The evolutionary significance of scale-like spines on the Australian and SW Pacific Cainozoic ostracods Ponticocythereis manis Whatley & Titterton, 1981 and Trachyleberis floridus sp. nov

Ponticocythereis species are a discrete phylogenetic group within the Trachyleberididae that evolved in the SW Pacific and Australasian regions during the Tertiary. The presence of similar scale-like spines on the unrelated species Ponticocythereis manis Whatley & Titterton, 1981 and Trachyleberis floridus sp. nov. is presented as an example of convergent evolution.


INTROD IJCTION
The ostracod species Ponticocythereis manis Whatley & Titterton, 1981 and Trachylebe,ris joridus sp. nov. both possess rather similar scale-like spines. These lateral surface carapace features are seen in few other ostracod species and are here disciissed in terms of their evolutionary significanct:.
Figured specimens of P.rnanis were collected from N E Shortland Island., near Rokau Island in the Solomon Islands chain, SW Pacific (Whatley & Titterton, 1981: sample no. 0.S.6;p.158) and are housed in the British Museum (Natural History).

SPINE TYPES AND SPINE POSITIONS
There are two types of spines found on the carapaces of P. Blade-like spines occur along the anterior and ventral margins of both species. Scale-like spines cover the rest of the external surl'ace of the carapace in each species, except in the vicinity of the anl.erior and posterior marginal depressions.
Spine positions are characteristic and distinct for each species. These are mapped for both valves of P. rnariis and for the right valve of T. fioridus (Fig. 1).

Systematic framework of Ponticocythereis
The genus Ponticocythereis probably includes a phylogenetically discrete group of species within the Trachyleberididae Sylvester-Bradley, 1948. This genus has a distinct palaeo/biogeographical distribution. It is restricted to the SW Pacific and Australasia. The earliest record of a Ponticoc,ythereis species is in the Tertiary of southeastern Australia where Pmilitaris McKenzie, 1967 makes its first appearance during the early Middle Miocene (Warne, 1987). Whatley & Titterton (1981) modified McKenzie's (1967) diagnosis of Ponticocythereis to accommodate species with intercostal spines, micropunctae. micropapillae or delicate open reticulation. It is here emphasized that the association of three spinose ribs, blade or scale-like spines, posteriorly pointing spines and the development on the posterior inner lamellae of an oval pit and corresponding knob in the opposite valve. is a combination typical of Ponticocythereis species. Other significant features of Ponticocytherris are a strongly developed shelf-like structure below the anteromedium part of the hinge, development of a small tooth anterior to the left valve antero-terminal socket and a denticulate postero-median bar that is elevated and expanded posteriorly (Whatley & Titterton, 1981).

The relationships between Ponticocythereis and A ctinocythereis
Spinose ribs are found on both Ponticocythereis and ActinocythrrPis s.1. species. Furthermore, P. munis, in common with some Actinocythereis species, has intercostal spines. A typical Australian Actinocythereis species is A. tetrica (Brady, 1880). A . tetrica differs from Ponticocythereis species in lacking posteriorly pointing blade or scale-like lateral surface spines (Fig. 2). The absence of these carapace characters distinguishes Actinocythereis s./.
from Ponticocythereis . Some specimens of A. tetrica possess a weakly developed oval pit or knob on the posterior inner lamellae. Actinocythereis rugihreuis (Hornibrook, 1952)  Trachy1eheri.s species lack lateral spinose ridges. The absence of these features distinguishes Trachyleheris from Ponricocythereis.
Within the Australasian/SW Pacific region two Trachyleheris species groups are common. The morphological relationship each has to Ponticocythereis is different.
The first includes the relatively small forms T. hrevicosta Hornibrook, 1952 and T. floridus sp. nov.. These two species lack a small tooth anterior to the left valve terminal hinge socket. a well developed posterior inner lamellae pit or knob structure and posteriorly pointing spines. The absence of these features, as well as lateral surface ribs distinguishes this group of Trachyleheris species from Ponticocythereis. T. ,Foridus and T. hrevicosta may possess blade or scale-like spines, similar to species of Ponticocytherris. The terminal expansions of the spines in some specimens of T. hrevicosta are similar to those found on some specimens o f T. .scuhrocuneata (Brady, 1880) s.1. sensu Hu (1986). In neither case are they as well developed as in T. floridus. McKenzie ef ul. (1991) referred to the spines on T. brevicostu as stud-like. Hu (1986) referred to the spines on some specimens of T. scahrocuneuta as being pimple-like.
The second group of Trachyleheris spp. includes relatively large, often robust forms such as, T. thornsoni Hornibrook, 1952;T. tridens Hornibrook, 1052 andT. careyi McKenzie, Reyment &Reyment, 1991. Blade or scale-like spines have not been observed on these species. In contrast to the first group of Trachylrheris species, but in common with Ponticocythereis species, these forms possess a small tooth anterior to the left valve terminal hinge socket. Like the first group of Truchyleheris species, they lack a posterior inner lamellae pit/knob structure and posteriorly pointing spines.

The relationship between Ponticocythereis and Acanthocythereis
The species Acarithocythereis uscoli (Puri, 1963) possesses bulb-like terminal spine expansions (pl. 27, fig. 9 in Bonaduce et a/., 1975). Unlike in Ponticocythereis species. these spines are arranged on the costae of a distinct and strong reticulation and are not posteriorly pointing or associated with ribs. They also occur in 'bunches' of 3-S bulbs to each spine.

The relationship between Ponticocythereis and Verrucocy thereis
Plate-likc spines are developed in the Quaternary Mediterranean species Verrucocythereis hulhi4spinarrz (Uliezny). These are not as well developed as in P. niunis and do not give V . l~ulbi~spiriata a scaly appearance. Furthermore, the spines on V . bulhuspinata are associated with a distinct reticulation, but not with ribs.

The relationship between Ponticocythereis and Pterygocythereis
Prerygoc,ythereis velivola (Brady, 1880) is an Australasian/ Indo-Pacific species that belongs to a phylogenetic genus possessing blade-like spines that also includes the  Warne, 1986. Species of all these genera have a well dcvcloped ala, clearly distinguishing them from other trachyleberid genera such as Ponticocytherris, Trachylehrris, Actinocytlwris and Acanthocytherris. Further distinctions relate to I he different internal features of Pterygocythereis as described by Guernet (1990). 'The common occurrence of blade-like spines in Ponticocytherris and Pterygocythereis therefore does not appear to indicate a close phylogenetic relationship. This conclusion is supported by the presence in adult Ponticocythereis specimens and lack in t'tc.rygocy/herris specimens of a median lateral surface rib.

CONVERGENT EVOLUTION
The aim of the previous discussions was to demonstrate the discrete phylogeny of Ponticocytherris and therefore the discrete phylogenetic relationship between P. manis and T. fio ridus.
As a Consequence of this discrete relationship, the co-occurrence of scale-like spines on P. nzunis and T. /toridus appears to be an example of convergent evolution.
Whatley & Titterton (1981) record P. munis from the Solomon Islands as inhabiting tropical intertidal regions in the vicinitj of coral reefs. Hartmann (1981) records P. munis (=Actinocythereis uerruciferu Hartmann, 1981) from northern Australia arb living on algae in coral reefs at aquatic temperatures of 26°C (tropical marine climate) in waters of normal marine !salinity. H e found dead specimens of this littoral species in coral sand.
The best preserved adult specimens of T. ,floridus sp. nov. encountered, were found in the absence of juvenile specimens in an earliest Middle Mliocene neritic (outer shelf) silty clay from southeastern Australia (type locality). A less well preserved, but autochthonous population occurs in a latest Lower Miocene shallower neritic (inner-shelf) marl ;ilso from southeastern Australia (see locality details under systematic palaeontology). These latest Lower Miocene to earliest Mliddle Miocene strata were deposited when southeastern Australia was experiencing subtropical to warm temperate palaeoclimatic conditions (Warne, 1988 and references therein).
In summary, both species appear to inhabit/inhabited warm temperate to tropical normal shallow marine environments. Their delicate ornament is presumed to indicate that they live/lived in :sheltered conditions away from the the influence of wave action. The scale-like ornament may serve/have served as armour against predators. 'These ecological similarities perhaps suggest that convergent evolution in spine typc between P. rnunis and T. floridits is a consequence of adaptation to similar cnvironments.
As P. nianis and ?I jloridus both belong to the family Trachyleberididae consideration needs to be given to the possibility that they inherited the genetic capacity to express scale-like spines from a common ancestor, with the phenotypic expression of this spine type in diverging lineages being similarly dependent on favourable environmental conditions. However, differences in the orientation of scale-like spines between the two taxa seems good evidence against the simple co-inheritance of this character. Furthermore, scale-like spines d o not occur early in the ontogeny of P. manis as might be apparent if these features were inherited from an ancestor in common with T. floridus.
The appearance of scale-like spines late in the ontogeny of P. rnunis could signify that it is a recent, independent acquisition within Ponticocythereis and similarities in spine type with T. floridus reflect convergence, nothing more. There is no fossil record of scale-like spines in Ponticocythereis, whereas within Trachyleberis, scale-like spines are known to occur as far back as the mid Tertiary.  (1976) or in any of Brady's (1880) figures of this species. The absence of raised ribs in I ruchyleberis species' is important with respect to their differentiation from Actinocythereis species'. Therefore, Hu's (1986) placement of T. citneatrlles as a junior synonym of the Truchyleberis type species diminishes the clarity of the differential diagnosis between Trachyleheris and Actinocythrreis. The concept adopted here for T. scabrocicneatu s .~. is based on the specimen of this species illustrated by Puri & Hulings (lY76). This specimen does not have any raised longitudinal ribs although a very poorly defined longitudinal alignment of spines is apparent in the ventral region of the external lateral surface. The absence of obvious longitudinal lateral surface ribs (usually three) distinguishes Truchyleberis from Actinocythereis. In order to stabilize the concept of the carapace morphology of 7'. scahrociuieutu and clarify the differential diagnosis between I ruchylthrris and Actinocytherris, the topotype of T. ~c~h r o c i i t~~a t~ illustrated by Puri & Hulings (1Y76) is here designated as a neotype. This is a practical measure taken in order to stabilize the definition of T. scubrociineatu and the differential diagnosis between Trachylrheris and Acrinocvrhrreis. However, it is not here implied that this differential diagnosis always clusters species into phylogenetically consistent groupings.  Description. Large, thick-shelled, opaque to semitranslucent: subrectangular in lateral view: subovate in dorsal view. Anterior margin evenly rounded with up to 13 blade-like spines; extremity at mid-height. Posterior margin gently rounded with up to 10, mostly blade-like spines (frequently abraded): extremity above mid-height. Dorsal margin with 10 scale-like spines; straight in RV; nearly straight in LV except for a slightly concave aspect just above and behind the terminal anterior hinge element. Ventral margin with numerous spines (blade-like in posterior) and with a slight medium concavity. Greatest length slightly above mid-height: greatest height at anterior cardinal angle: greatest width medianly. LV > RV.  nov., latc stage juvenilc (overall length of specimen = 0.72 mm). lcft valve. MV P122590, enlargement of a lateral surface spine in the postero-ventral region. The upper margin o f the figure is towards the posterior and the right hand margin is towards the venter, approximately X920. Fig. 10. Truchykcheris florrdrts sp. nov., female? left valve, paratype. MV P122.580. external lateral view, X U . (ct = abraded spine with clustered tubes: stabraded spine with single tube). Fig. 11. Trarhyl(4ierrs ,floridus sp. nov.. male'.' left valve, paratype. MV P122581, internal view. X45. Fig. 12. Trtrchjddicri.~ ,floridrcs sp. nov., male?. right valvc, holotype. MV P122.579, external view. X52. serrated edged scale-like spines which are minutely punctate. These are absent in the anterior and posterior marginal depressions. Subcentral tubercle is not visible externally, eye tubercle usually obscure, but in some specimens is prominent due to a glassy appearance. Marginal pore canals straight and numerous in both the anterior and posterior. Hinge amphidont. A snap knob and pit structure is weakly developed on the posterior inner lamellae. Adductor muscle scar pattern comprises 4 elliptical scars in a subcentral column. The frontal muscle scar is 'V'-shaped. Sexual dimorphism not obvious, with males having only a slightly greater length to height ratio than Elsewhere on the lateral surface of this juvenile specimen, scale-like spines are poorly developed or undeveloped. These undeveloped spines possess a relatively long stem on which a very small spherical ball sits and from which a number of delicate minute spines extend resembling a mace with its spherical spiked head.

Surface of carapace primarily smooth but covered with
In adults the spherical ball appears to develop into a larger disk. The delicate minute spines extending from the ball d o not appear to develop further in adults and may become the serrations on the edge of the scale-like disk. The overall effect of spines on the adult lateral surface is to give the species an armoured or scaly appearance. When spines are abraded in adults they usually appear as a single broken stem. Rarely, broken or undeveloped spines appear as a cluster of nodes on the carapace surface. Branching spines originating from a single stem, either as an originally preserved ornamental feature, or as an abraded feature have not been observed on the adult specimens examined in this study. They have, however, been observed in some of the Trachyleheris specimens illustrated by McKenzie et al. (1991). Adult specimens of T. ,floridus differ from other Trachyleheris species with scale or blade-like spines in being regularly subrectangular in shape, possessing a broadly rather than acutely rounded posterior and in having a different arrangement of spine positions. Some figured specimens designated as Ponticocythereis sp. aff. P. manis Whatley & Titterton by Neil (1994, pl. 2, figs 3 & 5) are here considered to belong to T. joridus. Age and stratigraphical range. Specimens examined for this study were collected from the latest Lower to earliest Middle Miocene marl, silt and clay lithologies of the Fyansford Formation, Victoria, Australia.
Genus Actinocythereis Puri, 1953 (1984) are similar in size and have almost identical spine positions o n the lateral surface of the carapace. This species is placed within Actinocythereis s.1. on the basis that some o f the lateral surface spines are aligned along slightly raised longitudinal ribs. These ribs are not very prominent. As previously remarked, the differential diagnosis between Trachyldxris and Actinocythereis is based on character associations that are possibly a less than exact reflection of phylogenetic groupings of species within these genera. Figured specimens designated as Actinocythereis tetricrr (Brady) by Yassini rt. ul. (1993) vary slightly from the illustrated specimens within works here referred to in the synonymy list for A . tetrica by being less elongate. more rounded posteriorly and by possessing a higher anterior half of the carapace relative to posterior half of carapace in lateral view. Hornibrook (1952) has recorded this species from the Oligocene of New Zealand.
Actinocythereis rugibreuis (Hornibrook, 1052) Hornibrook's (1952) monograph and from the inspection of subfossil material from New Zealand it is apparent that some of the spines on th!s species are aligned along longitudinal ribs. As a consequence it is here placed in the genus Actinocythereis are very similar in external ornament and shape although not in size. Scale-like spines are not apparent on these specimens and only go on to develop in the adult stage of P.  fig. 12) has had the scale-lik,: spines completely eroded off the carapace leaving only :stumpy nodes. I t is not possible to see the posterior pointing aspect of ;any carapace lateral surface spines on this specimen, although the longitudinal ribs and all spine positions are still evident. As a consequence, and without consideration of internal features, biostratonomic influencc:s have caused this specimen to superficially resemblc. ActinocythercG species Ponticnythrreis luhiutu (Brady, 1890) is very similar in size and shape to P. manis. McKenzie's (1986) illustration of the holotype of P. luhiatu indicates that it has fewer spines. As some of these spines are scale-like it is presumed that spine development was sufficient for all spine positions to be apparent o n the carapace. P. lahiuta therefore appears to be a discrete species from P. munix. A1 juveniles of both P. manis and the Recent Papua New Guinea specie., Ponticor,ythereis laingenesis (Wouters,I981 ) 162-164, PI. 2, figs 1LI2.