Lawrenciella sp.

Pradel, Alan, Maisey, John G., Mapes, Royal H. & Kruta, Isabelle, 2016, First evidence of an intercalar bone in the braincase of “ palaeonisciform ” actinopterygians, with a virtual reconstruction of a new braincase of Lawrenciella Poplin, 1984 from the Carboniferous of Oklahoma, Geodiversitas 38 (4), pp. 489-504 : 491-500

Lawrenciella sp.

EXAMINED MATERIAL. — AMNH FF 20852.

A

CHARACTERS

Lawrenciella sp. shares with Lawrenciella schaefferi the following diagnostic features of the genus and species ( Hamel & Poplin 2008): presence of paired prespiracular fossae; anterior edge of the posterior dorsal fontanelle overhung by the tip of a medial bony prominence; ventral otic fissure separated from the vestibular fontanelle by a bony bridge; posterior myodome unpaired, large and without a ventral fenestra; lateral cranial canals bulging blindly through the loop of the posterior semicircular canals; dorsal aortic canal extending from the occipital face to a level just behind the ventral otic fissure, with a single median bilobate anterior foramen for the lateral dorsal aortae and, at mid length, with a median opening for efferent branchial arteries; notochordal canal ending blindly, without reaching the ventral otic fissure; presence of wide and irregular intracranial cavities lateral to each jugular canal; absence of a cerebellar corpus at the metencephalic region of the braincast; optic nerves leaving the cranial cavity through a single foramen; canal for the lateralis nerve incompletely separated from the trigeminal canal; profundus nerve canal independent from the trigeminofacialis complex; absence of a distinct canal for the superficial ophthalmic branch of the facial nerve in the posterior wall of the orbit; no palatine foramen in the floor of the jugular canal; internal carotids penetrating into the braincase through two separate canals; anterior cerebral veins leaving the endocranial cavity at the level of the diencephalon, the left one always through an independent bony canal toward the left orbital cavity; inner wall of the unpaired posterior myodome with lateral deep grooves for the pituitary veins; parasphenoid with very short anterior ascending processes, and large and long posterior ascending processes reaching the spiracular canals.

Lawrenciella sp. differs from Lawrenciella schaefferi diagnosis in its smaller size (see below); the presence of posterior fossa bridgei with intramural diverticuli; no intracranial ossicles; absence of paired anterior myodomes for nonocular muscles below the olfactory nerve canal; absence of single median anterior myodome for ocular muscles; parasphenoid not extending to the ventral otic fissure posteriorly. However, we prefer not to erect a new species based on such differences, because their systematic significance is uncertain.

DESCRIPTION

General features

The braincase is fully ossified,except for the ethmoid region where the nasal capsules are not preserved. As many other “palaeonisciformes”, the endocranium shows no sutures. The ethmoidal, orbitotemporal and the upper part of the otic regions are separated from the occipital arch and the lower part of the otic region by the otico-occipital fissure, the vestibular fontanelle and the ventral otic fissure. The braincase is tropibasic, with the orbit separated by a narrow interorbital septum and the endocranial cavity mainly situated dorsal to the septum. The skull is about 24 mm long, 16 mm wide at the level of the postorbital process, and 12 mm high at the level of the basipterygoid processes. This new braincase is thus slightly smaller than the previously described Lawrenciella braincases ( Hamel & Poplin 2008).

Ethmoidal region. The ethmoidal region is largely missing anterior to the paired dorsal anterior myodomes ( Fig. 2 [d.ant.my]) for the superior oblique muscles.These depressions are dorsal to the canal for the olfactory nerves ( Fig. 2 [I]) and are separated by the incompletely ossified orbital septum ( Fig. 2 [iorb.s]).

Orbitotemporal and upper part of the otic regions. The orbits are surrounded by a wide supraorbital shelf ( Figs 2 [s.s]; 3[s.s]) dorsally, which is confluent with a massive postorbital process ( Figs 2 [po.p]; 4[po.p]) posteriorly. The supraorbital shelf is pierced by several foramina for the passage of the ramus ophthalmicus superficialis, the ramus ophthalmicus lateralis and vessels ( Figs 3 [f.ram.dors]; 5[c.r.d, f.r.d]). The orbits are separated by the interorbital septum ( Fig. 2 [iorbs.s])

dorsally. The posterior part of the orbit shows a high pars basisphenoidea ( Fig. 4 [bsph]) that contains paired ventral depression for the right and left superior, inferior and internal recti muscles ( Fig. 4 [o.rec.m]). The base of the pars basisphenoidea is flanked by the basipterygoid process ( Figs 2 [b.p]; 4[b.p]; 5[b.p]). The anterior face of the pars basisphenoidea displays two vertical and parallel grooves for the internal carotid arteries ( Fig. 4 [int.car]). The optic arteries ( Fig. 4 [opt.a]) were in the grooves situated above the entrance of the internal carotids and below the optic nerve foramen ( Fig. 4 [II]) that pierces the braincase above the pars basisphenoidea. Flanking the optic foramen dorsolaterally are two small foramina for the ophthalmic artery ( Fig. 4 [oph.a]). Between the optic nerve foramen anteriorly and the trigeminofacialis chamber ( Fig. 4 [tr.f.ch]) posteriorly, lie a series of foramina that are, from anterior to posterior, the oculomotor nerve ( Fig. 4 [III]), the profundus nerve ( Fig. 4 [V 0]), the trigeminal and the lateralis trunk of the facial nerve ( Fig. 4 [V+VII lat]), and the facial nerve ( Fig. 4 [VII]). Dorsal and slightly anterior to the oculomotor foramina is the trochlear nerve exit ( Fig. 4 [IV]). The trigeminofacialis chamber is delimited by the orbital exit of the jugular vein ( Fig. 4 [j.c]), the gasserian and lateralis ganglia recess dorsally ( Fig. 4 [ r.gass .lat.g]) and the geniculate ganglion recess ventrally ( Fig. 4 [ r.gen .g]). A bridge separates the later recess to the deep, unpaired posterior myodome ( Figs 2 [my]; 6[my]) for the left and right external recti muscles. The posterior wall of the myodome is pierced by the foramen for the abducens nerve ( Fig. 4 [VI 2]). A deep groove that housed the pituitary vein ( Figs 4 [pit.v]; 6[pit.v]) lies below and in front of the abducens nerve. The roof of the myodome is fenestrated by a large pituitary fontanelle ( Fig. 6 [pit.fon]) that is confluent with a space for the saccus vasculosus ( Fig. 6 [s.vasc]) and the pituitary ( Fig. 6 [pit]) respectively. These recesses are bordered by the prootic bridge ( Fig. 6 [pr.b]) dorsoposteriorly. The latter is not continuous: an opening is present between the pituitary and the floor of the optic lobe. Behind the basipterygoid process, the lateral edge of the floor of the posterior myodome displays a pair of pronounced notches ( Fig. 5 [VIIpal]). Antero-medial to these lies the ventral opening of the bucco-hypophyseal canal ( Fig. 5 [bhc]) that runs upward and exits dorsally into the anterior part of the myodome.

In front of the anterior level of hyomandibular facet, the roof of the braincase displays an anteroposteriorly elongated anterior dorsal fontanelle ( Fig. 3 [ant.d.fon]) medially. The fontanelle ends anteriorly with a constriction indicating its epiphyseal part ( Fig. 3 [epi]). The anterior dorsal fontanelle is followed by a median ridge ( Fig. 3 [m.r]) anteriorly, and which is flanked by two paired lateral ridges of irregular shape ( Fig. 3 [l.r]). From the posterior border of the fontanelle, two prominent posterior dorsal crests ( Fig. 3 [cr.csp]) diverge until the otico-occipital fissure ( Fig. 3 [ot.oc.f]) and cover the path of the posterior semicircular canal. The otico-occipital fissure is confluent with the posterior dorsal fontanelle ( Fig. 3 [post.d.fon]), the anterior border of which being marked by an intrusive process ( Fig. 3 [l.st.a]). At approximately half length, the posterior dorsal crest is flanked by a large and deep posterior fossa bridgei ( Fig. 3 [p.fb]), which possesses intramural diverticuli ( Fig. 8 ). A ridge covering the anterior semicircular canal diverges anteriorly from the medial edge of the posterior fossa bridgei until the anterior fossa bridgei ( Fig. 3 [a.fb]). The spiracular canal ( Fig. 5 [spic]) pierces the postorbital process vertically and exits dorsally through the anterior fossa and ventrally on the ventral surface of the postorbital process. In front of the anterior fossa and medial to the postorbital process lies another large pit, the prespiracular fossa ( Fig. 3 [p.spi.f]), the floor of which being pierced by a canal, maybe for an ophthalmic branch of the trigeminal or facial nerve ( Figs 5 [p.spi.c]; 8[p. spi.c]). The postorbital process is partly formed by a chondrified lateral commissure ( Fig. 2 [l.c]) that covers the jugular canal ( Figs 4 [j.c]; 7[j.c]) laterally. Posterior to the lateral commissure is an elongated and anteroventrally-directed articular facet for the hyomandibula ( Fig. 2 [f.hm]), below which lies the efferent hyoid artery foramen ( Fig. 5 [e.hy.a]; following Giles et al. 2015c for the interpretation of this foramen). Two openings for the hyomandibular ( Figs 5 [VIIhy]; 7[VIIhy]) and opercular ( Figs 5 [VIIop]; 7[VIIop]) branches of the facial nerve lie directly behind the hyomandibular articulation facet and below the posterior opening of the jugular canal. The hyo-opercular artery ( Fig. 7 [hy.o.a]) exits the jugular canal laterally via a foramen in the right side whereas it leaves only a groove extending outward from the posterior opening of the left jugular canal. The jugular vein probably lay in the jugular depression ( Fig. 2 [j.d]) that joins the posterior opening of the jugular canal to the vagus nerve exit ( Fig. 2 [X]), which forms a wider area of the otico-occipital fissure. Anteroventral to the vagus nerve exit, the lateral wall of the braincase is pierced by the glossopharyngeal nerve ( Fig. 2 [IX]) foramen, which is followed by a groove on the horizontal semicircular canal ridge for its supratemporal branch ( Fig. 2 [IXst]) that might ran in a notch on the lateral edge of the dorsal surface of the braincase ( Fig. 3 [IXst]). Between the glossopharyngeal foramen and the vestibular fontanelle ( Fig. 2 [ v.font ]), which is continuous with the otico-

occipital fissure, is an articular facet for the first suprapharyngobranchial ( Fig. 2 [art.1.s]; 5[art.1.s]). The vestibular fontanelle is separated from the ventral otic fissure ( Fig. 2 [v.ot.f]; 5[v. ot.f]; 6[v.ot.f]) by a bony bridge. Immediately behind the fissure, the aortic canal ( Figs 5 [ao.c]; 6[ao.c]; 7[ao.c]) exits and divides into paired grooves for the lateral aortae ( Fig. 5 [l. ao]). Each lateral aorta gave off the efferent hyoid artery ( Fig. 5 [g.e.hy.a, e.hy.a]) that bends outward and backward in front of the articular facet for the first infrapharyngobranchial ( Fig. 5 [art.1.i]), and the internal carotid artery that ran forward until the level of the bucco-hypophyseal ventral opening where it enters the floor of the braincase ( Fig. 5 [int.car]). Before penetrating the endocranium, the internal carotid artery gave off the efferent pseudobranchial artery ( Fig. 5 [e.ps.a]), which is housed in an anteriorly divergent deep groove, and the palatine artery ( Fig. 5 [pal.a]) that extended in a shallow anterior groove. More posteriorly, a large foramen marks the

B

divergence between the dorsal aorta, which is housed into the aortic canal, and the epibranchial arteries ( Fig. 5 [epi.a]). Anterior to this foramen are paired anteriorly divergent grooves probably for a common root of the epibranchial arteries III and IV ( Fig. 5 [epi III + IV]).

Occipital region. In the posterior view, the occiput possesses three openings that are, from dorsal to ventral, for the foramen magnum ( Fig. 7 [f.m]), the notochordal canal ( Fig. 7 [not]), and the aortic canal ( Fig. 7 [ao.c]) respectively. The floor of the aortic canal is not complete posteriorly behind the foramen for the epibranchial arteries, probably due to preservation artefact. Above the foramen magnum lies a rather low median occipital crest ( Fig. 7 [cr.o]). The dorsolateral edge of the occipital unit displays a pair of dorsolateral processes ( Fig. 7 [l.ex.oc]) and a pair of cranio-spinal processes ( Figs 2 [crs.p]; 5[crs.p]; 7[crs.p]), which bear the intercalar bones ( Figs 2 [int]; 5[int]; 7[int]). Although the posterior wall of the occipital ossification is damaged, it shows two pair of ridges ( Fig. 7 [oims1, oism2]) for the insertion of the first and second inter muscular septa. The foramina for the spino-occipital nerve ( Figs 5 [spio]; 6[spio]; 7[spio]; 8[spio]) lies at the base of the smallest of these two ridges, the medial ones ( Fig. 7 [oism2]). At the base of the lateral ridges ( Fig. 7 [oism1]) is a small foramen, probably for a branch of the occipital artery ( Figs 6 [b.oc.a]; 7[b.oc.a]). At the level of the roof of the aortic canal, are the foramina for the occipital artery ( Figs 5 [oc.a]; 6[oc.a]; 8[oc.a]).

Internally is located a medial ossification, the zygal plates ( Fig. 6 [zy]) formed by two thin and triangular blades covering laterally the anterior end of the notochordal canal. Each blade displays a small foramen for the abducens nerve ( Fig. 6 [VI 1]) at their medial wall.

Intercalar. Posterior and lateral to the vagus foramen, on the lateral edge of the occipital region, at the same level as the floor of the foramen magnum, paired intercalar bones ( Figs 2B [int]; 5B[int]; 7B[int]; 9[int]) are tightly attached to the cranio-spinal processes of the neurocranium. Their internal edges show a concavity posteriorly that surrounds the cranio-spinal process posterolaterally. The intercalar bones then taper and project anteriorly to cover the metotic fissure, but they do not extend farther than the anterior edge int of the fissure and the vagus foramen. Cranio-spinal processes are also present in the Lawrenciella specimen described by Hamel & Poplin (2008), but they do not project anteriorly. The intercalar bones of the specimen studied here are not spongious ( Fig. 9 ), unlike the endochondral bone of the neurocranium. These observations suggest that these anterior projections represent distinct bones that are not endochondral ossifications of the neurocranium, but rather independent bones that are tightly associated with the cranio-spinal processes of the braincase. It is however not possible to say whether these intercalars are membranous bones as in Amia , teleosts, Mesozoic caturids, pholidophorids and leptolepids.

Parasphenoid. The parasphenoid seems to be not entirely preserved, but still appears tightly attached to the ventral face of the endocranium. The CT scan images show numerous small denticles on the ventral surface of the parasphenoid. The latter extends and tapers anteriorly until the level of the mid-orbit, though it seems that its anterior end is missing. The preserved part of the posterior edge is rounded and does not reach the ventral otic fissure posteriorly. The parasphenoid covers the internal carotids and the palatine branches of the facial nerves. The anterior ascending processes ( Fig. 5 [ant.asc.p]) cover the base of the endocranial basipterygoid processes. Their anterior margin of the dorsal surface displays a short groove medially for the passage of the efferent pseudobranchial artery ( Fig. 5 [e.ps.a]). The posterior ascending processes ( Figs 2 [post.asc.p]; 4[post.asc.p]; 5[post.asc.p]) extend dorsally almost as far as the lateral commissure and terminate at the level of the spiracular canals. Two deep spiracular grooves ( Fig. 5 [spi.g]) extend along the posterior ascending processes. On the dorsal surface of the parasphenoid is a small bump located at the level of the opening of the bucco-hypophyseal canal.

Endocast anatomy. The telencephalon ( Fig. 10 [tel]) is small and slightly narrower than the paired olfactory bulbs ( Fig. 10 [olf.b]) in front. The olfactory bulbs extends forward as a long unpaired olfactory canal ( Figs 10 [I]; 11[I]; 12[I]), which is not preserved more anteriorly. A canal for the anterior cerebral ( Figs 10 [ant.c.v]; 11[ant.c.v]; 12[ant.c.v]) vein originates on the roof of the telencephalon and extends anterolateroventrally to reach the left orbit in front of the trochlear nerve foramen.

The dorsal part of the diencephalon ( Fig. 10 [die]) is very short and restricted to the area beneath the epiphyseal part of the anterior dorsal fontanelle ( Fig. 10 [epi]). Its floor comprises from dorsal to ventral respectively: paired canals for the small ophthalmic arteries ( Figs 11 [oph.a]; 12[oph.a]), a single, large optic nerve foramen ( Figs 11 [II]; 12[II]), paired internal carotid canals ( Figs 11 [int.car]; 12[int.car]), a large hypophyseal recess ( Fig. 12 [pit]), which opens in the roof of the posterior myodome via the pituitary fontanelle ( Fig. 11 [pit.fon]) and which extends backward with the saccus vasculus ( Figs 11 [s.vasc]; 12[s.vasc]).

The anterior dorsal fontanelle communicates ventrally with two large optic lobes ( Figs 10 [opt.l]; 12[opt.l]) of the mesencephalic region. The floor of the optic lobes is pierced by the trochlear ( Figs 11 [IV]; 12[IV]) and oculomotor ( Figs 11 [III]; 12[III]) canals, the latter flanking the hypophyseal recess.

The metencephalic region is mainly represented by paired, anteriorly divergent, large cerebellar auricles ( Figs 10 [aur]; 12[aur]). There is an antero-posterior ridge for the longitudinal dorsal vein ( Fig. 10 [l.d.v]) between the cerebellar auricles. The middle cerebral vein ( Figs 10 [md.c.v]; 11[md.c.v]) forms a ridge along the anterodorsal part of the cerebellar auricles that extends downward. Three canals for the profundus ( Figs 11 [V 0]; 12[V 0]), trigeminal and lateralis (V + VII lat) and facial (VII) emerge between the floor of the cerebellar auricles and the utricular recess. The abducens nerve ( Figs 11 [VI 2]; 12[VI 2]) pierced the floor of the metencephalic region between the two saccular chambers ( Fig. 12 [sac]).

Behind the cerebellar auricles is a slight and smooth swelling that might represent the area octavolateralis of the rhombencephalon ( Fig. 10 [a.octl]) rather than a cerebellar corpus because the latter is anterior to the cerebellar auricles in recent sharks and actinopterygians. This area then widens backward until the posterior dorsal fontanelle. A small intramural pocket that represent the lateral cranial canal ( Figs 10 [l.cr.c]; 12[l.cr.c]) projects through the loop of the posterior semicircular canal. In front of the otico-occipital fissure, the lateral cranial canal communicates posteriorly with the myelencephalic cavity ( Fig. 12 [mye]).

Two canals open on each side of the inner wall of the foramen magnum: the spino-occipital nerve canal ( Figs 10 [spio]; 11[spio]; 12[spio]) posteriorly and the canal for a branch of the occipital artery ( Fig. 11 [b.oc.a]). Then the foramen magnum extends anteriorly as the endocranial cavity. An occipital artery canal ( Figs 11 [oc.a]; 12[oc.a]) opens on the wall of the aortic canal, which remains unpaired, but is enlarged anteriorly as it opens into two deep and divergent grooves for the lateral aorta. The notochord canal ( Fig. 12 [not]) tapers anteriorly and does not reach the ventral otic fissure.

The inner ear comprises three semicircular canals: the horizontal ( Figs 10 [h.s.c]; 12[h.s.c]), the posterior ( Figs 10 [post.s.c]; 12[post.s.c]) and the anterior ( Figs 10 [ant.s.c]; 12[ant.s.c]). The two latter semicircular canals meet dorsally at a crus commune ( Fig. 10 [c.c]) that extends downward as a sinus superior ( Fig. 12 [s.su]), which communicates with the saccular chamber ( Fig. 12 [sac]) and the utricular recess ( Figs 7 [utr]; 12[utr]). The horizontal ampullae ( Figs 10 [h.amp]; 12[h.amp]) is situated at the antero-dorsal corner of the saccular chamber, just behind the anterior ampullae ( Fig. 12 [ant.amp]), while the posterior ampullae ( Fig. 12 [post.amp]) is at the postero-dorsal corner of the saccular chamber. No otoliths are preserved.

Figures

FIG. 2. — Surface rendering generated from CT Scan slices of the left lateral surface of AMNH FF 20852: A, braincase (blue); B, braincase (blue) and associated bones (intercalar in pink; parasphenoid in red). Grey area, not perichondrally lined surface. Abbreviations: see Material and methods. Scale bar: 5 mm.

FIG. 3. — Surface rendering generated from CT Scan slices of the dorsal surface of the braincase (blue) of AMNH FF 20852. Grey area, not perichondrally lined surface. Abbreviations: see Material and methods. Scale bar: 5 mm.

FIG. 4. — Surface rendering generated from CT Scan slices of AMNH FF 20852 in anterior view: A, braincase (blue); B, braincase and associated bones (parasphenoid in red). Grey area, not perichondrally lined surface. Abbreviations: see Material and methods. Scale bar: 5 mm.

FIG. 6. — Surface rendering generated from CT Scan slices of the braincase (blue) of AMNH FF 20852 in sagittal view, left side. Black area, braincase section Abbreviations: see Material and methods. Scale bar: 5 mm.

FIG. 5. — Surface rendering generated from CT Scan slices of the ventral surface of AMNH FF 20852: A, braincase (blue); B, braincase and associated bones (intercalar in pink; parasphenoid in red). Grey area, not perichondrally lined surface. Abbreviations: see Material and methods. Scale bar: 5 mm.

FIG. 8. — Surface rendering generated from CT Scan slices of AMNH FF 20852: A, postero-lateral view of the braincase (blue); the transverse microtomographic slices of B and C are indicated in red line. The red colored area of the braincase in B and C represent the intramural diverticuli of the posterior fossa bridgei. Abbreviations: see Material and methods. Scale bars: 5 mm.

FIG. 7. — Surface rendering generated from CT Scan slices of AMNH FF 20852 in posterior view:A, braincase (blue); B, braincase and associated bones (intercalar in pink; parasphenoid in red). Grey area, not perichondrally lined surface. Abbreviations: see Material and methods. Scale bar: 5 mm.

FIG. 9. — Microtomographic slices through the intercalars of AMNH FF 20852: A, cross section; B, saggital section through the left intercalar. Abbreviations: see Material and methods. Scale bar: 5 mm.

FIG. 10. — Surface rendering generated from CT Scan slices of the dorsal surface of the endocranial cavity (pink) of AMNH FF 20852. Grey area, cast section. Abbreviations: see Material and methods. Scale bar: 5 mm.

FIG. 11. — Surface rendering generated from CT Scan slices of the ventral surface of the endocranial cavity (pink) of AMNH FF 20852. Grey area, cast section. Abbreviations: see Material and methods. Scale bar: 5 mm.

FIG. 12. — Surface rendering generated from CT Scan slices of the left lateral surface of the endocranial cavity (pink) of AMNH FF 20852. Grey area, cast section. Abbreviations: see Material and methods. Scale bar: 5 mm.