The Relationship of Agnostina to Eodiscina
This page last revised 30 June 2007 by S. Gon III
Summarized here are the main observations
in Cotton & Fortey 2005 "Comparative morphology and relationships
of the Agnostida," that together arugue for unity of the Agnostina and
Eodiscina within Order Agnostida:
The result of the phylogenetic analysis conducted indicated that there are
two large clades in the Agnostida corresponding roughly to Agnostina and
Eodiscina, but that the agnostine clade includes eodiscines of the family
Weymouthiidae. This strongly infers that the Agnostina and Weymouthiidae
are closely related, uniting the Eodiscina and the Agnostina. Moreover, the
Agnostina form a monophyletic clade nested within a clade that includes the
weymouthiid genera Mallagnostus, Chelediscus, Tannudiscus
and Jinghediscus, and this group falls within a larger weymouthiid
To reiterate, the conclusion is that there is a strong link between the Agnostina
and the Eodiscina via the family Weymouthiidae, and Agnostina are considered
part of the order Agnostida, and thus are true trilobites.
- All Agnostina are eyeless and sutureless,
as are some Eodiscina, allowing a hypothesis that Agnostina arose from a
clade of blind, sutureless Eodiscina.
- Primitive Eodiscina have a cephalon
wider than long, but advanced eodiscines such as
Weymouthiidae (and most Agnostina) bear longer cephala.
- The obvious synapomorphy of very strong
isopygy is seen in all Eodiscina and Agnostina, and unites all Agnostida.
- Although in the past it was argued
that Agnostina lack genal spines, a review of the taxa show this supposed
synapomorphy of Agnostina is not true. There are species of Agnostina and Eodiscina
that bear genal spines; e.g., in Eodiscus & Acidicus
(in Eodiscina), and in the Condylopygoidea (Agnostina).
- In Agnostina, there is a distinctive
glabellar division into an anteroglabella and posteroglabella, and a
pair of basal lobes corresponding to the occipital band. The furrow dividing
antero- from posteroglabella corresponds to S3 in a typical trilobite,
and the anteroglabella and posteroglabella consist of two segments each,
as shown by muscle scars of underlying limbs in some specimens. There
are transglabellar furrows of similar type in some Eodiscina, and in
no Agnostida are there furrows anterior to the transglabellar furrow.
So complete effacement of the S4 furrow also unites Agnostina and Eodiscina.
- In Eodiscina, the structure of the
cephalic axis is highly variable, and in primitive Eodiscina, differs
from a typical polymeroid trilobite only in effacement of S4 and a transglabellar
S3. The occipital lobe in some Eodiscina is similar to the paired basal
lobes of Agnostina, bearing a narrow band connecting two lateral triangular
basal lobes, the medial connecting band being hidden by the adjacent
anterior glabellar lobe. The occipital lobe of Eodiscina is thus considered
homologous to the agnostine basal lobes.
- Many agnostines bear a median glabellar
node on L2 (although effacement makes it difficult to ascertain the exact
lobe in many cases). This is considered homologous to L2 spines or nodes
on some Eodiscina, including several weymouthiids.
- A preglabellar sagittal furrow is
seen in both agnostine and eodiscines, but is effaced in many taxa.
Nonetheless this is another unifying feature.
- Although the hypostome of Agnostus
pisiformis is striking distinctive from the typical polymeroid hypostome,
that of another agnostine (Oidalagnostus trispinifer) is of typical
polymeroid form, and that of Peronopsis interstricta is intermediate
between A. pisiformis and O. trispinifer. The hypostome
of an eodiscine, Pagetia ocellata, is of typical polymeroid type.
Taken altogether, the known hypostoma of Agnostida are variable, and
most are of polymeroid form.
- Presence of only two or three thoracic
segments in Agnostida was the basis of the old division of Trilobita
into subclasses Miomera and Polymera. However, there are polymeroids
with few thoracic segments, such as Thoracocare (Corynexochida)
and Taklamakania (Asaphida), so the distinction on the basis of
thoracic segment number alone is not valid. All agnostines bear only
two segments, and eodiscines bear either three, or sometimes two segments.
The structure of the thoracic segments of Agnostida is distinctive,
and seems related to the function of enrollment.
- The pygidial axis of Agnostina is
distinctive, bearing furrows only anteriorly, with three pairs of axial
ring furrows in condylopygoids, and two in all other Agnostina. Moreover,
an axial node on the second pygidial segment (sometimes a spine) was considered
an autapomorphy of agnostines. In contrast, the pygidial axis of eodiscines
is variable, usually bearing several clear axial rings. When axial spines
are present, they are often most prominent anteriorly, and often most
prominent on the second segment. This is another uniting factor,
though not consistently present among all eodiscines and agnostines.
- The pleural fields of agnostine pygidia
are effaced, while in most Eodiscina, there are pleural furrows. However,
in most Weymouthiidae, pleural furrows are also effaced, making this
a good synapomorphy of a weymouthiid-agnostine clade.
Cotton, T. J. & R.A.
Fortey. 2005 Comparative morphology and relationships of the Agnostida. In: Koenemann, S. & Jenner,
Crustacean Issues 16, Crustacea and Arthropod Relationships (CRC
Press: Boca Raton).
Resser, C.E. 1938. Cambrian
system (restricted) of the Southern Appalachians. Geol. Surv. Amer. Spec.
Shergold, J.H. 1991. Protaspis
and early meraspis growth stages of the eodiscoid trilobite Pagetia ocellata
Jell, and their implications for classification. Alcheringa 15: 65-86.
Walossek, D. & K.J.
Müller. 1990. Upper Cambrian stem-lineage crustaceans and their bearing upon
the monophyletic origin of Crustacea and the position of Agnostus. Lethaia