Sylvienodus nov. gen. is a tiny fish; the estimated standard length (SL) of the lectotype, LNEG–MG 6659, is 57 mm. In spite of this, it does not seem juvenile, as it is well ossified and the dermal bones show remarkable ornamentation of tubercles and grooves. Dorsal and anal fin rays are barely bifurcated, just once or twice, but this also occurs in Pycnodus, Polazzodus, and Tergestinia (Blot, 1987 and Poyato-Ariza, 2010; pers. obs.). This is not considered a juvenile feature for these genera; even the largest specimens of Pycnodus, well above 200 mm, have barely bifurcated dorsal and anal fin rays. Caudal fin rays in all these genera are more bifurcated, especially in larger specimens, although the exact number of bifurcations in Sylvienodus is unknown because all specimens lack the distal part of this fin. Caudal fin rays are bifurcated 4–5 times in the specimen illustrated by Jonet (1964, p. 15–17; pl. 2, fig. 1), comparable to the preserved part of the caudal fin of the lectotype, which is additional evidence that they are adults. Paralectotypes LNEG–MG 4716 and 6658 of Sylvienodus are smaller, less ossified and less ornamented, probably subadult to juvenile, as are most specimens from IST–MDT except 527, 580, and 592.

The overall shape of the body, without the fins, in larger specimens is ovoid, more similar to Polazzodus than to the typical truncated shape of Pycnodus and Tergestinia. Sylvienodus is low for a pycnodont, about 41% of the estimated SL in the lectotype; it is 47–46% in younger specimens, so the body is always less than half as high as it is long. The maximum body height is reached at the posteriormost part of the skull or slightly posterior to it, always well before the point of insertion of the dorsal fin. The head is about 32% of the standard length in the lectotype and very similar in other specimens; head height is about 123% of head length in the lectotype, and 115–130% in other specimens. In other words, the head is higher than it is long (see Table 1 for further measures, proportions, and meristic characters).

Size itself is a feature with little taxonomic value; however, the difference between S. laveirensis and P. apodus is really remarkable. In percentage, the SL ratio of the only remaining syntype of the latter and the lectotype of the former is about 360%. This is a remarkable size difference, certainly not of intrageneric variation in pycnodonts. The SL ratio of the holotype of Polazzodus and the lectotype of Sylvienodus is about 130%; the only other advanced pycnodontid of comparable small size is Tergestinia.

The endocranium, as in all pycnodontins, is posteriorly exposed, the supraoccipital bone largely showing behind the posterior border of parietal and dermopterotic. The exposed portion is large but delicate, never entirely preserved; the postcephalic lacuna present in Pycnodus and Oropycnodus seems absent in Sylvienodus nov. gen.

The skull roof consists of frontal, parietal, small dermosphenotic, and large dermopterotic. The frontal is elongated, as in most pycnodonts, its postorbital part short, only slightly expanded. The parietal is larger than in Pycnodus, where the main body of this bone is comparatively slender. Unlike other pycnodonts with parietal peniculus, in Sylvienodus it is very small, short, with very few branches; only 4 in the lectotype. The posterolateral part of the skull roof is incomplete and/or imperfectly preserved, but a brief description can be provided from observations in the lectotype and LNEG–MG 4716. The dermosphenotic is very small, but present; it is apparently absent in Pycnodus. It forms the posterodorsal corner of the orbit, versus the posteroventral corner in Polazzodus. In turn, the dermopterotic, placed lateral to the parietal, is, as in Polazzodus, remarkably large, unlike the much narrower dermopterotic of Pycnodus (Blot, 1987 and Nursall, 1999; pers. obs.).

All published and observed specimens of Sylvienodus lack the dermocranial fenestra of Pycnodus (“fosse supratemporale” of Blot, 1987, fig. 6; pers. obs.). This is a very distinctive feature among pycnodonts. Its observation in Sylvienodus can be misleading because there are areas of the skull roof heavily ornamented, with deep grooves filled with matrix, and some parts of the bones may be missing, forming small, irregular gaps (e.g., lectotype). This is the reason why an accurate figure of the skull roof is not provided. However, the regular, well-defined dermocranial fenestra anterior to the parietal, posterior to the frontal, and ventral to the dermal supraoccipital (Blot, 1987, fig. 6; Poyato-Ariza and Wenz, 2002, fig. 11B) never appears in Sylvienodus, where the corresponding area is always ossified. When present, this fenestra is large, about half the size of the orbit, so it is conspicuous and very characteristic. This remarkable difference provides strong support for removal of the Portuguese species from Pycnodus. It is also absent in Polazzodus and Tergestinia, but present in Oropycnodus. The dermal supraoccipital bone closes the skull roof posteriorly, as in all pycnodonts; it is long, with a pointed anterior end, and posteriorly large, encasing the transversal extrascapular commissure (which partially pierces the parietal as well); extrascapulars are absent as independent ossifications.

The premaxilla and the dentary, as seen in LNEG–MG 6658, IST–MDT-580, 592 and the specimen described and illustrated by Jonet (1964, p. 15–17, fig. 2; pl. 2, figs. 1–2) have, respectively, 1 and 2 teeth. A single premaxillary tooth is another difference from Pycnodus, which has 2. As in all Pycnodontoidea but Akromystax, the premaxillary and dentary teeth are incisiform; in Sylvienodus, the premaxillary tooth is larger than the dentary teeth, of which the 1st one is much larger than the 2nd one. Polazzodus has only 1 premaxillary tooth as well, but its crown is shorter, larger, and thinner than in Sylvienodus. The premaxillary process of Polazzodus is relatively longer and thinner, and it has an olfactory fenestra that is absent in Sylvienodus. The premaxilla of the new genus is more similar to that of Tergestinia, the only other pycnodont with a single premaxillary tooth. The main difference is that this tooth is comparatively shorter in meso-distal dimension and higher between the insertion and the occlusal border in Tergestinia.

The morphology of the maxilla is another remarkable feature of the new genus (Fig. 2). It has a conspicuous dorsal process, with a distinct and robust articular head for the premaxilla; the posterior and ventral borders of the bone are nearly straight, and the anterior one, very concave (lectotype, LNEG–MG 6658, IST–MDT-580, 583(2), 591). The maximum width is reached in the ventralmost part, which presents a long, pointed anterior projection. This peculiar “ax-shaped” morphology of the maxilla is known in Polazzodus only among pycnodonts; the articular head is shorter and stouter in Sylvienodus than in Polazzodus (Fig. 2A-B). The maxilla is reniform in Pycnodus (Fig. 2C). A unique character of Sylvienodus is the crenulated ornamentation on the anterior border, presented since early ontogenetic stages (e.g., juvenile IST–MDT-591).

Vomerine and prearticular teeth are seldom visible; IST–MDT-580 shows 9 exposed vomerine teeth, all small, rounded to oval in occlusal contour and bearing crenulations where unworn (also IST–MDT-592). These crenulations are strong, placed around the crown. Two vomerine and one prearticular teeth exposed in the juvenile IST–MDT-583(6) confirm such shape and ornamentation.

The coronoid process is large, with a straight, inclined anterodorsal border (IST–MDT-580); it is so high that the mandible is nearly as high as it is long. This shape is quite similar to that of derived pycnodontids such as Polazzodus and Pycnodus.

The opercular bone is present, but greatly reduced (lectotype, IST–MDT-592); it is placed rather high, not reaching the preopercular ventrally (Fig. 3). In shape, extension and position, it is much more like that of Polazzodus and Oropycnodus than that of Pycnodus. The preopercular is about as high as the exposed, superficial part of the dermohyomandibula, and, curiously enough, the two bones are partially fused anteriorly. This partial fusion, first reported in pycnodonts, is observed in juvenile (LNEG–MG 4716) and adult individuals (lectotype; Fig. 3), although there is individual variation; IST–MDT-592 and juvenile IST–MDT-591 have an autogenous preopercular showing a very long, thin anterior ascending process. The relationship between the preopercular and the dermohyomandibular is very peculiar in pycnodonts; the former is progressively smaller from primitive to derived forms, and the latter is progressively more exposed superficially, with an increasing ornamented portion tightly articulated with the preopercular (“dermalization of the hyomandibular body” sensu Nursall, 1996, p. 138–139). In this context, this partial fusion in some specimens of Sylvienodus is unique, but not very surprising.

There are 23 vertebrae in the lectotype, counted, as in other pycnodonts, as neural spines not forming part of the caudal skeleton. This includes 12 abdominal and 11 caudal vertebrae (Table 1). This number is lower than in Pycnodus, Polazzodus and Tergestinia (mostly 29). Paralectotype LNEG–MG 4716 has 23, although not as clearly countable as the lectotype. Other specimens (LNEG–MG 6658 and IST–MDT-4716) have 23. There are 7 anterior autogenous neural spines (IST–MDT-617; the 7th spine is arranged very tightly to the corresponding arch, with a suture between them). The number of caudal centra, excluding caudal endoskeleton, is 11–12.

Neural and haemal spines have anterior sagittal flanges, as in most pycnodontids. The arcocentra surround the notochord, closing the notochordal canal in lateral view, as in some other pycnodonts. Caudal arcocentra surround the notochord completely since earlier ontogenetic stages, but abdominal vertebrae leave a little aperture for the notochordal canal in lateral view in younger specimens (e.g., LNEG–MG-4716). The contact of each arcocentrum with the adjacent ones is simple, not interdigitated, by means of a single projection; quite different from the complex, interdigitated contact of Pycnodus. This dissimilarity is very significant for pycnodonts.

Paired fins are rarely and partially preserved. There are at least 20 pectoral fin rays in the lectotype, evidence of a much larger pectoral than pelvic fin. IST–MDT-580 shows 22 right and 20 left long, robust pectoral radials, a confirmation of the ample width of the fin. IST–MDT-583(1) and 583(7) show remains of a tiny pelvic fin, with very few lepidotrichia; it is inserted before the first scale row anterior to the cloaca.

Another unique character of Sylvienodus nov. gen. is the shape of the dorsal and anal fins (Fig. 1 and Fig. 4). The lectotype, the only observed specimen where the very delicate fin rays are entirely preserved, shows extremely acuminate fins. The dorsal one has 3 short precurrent fin rays, unsegmented and unbranched. Then, there are 37 segmented fin rays; the first 6–7 of them are much longer than the rest. These fin rays are so long that they make the dorsal fin higher than it is long; these rays are about 8 times longer than the shortest fin rays, which form most of the fin (e.g., the 4th segmented fin ray is about 7,8 times longer than the 17th). The anal fin is very similar, with only 1 precurrent, unsegmented fin ray and 33 segmented fin rays, the longest of which are the first 4–5; they are not as long as the dorsal ones, so that the fin is a little lower than it is long. This dorsal and anal fin morphology is an autapomorphy of Sylvienodus among pycnodonts; compare Fig. 4 herein with fig. 34 in Poyato-Ariza and Wenz (2002). The most similar is the acuminate shape, in, for instance, Gyrodus or Proscinetes (e.g., op. cit., figs. 5, 34B), but the acumination in Sylvienodus is extreme, unique in pycndonts. In Pycnodus, the dorsal and anal fins are slightly falcate, with the anteriormost segmented fin rays only about twice longer than the rest of the series. Yet another remarkable difference: Sylvienodus has fewer axonosts than Pycnodus; about 37 dorsal and 33 anal in the former, and about 58 and 45 in the latter.

The caudal fin of the lectotype lacks the distal part. This fin is entirely preserved only in the specimen illustrated by Jonet (1964, p. 15–17; pl. 2, fig. 1); its distal contour shows that it is double emarginated, very much as in Pycnodus and Oropycnodus, unlike the forked caudal fin of Polazzodus or the straight distal border of Tergestinia. Fringing fulcra are absent; there are 5 dorsal and 7–8 ventral precurrent fin rays in the lectotype; respectively, 4 and 4 in juvenile LNEG–MG 4716. Adults of Pycnodus have 9–11 precurrent rays on each lobe, yet another difference. In Sylvienodus, there is 1 longest, segmented, unbranched principal caudal ray on each lobe plus 6 segmented, branched rays in the upper one, and 9–10 in the lower one, a total of only 17–18 principal caudal fin rays (about 21 in Pycnodus and Polazzodus). One large, thin, irregular urodermal with several projections is seen on the lectotype and LNEG–MG-4716. The former also shows a dorsal caudal scute (corresponding ventral part missing).

The caudal endoskeleton (Fig. 5) is close in some features, but dissimilar in others, to that of Pycnodus and similar genera. There are 5 epichordal elements (e.g., lectotype), versus 6 in Pycnodus, Oropycnodus, and Polazzodus. There are only 9 hypochordal elements (e.g., lectotype, IST–MDT-583(5), 617), unlike 10–11 in the mentioned genera. The most developed hypochordal elements are the 8th and 9th, as in Pycnodus and the others; however, these hypochordal elements are only enlarged in Sylvienodus, not hypertrophied as in Pycnodus and other genera, that is, extremely wide, with a central, additional ridge (compare Fig. 5 herein with, for instance, figs. 27, 28 and Plates 14, 15 in Blot, 1987 and figs. 26 and 29 in Poyato-Ariza and Wenz, 2002).

Sylvienodus nov. gen. has a clathrate squamation pattern, like most derived pycnodonts (Nursall, 1996). The lectotype and LNEG–MG-6658 show 6 complete flank scale rows plus at least 2 incomplete rows. Each complete flank row is composed of about 6 thin, long scales.

Contour scales are never well preserved, but their main features can be outlined from different specimens (e.g., lectotype, paralectotype IST–MDT-527). There are 12 dorsal ridge scales. Their morphology is mostly rather simple: thin and elongated, with a crenulated dorsal border. They are different from the elaborated scutellum-like dorsal ridge scales of Pycnodus. The 1st dorsal ridge scale is incorporated into the skull roof, as in all pycnodonts. The 2nd dorsal ridge scale has a long, stout anteroventral projection; the dorsal part resembles the scutellum-like scales of Pycnodus more than the rest of the series. The anterior and posterior borders of this scale are rather curved, the anterior one forming an oval notch with the posterior border of the first scale (MGB 6658, IST–MDT-527, 583(1); in the lectotype, the notch is deformed due to displacement of the 2nd dorsal ridge scale). Such a notch (Fig. 6A) is apparently unique among pycnodonts.

There are 15 ventral keel scales, 2 post-cloacal. IST–MDT-580 shows 5 under the cleithrum, 8 between cleithrum and cloaca, and 2 after the cloaca. Unfortunately, the specimen is broken, so their distal border is missing. In general, these scales are very simple, although larger than the dorsal ridge scales. The anteriormost ones seem to bear a single spine on the center of the distal border, whereas the rest of the series bear spines in increasing number, from 2 short to 3–4 long, curved spines, placed on the posterior part of the distal border, of increasing size and in close contact with each other (e.g., IST–MDT-583(2), 583(7), lectotype). In sum, the ventral keel scales are very much like those of many pycnodonts, such as Ocloedus, Turbomesodon and Polazzodus, but a bit smaller. They are not like the scutellum-like ventral keel scales of Abdobalistum; the ventral keel scales of Pycnodus are even simpler, with no more than 3 small spines.

The cloaca (Fig. 6B) is seldom preserved. Specimen IST–MDT-580 shows a cloaca with a bifid scale, a derived character state within Pycnodontidae (Poyato-Ariza and Wenz, 2002). In addition to the bifid scale, Syvienodus has 1 long, robust anterior and, differently from other genera, 2 short and thin posterior cloacal scales, all of them long and straight (Fig. 6B).