraspid trilobite
(with cephalon and pygidium, but no thoracic segments) was noted, and was
part of an ontogenetic argument for including Naraoia as a "soft-bodied
trilobite." A fuller discussion of the relationship of naraoiids to trilobites
is presented on a separate page
of this website.
Liwiidae
Tarricoia arrusensis Puddinga Fm,
Sardinia
Liwia convexa Cambrian, Poland
Soomaspis splendida Soom Shale,
Africa
Buenaspis forteyi Sirius Passet
The family Liwiidae (sometimes subfamily Liwiinae of the family Naraoiidae)
is comprised of naraoiid-like species that are distinguished from naraoiids
in bearing thoracic tergites. Some, like Buenaspis (above, far right),
are remarkably similar in general form to some advanced trilobites (e.g.,
Nileidae), but as in naraoiids, are primarily eyeless, and lack a well-defined
axis (in addition to being uncalcified). The Liwiidae are geographically
diverse, being recorded from a variety of sites outside of either the Burgess
Shale or Chengjiang deposits, including one other Cambrian konservat-lagerstätte,
Sirius Passet, North Greenland (where Buenaspis was found). Despite
a diversity of form, the Liwiidae are considered most closely related to
the Naraoiidae. However it remains to be seem whether the general similarities
reflect true homologies.
Xandarellidae
Xandarella spectaculum Chengjiang
Cindarella eucalla Chengjiang
Sinoburius lunaris Chengjiang
Squamacula clypeatus Chengjiang
Pygmaclypeatus daziensis Chengjiang
The xandarellids include arachnomorphs with a well-defined head shield,
stalked, ventral eyes (but sometimes furnished with dorsal fenstrations,
as in Xandarella and Sinoburius), and numerous dorsal tergites.
The axis is either completely undeveloped (e.g., Squamacula, Pygmaclypeatus),
or poorly defined (e.g., Sinoburius, Cindarella, Xandarella).
Size of the tail shield varies considerably between species, and is quite
trilobite-like in Sinoburius. Even where the tail shield is not well-developed,
there is a decoupling of tergites and somites, so that posterior tergites
cover increasing numbers of limbs (each limb pair corresponding to a somite).
There are no xandarellids documented from the Burgess Shale. All the known
species are from Chengjiang. They point to the value of this exceptional
Cambrian konservat-lagerstätte, from which an entire clade
of trilobite-like arachnomorphs was added to the fossil record.
Of course, no matter how similar the other trilobite clade arachnomorphs
are to true trilobites in morphology, there are a number of features that
trilobites share with no other arthropod group. These are discussed below:
What distinguishes Trilobites
among Arthropods?
Trilobites are the most diverse of the extinct arthropod groups,
known from about 5000 genera (e.g., see Jell & Adrain 2003). The classification of trilobites within
the Arthropoda has generated much controversy, much of which is still not
completely resolved (see above). Whatever their higher position among arthropoda,
there are a number of characters that distinguish trilobites from
within its Arachnomorph clade, the most significant noted below:
eye ridges: These are consistently present in
primitive trilobites,
connecting the front of the
palpebral lobe with the
axial furrow (a feature
lost in many post-Cambrian
trilobites)
pygidium: The
posterior tagma of greater
than one segment is a
conspicuous feature
of all trilobites (but
not restricted to Trilobita).
Pygidia are typically
very small in primitive
forms (e.g., Olenellina)
Together with the organization
of the body into three anterior-posterior divisions (cephalon, thorax,
and pygidium), and the three longitudinal lobes (axial lobe and two flanking
pleural lobes), the body features on this page serve to readily distinguish
trilobites from all other known
arthropod groups.
calcitic compound eyes: While other compound eyes are found
in Cambrian arthropods, only those of trilobites have corneal surfaces composed
of prismatic calcite lenses (with the crystallographic axis normal to the
lens surface).
circumocular sutures:
In Cambrian holochroal
trilobite eyes, a suture
around the edge of the shared
corneal surface assisted
in molting of holaspid trilobites.
In post-Cambrian trilobites
this feature is secondarily
lost, leaving the corneal
surface attached to the
fixigena.
rostral
plate: a ventral anterior plate separated
from the rest of the
cephalic doublure by sutures
is very well developed
in primitive trilobites (e.g.,
Redlichiida), narrower
in other trilobite orders, and
secondarily lost in some
advanced forms (e.g.,
Asaphida and Phacopida)
hypostomal
wings: The trilobite
hypostome may be homologous
to
the labrum in Crustacea,
and other
arachnomorphs bear hypostomes,
but all trilobite hypostoma
bear a
pair of anterior wings
which fit in
pits in the anterior
axial glabellar
furrows (or homologous
locations).
calcified cuticle:
Trilobites bear a
rather pure calcareous
cuticle that
ends ventrally at the
inner edge of
the doublure. Although
a few other
arthropod groups calcify,
none do
so the same way as trilobites.
Crustacea, for example,
are calci-
fied ventrally and post-orally,
so
appendages are calcified.
Relationships Between the Trilobite
Orders Although higher level classification of trilobites is an unsettled
subject, Fortey's classification outline in the 1997
Trilobite Treatise,
his 2001 review of trilobite classification, the 2002 splitting of
the Harpetida from the Ptychopariida, and recent acknowledgement
of the order Odontopleurida suggests an arrangement that looks
something like the figure below. A discussion of
the evolutionary trends and relationships of the orders follows.
The Redlichiida (particularly the
Suborder Olenellina) is considered primitive, appearing in Series/Epoch 2 of the Cambrian, and not persisting into the latest Cambrian (Furongian). The Agnostida
also appear early and persist to the end of the Ordovician. The Redlichiina
give rise to at least the Corynexochida,
and the Ptychopariida in the Cambrian. The Lichida
and Odontopleurida may have arisen from early Corynexochida or Redlichiida, indicated with a "?". The
Redlichiida, Corynexochida, and the Ptychopariida, as large primitive
groups including the ancestors of
other orders, must be paraphyletic, which is indicated by dashed lines
between these three orders. In 2002, another order was split
out of the Ptychopariida; the Order Harpetida (formerly suborder Harpina of the order Ptychopariida)
The Ptychopariida, Harpetida, Asaphida,
and Proetida share (in at least the primitive
forms) species with a natant hypostomal condition, leading Fortey to suggest the Subclass Libristoma for these
orders combined. The recognition of Asaphida, Proetida, and Harpetida as
orders is a relatively recent thing; in the 1959 Treatise they were all included
within the very large and paraphyletic Ptychopariida. The Ptychopariida and
Harpetida maintain the natant state until their extinction at the end of
the Devonian, but both the Asaphida and the Proetida
develop conterminant and impendent hypostomes in their advanced forms. The major extinction event at the end of the Ordovician greatly affected trilobites,
ending the Olenina, Agnostida and the vast majority of Asaphida. The remainder
of the Asaphida (superfamily Trinucleioidea) are lost before the end of the
Silurian and all other orders except Proetida are
lost by the end of the Devonian (most in the major
extinction event between the Frasnian and Famennian ages in the Late Devonian,
but the Phacopidae hang on until nearly the Devonian-Carboniferous transition).
The Proetida persist until the end of the Permian,
the last of the orders of trilobites to go extinct. Only Order and Suborder
epithets are provided above. There wasn't room for all of the superfamily
figures and labels for the Proetida, Asaphida and Lichida.
The origin of the Phacopida is uncertain.
The three suborders (Phacopina, Calymenina, and Cheirurina) share a distinctive
protaspis
type; this similarity in development suggests phylogenetic closeness.
The Calymenina is perhaps the most primitive of the Phacopida, and
share some characters with the Ptychopariida (including a few species
with natant hypostomes), so although the hypostomal condition of the
Phacopida is typically conterminant (and impendent in some advanced
Phacopina), they may have had their origins with the natant
Ptychopariida (which would make the Phacopida another addition to the
Libristoma). Others point out the overwhelmingly conterminant
hypostomal condition among Lichida, and similarities in the exoskeleton
tuberculation of Phacopida and Lichida, so the ancestral sister group
of the Phacopida remains unclear, and its clade in the chart above is
placed between Ptychopariida and Lichida.
The status of the Order Lichida is likewise not entirely clear.
In the first trilobite Treatise (1959), an Order Odontopleurida was
recognized, including the large
family Odontopleuridae. Thomas & Holloway (1988) acknowledged that
the Lichidae and
Odontopleuridae are related, but that their post-Cambrian evolutions
have been
distinct. When Fortey (1997) added Damesellidae to the order Lichida,
it was indicated that they are more similar to Odontopleuridae than to
Lichidae. If two orders are recognized, it may be
Lichidae+Lichakephalidae=Lichida, and
Odontopleuridae+Damesellidae=Odontopleurida. However, the Cambrian family
Lichakephalidae may be paraphyletic, with some of its genera similar to
Lichidae and others more similar to Odontopleuridae. In particular,
some workers reject the synonymy of Eoacidaspididae with
Lichakephalidae, and note that at least some genera in Eoacidaspididae
are close to Odontopleuridae.
notes on the use of this figure: This
diagram was initially designed by Sam Gon III, based on information available in
the literature, including the 1997 revision of the Treatise, and
Fortey's 2001 synopsis of trilobite systematics. The intention is to share with
others interested in trilobites my slowly growing understanding of the
relationships between the higher taxonomic units. Any similarity to
figures published elsewhere is unintentional, and I have not seen any
handling of quite this
sort in the literature. Please contact me before using this image.
Any inaccuracies are entirely mine, and there may be future revisions. For
example, since 1999, I adjusted the extinction of the Asaphida
to the early Silurian, set the origin of the Phacopida to the
Cambrian-Ordovician boundary, adjusted all of
the Devonian extinctions to reflect the Frasnian-Famennian boundary,
and
added the small persistent tail on the Phacopida until its last family
(Phacopidae) disappeared
near the Devonian-Carboniferous boundary. When the Order
Harpetida was recognized in late 2002, that order-level clade had to be added!
In May of 2006, adjustments were made on the geological time scale
to reflect the emerging
treatment
of the Cambrian, in which the appearence of trilobites is considered
the bottom of Series/Epoch 2, and dashed lines
indicating paraphyly in the primitive orders were added. In
2007, working with Nigel Hughes, a modification of this figure was
formally published (see citation for Hughes 2007, below). In 2008, the
diagram was adjusted to split Odontopleurida out from Lichida. I
intend to make continuing
revisions to
this figure as I receive
feedback or
learn more. Thank you for your understanding!
On what basis, then, are the orders defined? No
single character (e.g., facial sutures) dominates in higher level
classification. Instead, such characters as facial sutures, glabellar shape
and pattern of lobation, eyes, thoracic features and numbers of thoracic segments,
pygidial shape, size and segmentation, and spinosity all play a role in helping
define the orders. In addition, note that hypostomal conditions and shared
ontogeny play an important role in defining the orders of trilobites. That
is why for each of the order fact sheets, as many of the above details are
provided as possible. As a synopsis, here are brief statements regarding each
of the trilobite orders:
AGNOSTIDA - Among the early trilobites, with a basic, clamshell-like
appearance.
Suborders Agnostina and Eodiscina.
Representative species pictured here: Ptychagnostus
akanthodes (Agnostina)
REDLICHIIDA - Including the most primitive trilobites from the
lower Cambrian.
Suborders Olenellina and Redlichiina.
Representative species pictured here: Redlichia
sp.(Redlichiina)
CORYNEXOCHIDA - An often spiny group united by a shared hypostomal attachment.
Suborders Corynexochina, Illaenina, and Leiostegiina.
Representative species pictured here: Kootenia
sp. (Corynexochina)
.
ODONTOPLEURIDA - Very spiny trilobites, a sister group to the Lichida.
Suborder Odontopleurina; superfamilies Dameselloidea and Odontopleuroidea.
Representative species pictured here: Selenopeltis buchii.
LICHIDA - Some of the most ornately
sculptured species fall into this group.
Suborder Lichina; families Lichidae and Lichakephalidae.
Representative species pictured here: Arctinurus
boltoni (Lichioidea)
PHACOPIDA- The well-known Phacops,
with its beautiful compound eyes belongs here.
Suborders Calymenina, Phacopina, and Cheirurina.
Representative species pictured here: Phacops
sp.(Phacopina)
PROETIDA - Includes some of the last
trilobite species before the Permian Extinction.
Suborder Proetina, with three Superfamilies.
Representative species pictured here: Proetus
granulosus (Proetoidea)
ASAPHIDA - All share a ventral median
suture, and most a similar development.
Suborder Asaphina, with six Superfamilies comprising
~20% of all trilobites.
Representative species pictured here: Homotelus
sp. (Asaphoidea)
PTYCHOPARIIDA - Bearing the "generic
trilobite" body plan, but many weird variations!
Suborders Ptychopariina and Olenina (Harpina
has been elevated to order Harpetida; see below)
Representative species pictured here: Modocia
sp. (Ptychopariina)
HARPETIDA - Bearing the distinctive, broad, often intricately
pitted, cephalic fringe.
In 2002, split out of the Ptychopariida and elevated from
suborder to full order.
Representative species pictured here: Eoharpes sp.
NEKTASPIDA - The so called "soft-shelled trilobites" such
as Naraoia have been classified as an order of trilobites by
some. Click on the image or link to learn more about them, and to see how
they are handled in the 1997 Treatise.
If you would like to see a listing of the families of each of the
orders in phylogenetic arrangement according to the 1997 Treatise, please
click here. Literature Cited:
Chen, J. & G. Zhou. 1997. Biology of the Chengjiang Fauna. in The
Cambrian Explosion and the Fossil Record. Bulletin of the National Museum
of Natural Science 10:11-106.
Cotton, T.J., and S.J. Braddy. 2004. The phylogeny of arachnomorph arthropods
and the origins of the Chelicerata. Transactions of the Royal Society
of Edinburgh: Earth Sciences, 94: 169–193,
Ebach, M.C. & K.J. McNamara. 2002. A systematic revision of the family
Harpetidae (Trilobita). Records of the Western Australian Museum 21:
235-67.
Edgecombe, G. & L. Ramskøld. 1999. Relationships of Cambrian
Arachnata and the systematic position of Trilobita. J. Paleontology.
73(2):263-87.
Fortey, R.A. 1997. Classification. In
Kaesler, R. L., ed. Treatise on Invertebrate Paleontology, Part O, Arthropoda
1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida.
xxiv + 530 pp., 309 figs. The Geological Society of America, Inc. & The
University of Kansas. Boulder, Colorado & Lawrence, Kansas.
Fortey, R.A. 2001. Trilobite systematics:
The last 75 years. J. Paleontology. 75(6) 1141-51.
Lauterbach, K.-E. 1983. Synapomorphien swischen Trilobiten- und Cheliceraten-sweig
der Arachnata. Zoologischer Anzweiger 210:213-38.
Størmer, L. 1944. On the relationships and phylogeny of fossil and
recent Arachnomorpha. A comparative study on Arachnida, Xiphosurida, Eurypterida,
Trilobita, and other fossil Arthropoda. Skrifter Utgitt av Det Norske
Videnskaps-Academi I Oslo. I. Matematisk-Naturvidenskapelig Klasse 5:1-158.
Wills, M.A., D.E.G. Briggs, R.A. Fortey, M. Wilkinson & P.H.A. Sneath.
1998. An arthropod phylogeny based on fossil and Recent taxa. In: G.D.
Edgecomb, ed. Arthropod Fossils and Phylogeny. Columbia University
Press, N.Y.
