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Introduction to the Bathynotidae
The small, trilobite family Bathynotidae is known from late lower Cambrian
and the earliest Middle Cambrian sites in North America and Eurasia. This
is a time in early Paleozoic
history when the orders of trilobites were restricted to the Redlichiida,
Ptychopariida, Agnostida, and Corynexochida. The 1959 Treatise placed the
family in a suborder of the Redlichiida (Bathynotina). The 1997 Treatise
treats the Bathynotidae as a family of "uncertain order." What is it about
the Bathynotidae that leads to this uncertain status? Given what we know
of the characters of the orders, why is it that the Bathynotidae confound
ordinal placement? This page explores the morphological characters of the
Bathynotidae, dorsal and ventral, in search of an explanation.
Bathynotus holopygus
Bathynotus holopygus, Hall 1860
E. North America &Arctic Eurasia
Redlichiine dorsal characters?
Based on dorsal morphology alone (see figure, left), the Bathynotidae
share many characteristics with members of the Order Redlichiida,
Suborder Redlichiina, Superfamily Redlichioidea. The glabella is
simple
and tapering forward, eyes long and cresentic,
long
genal spines, thoracic axiswide, with 13 spinose
thoracic segments, 11th segment macrospinose.
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This set of characters is not unheard of in the Redlichioidea. A wide
axis is seen in the Saukiandidae. Several redlichioid families bear 12
to 14 thoracic segments (Menneraspididae bear exactly 13). Macrospinosity
is seen in the Metadoxididae (e.g.,
Onaraspis) and Saukiandidae
(indeed
Despuljosia is notably similar in spine pattern to Bathynotus).
The dorsal facial sutures of Bathynotus are opisthoparian, with
the anterior sutures converging in arcs, and merging smoothly with an anterior
marginal suture. This is also seen among a few Redlichioids (e.g., Enantiaspis
and
Onaraspis (Metadoxididae)). There is little or no preglabellar
area in Bathynotus, and a few redlichioids share this (e.g.,
Hamatolenus,
Saukiandia,
and Perrector). The eyes are large and cresentic, as in many Redlichiida
(and Corynexochida).
The spinous nature of Bathynotus is also shared by early trilobites
in both the Redlichiida and Corynexochida. Many of these features are plesiomorphic,
defining the "primitive" set of characters applying broadly to the paraphyletic
Redlichiida. The row of axial nodes on Bathynotus can be found in
Redlichiida, Corynexochida, and even Ptychopariida.
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The general similarity of form between Bathynotus and a selected
set of redlichioid species is rather easily seen below. Based on the dorsal
morphology alone, it would be reasonable to place Bathynotus in
one of the advanced redlichioid families, such as Saukiandidae or Metadoxididae.
Jell & Adrain (2003. Available generic names for trilobites. Memoirs
of the Queensland Museum 48(2):331-551) place Bathynotus in
the Redlichioid family Chengkouaspidae.
Bathynotus holopygus
Bathynotidae
Despujolsia rochi
Saukiandidae
Perrector (Richterops) falloti
Saukiandidae
Onaraspis somniurna
Metadoxididae
Specific similarities to Bathynotidae are noted in the various captions of
the redlichioid taxa to the right. Compare to Bathynotus above.
Cresentic eyes, similar pattern of thoracic spines, including a macrospinose
11th segment followed by subspinose segments.
Glabella wide, tapering, simple;
macrospinose segment followed by
subspinose segments, axial nodes,
pygidium simple, multisegmented.
"Unique" ventral cephalic morphology
While the dorsal morphology of Bathynotus is not hard to ally
with that of Redlichioidea, the ventral
morphology of Bathynotidae, as shown by Bathynotus holopygus (see
images below), is a different matter entirely. It is apparently unique
among trilobites. The 1997 Treatise goes so far as to state: "No other
species is known with such a ventral structure, or with a triangular rostral
plate in which the apex of the triangle is directed forward." The hypostome
is conterminant, that is, it meets the anterior cephalic doublure and its
placement is congruent with the anterior edge of the overlying glabella.
This much is typical of primitive trilobites (Redlichiida and Corynexochida),
but unlike other Redlichiida (which typically bear wide rostral plates),
there is no obvious rostral plate in Bathynotus, and the sutures
that separate the hypostome from the anterior doublure diverge at a roughly
100° angle from the anterior midline, giving the hypostome a roughly
pentagonal shape (see below, left). In his review of the hypostomes of
Cambrian trilobites, Whittington (1988) suggested that the triangular area
anterior of the median body in the hypostome of Bathynotus may represent
a remnant of the rostral plate, fused with the hypostome, which would make
the two sutures with the anterior doublure connective sutures (e.g., separating
the doublure from the rostral plate, rather than separating doublure from
hypostome). There is certainly no clear indication of a suture between
the purported rostral remnant and the hypostome proper, and even so, in
nearly all other trilobites with small rostral remnants, the typical pattern
is for the sutures to diverge forwards, rather than converge toward an
apex. Thus, while the dorsal features of Bathynotus sit confortably
in the Redlichioidea, its ventral features seem difficult to reconcile.
.
In the image below, the dorsal facial sutures
are shown as dotted red lines, and the unusual anterior sutures of the
hypostome are shown in solid red. Most lower Cambrian trilobites bear wide
rostral plates that dominate the anterior doublure.
The explode view of the hypostome, librigenae
and cranidium of Bathynotus show clearly the pentagonal shape of
the hypostome. The apex of this pentagon may represent a remnant, fused
rostral plate, but this is by no means clear.
What trilobites come closest to Bathynotus in ventral structure?
While no trilobites exactly match the lost or greatly reduced rostral
plate of Bathynotus, among trilobites with conterminant hypostomes
bearing small rostral plates there are two examples in which the rostral
sutures converge anteriorly. The first example is in the Redlichiida, in
the Emuelloidea. The ventral structure of the Australian emuellid Balcoracania
dailyi is shown below. The rostral plate (pink) is not fused with the
hypostome (orange), but bears a functional suture. The two connective sutures
initially converge anteriorly, then diverge to meet the dorsal sutures
along the anterior margin.
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In the image below, the dorsal facial sutures are shown as dotted red
lines, and in solid red lines are the connective sutures on either side
of the rostral plate (pink). The hypostome is attached to the rostral plate
by a functional transverse suture. The rostral plate of
Balcoracania
is narrower than the hypostome.
The explode view of the hypostome and rostral plate, librigenae and
cranidium of Balcoracania show some similarities to that of Bathynotus.
It is not too difficult to imagine the rostral plate shrinking until it
is essentially gone. In that case, the connective sutures would have to
meet at the anterior midline as in Bathynotus.
The other example is in the trilobite Fieldaspis furcatus, A
trilobite in the order Corynexochida, Family Zacanthoididae. The details
of the structures of this trilobite are shown below. As is apparent, the
rostral plate (pink) is fused with the hypostome (orange), so that the
median body marks the border between the hypostome and the rostral plate
(dotted red line). The two rostral sutures converge anteriorly, but meet
the dorsal sutures along the anterior margin, which bears a marginal suture.
If the right and left rostral sutures were to migrate medially, they would
converge on the hypostome and create an apex as shown in Bathynotus.
In the image below, the dorsal facial sutures are shown as dotted red
lines, and in solid red lines are the connective sutures on either side
of the rostral plate (pink). The hypostome is fused with the rostral plate
without any sign of a functional transverse suture. The rostral plate of
Fieldaspis
is rather narrow, being just slightly wider than the hypostome.
The explode view of the fused hypostome-rostral plate, librigenae and
cranidium of Fieldaspis show some similarities to that of Bathynotus.
It is not too difficult to imagine the rostral portion of the fused hypostome-rostral
plate shrinking until it is essentially gone. In that case, the connective
sutures would have to meet at the anterior midline.
Considering the full complement of characters shown by Bathynotus,
the dorsal features do not seem to justify placement in the Corynexochida,
while its ventral features show characters that might be considered compatible
with a few Corynexochida or Redlichiida, as shown above. It is particularly
interesting that another redlichiine, Balcoracania (albeit in the
Superfamily Emuelloidea, rather than Redlichiioidea), bears a ventral suture
arrangement that is arguably similar to that of Bathynotidae. If any of
the ventral structures of the advanced redlichioids (say, Saukiandidae)
were shown to be similar to Bathynotus, that would argue for placement
of Bathynotidae in the Redlichiida. Unfortunately, our knowledge of the
hypostomes and ventral sutures of Redlichiida are limited, so it will be
difficult to investigate for related taxa. For example, the hypostomes
and ventral sutures for redlichioids that seem dorsally most similar to
Bathynotus
(e.g.,
Despuljolsia,
Perrector,
Onaraspis) are not
known. If their ventral sutures should prove similar to Bathynotus,
perhaps placement of Bathynotidae in the Redlichiida is reasonable. Similarly,
the ventral and hypostomal structures of members of the Chengkouaspidae
should also be investigated.
Irvingella sp.
Elviniidae
Ptychopariida
Ptychopariida connections:
If Bathynotus resembles some advanced Redlichioids, it also
has been compared to some of the Ptychopariida, such as Irvingella (left).
Comparing the spine patterns, facial sutures, glabellar structure, and
pygidial structure of Irvingella (and other Elviniidae/Komaspididae)
to Bathynotus is very easy to do, and it has even been argued that
the Bathynotidae are the ancestors of the Komaspididae. Shergold, J.H.
& Whittington, H.B. (2000) in The Cambrian trilobite Bathynotus
(?Redlichioidea) in the Northern Territory, Australia. Alcheringa
24:1-10, state that "Kobayashi (1935) allied Bathynotus with Upper
Cambrian forms such as Komaspis and Irvingella in his new
family Komaspididae (in the Ptychopariida). Later, Hupe (1953), while acknowledging
that Bathynotus may have been ancestral to komaspidids, proposed
the family Bathynotidae and placed it with the Redlichioidea.
Trevor Cotton (in a trilobite discussion thread, March 2003) suggested
recently that "Bathynotus is likely to belong to a clade of trilobites
including some higher redlichioids, corynexochoids, ptychoparioids (and
the other libristomates) and paradoxidoids. Bathynotus shares
characters with some redlichioids, corynexochoids and paradoxidids."
He noted that "a close relationship between Ptychopariida and Corynexochiida
is a long-standing hypothesis in trilobite phylogeny (based e.g. on protaspis
morphology, among other things). The similarity in dorsal morphology of
adult Ellipsocephaloidea and corynexochids supports this. The corynexochid-ptychopariid
clade originated from "advanced redlichioids" - and I think there has been
some discussion of which redlichioids may be close to these more modern
trilobite groups." Given the position of Bathynotus among the diversification
of the Corynexochida, the Ptychopariida, and the advanced Redlichioidea,
Cotton suggests finally that "the advanced redlichioids (probably including
Bathynotus)
could not form a clade excluding these groups," as they are descendants.
