GLADIOBRANCHUS RELATIONSHIPS (HISTORY AND HYPOTHESES) In tro d u ctio n
The systematic relationships o f Gladiobranchus are discussed to varying degrees in the works o f Bemacsek and Dineley (1977: 13, 16-17), Denison (1979: 32), Long (1983: 52), and most recently in Young (1989 19-20). Bemacsek and Dineley tried to establish a close affinity between Gladiobranchus and Uraniacanthus founded mainly on shared opercular similarities and a perception that both genera shared a dermal shoulder girdle.
Their proposal to tie Gladiobranchus to the Ischnacanthiformes, thereby
automatically linking it as a close ally o f Uraniacanthus based on Miles’s (1973a) amended character statements was hampered by: i) the lack o f complete fossil material; ii) incorrect ad hoc assumptions about the presence o f important features that have been falsified by this account; coupled with the iii) problems o f ‘shoehoming’ Gladiobranchus into higher
The following accounts trace the problems historically then test the taxonomic efficacy o f these relationship hypotheses.
Gladiobranchus as an Ischnacanthid (historical assessment)
Since Bemacsek and Dineley (1977) focused on allying Gladiobranchus with
Uraniacanthus which was classified by Miles (1973a) as an ischnacanthiform, it is important to know what characters underpin this acanthodian order before testing their potential as apomorphies. Miles (1966: 166) erected these characters to define the order
Ischnacanthiformes. They are:
1) Acanthodes-type body scale microstructure bearing Poracanthodes-iypQ scales along some o f the cephalic lateral lines.
2) dorsal fins.
3) no intermediate spines
4) spines deeply inserted into body wall 5) no ventral shoulder girdle plates
6) large gill cover covering the whole o f the gill chamber laterally 7) strong jaw bones (upper and lower) to which teeth are ankylosed 8) symphysial tooth whorls in mouth cavity
Miles (1973a: 150-151) considered the diplacanthid attributes (i.e. deeply inserted dorsal fin-spines and 2 pairs o f intermediate spines) o f Uraniacanthus as ‘superficial’ because they were also present in climatiiforms. He based his decision to ally Uraniacanthus on the novel presence o f dentigerous bones, lack of dermal ventral armour including prepectoral spines, and the deep insertion o f the median fin-spines.
In order to ally Uraniacanthus ischnacanthiforms. Miles needed to amend his 1966 definition o f the order. To do this, he reversed the character ‘non intermediate spines’ (character 3 in the above list) to ‘intermediate spine present’.
He also deleted characters 1, 6, and 8 because he did not know how widely distributed these characters were within ischnacanthids. It is now known that histological character 1 is not very informative and characters 6 and 8 are simply primitive for gnathostomes in general (Janvier 1996).
After deleting these morphologically incongruent characters and discarding character 4: deeply imbedded fin-spines (he failed to mention it in his revised diagnosis of
ischnacanthiforms). Miles erected his amended definition of Ischnacanthiformes based on one modified feature, a new character, plus the remaining original characters. They are:
COMPARATIVE MORPHOLOGY (Gladiobranchusprobaton)
1) presence o f dentigerous jaw bones in both upper and lower jaws (original character 7). 2) intermediate spine present (modified character 3)
3) two dorsal fins (original character 2)
4) dermal plates (original character 5) and prepectoral spines absent (new character)
Miles (1973a: 151) had successfully taken some salient features o f Uraniacanthus and thus integrated them into a revised Ischnacanthiformes. This classification was not
challenged until Bemacsek and Dineley (1977).
Testing Bem acsek and Dineley (1977) with Miles (1973a) (Ischnacanthid affinity for Gladiobranchus)
Bemacsek and Dineley (1977) believed that Gladiobranchus was closely related to Uraniacanthus (see family diagnosis on p. 13). Since Miles (1973a) amended his definition o f Ischnacanthiformes to include all o f the characters supporting Uraniacanthus, Bemacsek and Dineley, by default, had to argue for its inclusion within the Ischnacanthiformes. Taking Miles’s (1973a) amended character list as a template, except for character 2 (presence o f intermediate spines which they ignore in their account), Bemacsek and Dineley (1977: 16) set out to compare characters which would provide the basis for a
gladiobranchid/ischnacanthid affinity. They also combined characters 4 and 5 o f Miles (1973a) into their character 3 (p. 16), thereby reducing the overall character count to three.
In short, Bemacsek and Dineley failed to amalgamate Gladiobranchus into the Ischnacanthiformes based on the three existing ischnacanthid characters. Combined
characters: absence o f dermal plate and prepectoral spines (their item 3, p. 16) was discarded because there was no established ischnacanthid taxon which shared either feature with Gladiobranchus.
Although new specimens reveal that dermal ventral plates do not exist for Gladiobranchus, this feature is not informative when used as a character for uniting this genus with ischnacanthids when it is known that non-ischnacanthid taxa such as
Lupopsyrus spp. and Brochoadmones coupled with acanthodids have the same pectoral condition. The presence o f 2 dorsal fins (item 2) is also considered too primitive (this feature is also found in climatiids and diplacanthids) to be informative.
Thus they were left with character 1 (dentigerous jaw bones in upper and lower jaw). The authors discarded this feature because a form genus, climatiid Nostolepis, is described as having dentigerous jaw bones. Bemacsek and Dineley (1977: 16) agreed that the presence o f dentigerous jaw bones in both climatiid and ischnacanthid taxa it would be untenable to include Gladiobranchus within what they considered an unsupported putative ischnacanthid suborder o f the Climatiiformes.
Despite the apparent lack o f shared characters provided by the three remaining characters, and the apparent dissolution o f the Ischnacanthiformes, Bemacsek and Dineley decided to maintain Gladiobranchus as an unsupported ischnacanthiform within the family Gladiobranchidae (p. 13).
Detailed character comparisons in this thesis between Gladiobranchus and the Ischnacanthiformes (represented by articulated taxa I. gracilis and Poracanthodes menneri, and Uraniacanthus) reveal very little in terms o f useful synapomorphies. A list o f additional character comparisons to those listed above are as follows:
1) orbit surrounded by an array o f circumorbital ossicles; ischnacanthids possess only sclerotic ossicles; orbital region unknown in Uraniacanthus.
2) spine morphologies: tapered-subcylindrical spines with mostly smooth longitudinal ribs directed toward distal tips; feature found in both diplacanthids and ischnacanthids. 3) spine insertion points: absent in ischnacanthids.
4) enlarged rostral bone: absent in ischnacanthids, but found in both diplacanthids and mesacanthids.
5) perichondrally-ossified Meckel’s cartilage: present also in ischnacanthids and climatiids, therefore primitive for acanthodians.
6) Lateral attachment o f pectoral spine to coracoid: absent in ischnacanthids, but present in diplacanthids and climatiids; again a primitive feature.
7) Straight scapular process with anterodorsal angle: this character is shared with the ischnacanthids, but this is just one character among many others that point to a possible diplacanthid affinity.
Shared featu res o f Gladiobranchus and Uraniacanthus
As delineated above, Bemacsek and Dineley (1977) attempted to place
Gladiobranchus within the order Ischnacanthiformes mainly because o f some remarkable resemblance between it and Uraniacanthus.
However, when they used Miles’s (1973a: 151) re-defined ischnacanthiform framework in its entirety to support a relationship with both Ischnacanthiformes and Uraniacanthus, the analysis fell apart due to the mostly plesiomorphic nature o f Miles’s (1973 a) characters.
It should be noted, however, that Bemacsek and Dineley (1977: 16) did find some characters which are shared by Gladiobranchus and Uraniacanthus. They listed them as: i) spines, ii) scales, and iii) branchiostegal rays.
COMPARATIVE MORPHOLOGY (Gladiobranchusprobaton)
Although scales and spines (which were more precisely described by Bemacsek and Dineley) can be discounted as general diplacanthid patterns, the branchiostegal rays (‘opercular rays’ o f this account) are practically identical in both genera (compare Miles 1973a: PI 13, Figs. 1-2 with Pis. 2.3.1, 2.3.2B, 2.3.3, and Fig. 2.3.1 in this work). Furthermore, both genera share a postorbital plate with lateral tubercles.
Another feature which Gladiobranchus does share with Uraniacanthus but is not mentioned in the author’s taxa comparison, is the postorbital plate. The postorbital plate o f Uraniacanthus was misdiagnosed by Miles (1973a: 150 and PI. 13, figs 1-2, dg.b, dentigerous bone ) as a ‘marginal jaw element’ Its placement on the cheek directly above the opercular plates and posterior to the jaws, coupled with close similarities between lateral and mesial surface morphologies, make for an ideal homology between the two taxa.
It is notable that Gladiobranchus does not share these opercular plate arrangement and morphologies, and postorbital-plate morphologies with any other taxon. One could argue that the operculum shares a superficial resemblance to Parexus, but it should be noted that Gladiobranchus lacks branchiostegal rays (gular division) which are present in a ORS
climatiid taxa. Since these two taxa share many other morphological characters deemed to be diplacanthid in nature (Miles 1966, 1973a, Long 1983), I conduct an historical analysis below to determine if an affinity between Gladiobranchus and diplacanthids warrants congruence testing.
Diplacanthid classification (an historical assessment)
Berg (1940: 129) erected the order Diplacanthiformes which was largely ignored because the diplacanthids were considered by other acanthodian workers to be an offshoot of the climatiid lineage, therefore ordinal-level ranking for diplacanthids was not considered appropriate at that time. However, in 1964, Novitskaya and Obruchev (p.273-274) reversed tradition and erected a different diplacanthid order, Diplacanthida, which combined both Chmatiidae and Diplacanthidae. They diagnosed this order using the following features:
1)2 dorsal fins
2) pectoral girdle with dermal bones 3) cartilaginous endocranium
4) one or two bones developing in each jaw 5) deeply inserted fin-spines
6) and anterior dorsal fin-spine supported by dorsals and radiais (after Watson 1937). Miles (1973a: 190) erected the suborder Diplacanthoidei without providing more precise character statements. The family Diplacanthidae was erected by Woodward (1891) and re-published by Novitskaya and Obruchev (1964: 276).
The Diplacanthidae was supported by: single pair o f intermediate spines on ventral surface; ossified brachial and neural arches; single operculum on branchial opening; 2
ossification centres on each jaw; extramandibular (mandibular splint o f this account) present; teeth absent; anterior dorsal spine with deeply inserted into musculature, with endoskeleton; pectoral girdle with pair o f accessory spines (pectoral fin-spines o f this account); and scale micro structure o f Acanthodes type.
Unfortunately, the higher taxonomic levels formulated for the diplacanthid
acanthodians suffered fi^om a lack o f precise differential characters {sensu Grande and Bemis 1998) to define each tier, whilst including the features from taxonomic ranks. The
diplacanthid characters provided by Denison (1979: 31) did not improve the situation. Instead he added more characters to an already long list o f features, e.g. relative
measurements o f head and cheek region, scale ornamentation, and problematic histological comparisons.
To include the genus Culmacanthus within the diplacanthid ranks. Long (1983: 52) defined more precisely the suborder Diplacanthoidei. He defined it by:
1. the presence of a deep body form. 2. body scales with a low fiat crown.
3. high scapulocoracoid (dorsal scapular process in this work). 4. large dermal plates bearing laterosensory canals on cheek.
5. one pair o f pinnal plates with anterior median bone (lorical plate in this work) or paired pinnal plates only.
6. deeply inserted fin-spines
7. less than 3 pairs o f intermediate fin-spines.
Since Long’s (1986b) publication, many new taxa have been discovered, e.g. MOTH acanthodians. Based on this wealth o f new information, for the seven characters listed above, only character 3 (high scapulocoracoid) appears to be attributed to diplacanthid acanthodians. Character 1 is found in non-diplacanthid genera from the MOTH formation, namely
Kathemacanthus and Brochoadmones', diplacanthids share character 2 with ischnacanthids; character 4 is an autapomorphy o f Culmacanthus and is not shared with other diplacanthids; and both characters 6 and 7 are shared by non-diplacanthid taxa.
Character 6 is shared with Brochoadmones, mesacanthids and acanthodids, whilst the last feature is known to exist in mesacanthids.
COMPARATIVE MORPHOLOGY (Gladiobranchusprobaton)
Gladiobranchus as a putative d iplacanthid
Young (1989: 19) rejected a Gladiobranchus-Uraniacanthus affiliation based on what he considered to be questionable grounds. For example. Young stated that Bemacsek and Dineley based this relationship by ‘suggesting that Uraniacanthus may have possessed dermal plates and prepectoral spines even though Miles (1973a) expressed doubt from a detailed study of the specimens that these structures were absent in Uraniacanthus".
This is incorrect. Bemacsek and Dineley (1977: 16) stated emphatically that ‘there are however a number o f weaknesses in this taxonomic scheme’. Furthermore, they made the point on page 13 in their generic diagnosis that ‘this genus {Gladiobranchus) may be
distinguished from Uraniacanthus by the above characters (i.e. dermal shoulder girdle plates and two pairs o f prepectoral spines) which are lacking in the latter (i.e. Uraniacanthus.
Although Young misinterpreted Bemacsek and Dineley’s generic diagnosis and taxonomic intent for Gladiobranchus, he was correct in pointing out a particular morphological mistake made by Bemacsek and Dineley when they described the
scapulocoracoid. Young pointed out that the scapulocoracoid is not ‘a low broad element’ but it possesses ‘a higher dorsal termination, with an anterior ridge and expanded posterior ventral part. Although I argue that the ridge is in fact more centrally located (see UALVP 38679, Fig. 2.3.2B), Young is correct in steering Gladiobranchus towards a diplacanthid taxonomic association. When combining Young’s scapulocoracoid data with the shared presence and morphology o f their opercular plates and postorbital plates, the weight o f apomorphic evidence shifts in favour o f a gladiobranchid-diplacanthid affinity.
The relationships o f Gladiobranchus and the Diplacanthiformes are tested and discussed in Chapter 5.2.
Composite reconstruction using the anterior region o f UALVP 41862 merged with the posterior region o f UALVP 41858. Anterior dorsal fin and spine reconstruction based on UALVP 38679, and operculum based on UALVP 41857.
a. Camera lucida drawing right side of the head featuring the dermal plates of the orbit, cheek, and rostrum of UALVP 41862 in dorsolateral view. Scale bar = 1 .0 cm.
b. Camera lucida drawing of the right endoskeletal shoulder girdle and pectoral fin-spine base and insertion area of UALVP 41862 in lateral view. Scale bar = 0.25 cm.