A late Permian ichthyofauna from the Zechstein Basin, Lithuania-Latvia Region

The late Permian is a transformative time, which ended in one of the most significant extinction events in Earth’s history. Fish assemblages are a major component of marine foods webs. The macroevolution and biogeographic patterns of late Permian fish are currently insufficiently known. In this contribution, the late Permian fish fauna from Kūmas quarry (southern Latvia) is described for the first time. As a result, the studied late Permian Latvian assemblage consisted of isolated chondrichthyan teeth of Helodus sp., ?Acrodus sp., ?Omanoselache sp. and euselachian type dermal denticles as well as many osteichthyan scales of the Haplolepidae and Elonichthydae; numerous teeth of Palaeoniscus, rare teeth findings of ?Platysomus sp. and many indeterminate microremains. This ichthyofaunal assemblage is very similar to the contemporaneous Lopingian complex of the carbonate formation from the Karpėnai quarry (northern Lithuania), despite the fact that Kūmas samples include higher diversity and abundance in fossil remains. The differences in abundance of microremains could possibly be explained by a fresh water influx in the northeastern Zechstein Basin margin, which probably reduced the salinity of the sea water. The new data enable a better understanding of the poorly known late Permian fish diversity from the Lithuania-Latvia Region.


Introduction 34
The late Permian is currently one of the most studied time periods in the whole 35 Phanerozoic, since it is evidenced by two phases of the end-Permian mass extinction, 36 which significantly affected long term macroevolution of biota [1]. Despite an overall low 37 level of detailed taxonomic palaeoichthyofaunal analysis there is an increased interest 38 on the late Permian fossil fish assemblages, which is mostly focused on the isolated 39 chondrichthyan and osteichthyan remains (dermal denticles, scales, fins, vertebral   In the studied area, we found many unidentified vertebrate fossils (e.g. fragment 123 of a conodont element, isolated fish vertebral centra, otoliths and tooth plate), 124 invertebrate and plant fossils (e.g. spores and pollens), which need additional detailed 125 taxonomic analysis. norther Lithuania [25]. The geographical distance between these locations is ~70 km.

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The total weight of the analysed samples from Karpėnai quarry reached ~214 kg and 133 from Kūmas quarry -~220 kg. These samples yielded ~2,400 isolated fish microremains 134 from both quarries (Table 1).

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The samples were chemically prepared using the optimal acetate 10 per cent 136 buffered acetic acid technique for extracting phosphatic fossils [32]         single blunt, small peg located in the labial margin (Fig 3C 2 ). The neck is massive, 230 broad, with some foramina canal openings filled by 'rubbish' (Fig 3C 3 ). The basal plate 231 is ellipse shape, reaches 0.7 mm in length and 0.3 mm in width. are similar -about 0.5 -0.6 mm -but direction of the growth is opposite (Fig 3E). The 261 crown is slightly narrower than the neck -0.2 mm. The pedicle base is sometimes 262 absent ( Fig 3F) or, in the case that is present here, has some foramina of vascular  Very rare dermal denticles looks as 'tripod boomerang', with three main lateral cusps, 290 two of which separated by two grooves more (Fig 3 N). The crown sits strongly

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The longitudinal and transversally sectioned dermal denticles of euselachian

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The inner structure of euselachian type dermal denticle based on 3D-data is Rare type of crown surface in studied quarry ( Fig 3Q 1 -Q 2 ). Mirror reflect 'twins' crown 351 has many convex lateral short ridges and two long mesial ridges which reached the 352 posterior margin (Fig 3Q 1 ). The crown sits vertically on its base, creating ~90° between 353 these axes, thereby missing a neck considered to be an important morphological 354 feature of this type. The subcrown is smooth, without any ornament (Fig 3Q 2 ) The poorly-preserved isolated tooth crown fragment is smooth, massive, has convex crystallites randomly arranged (Fig 4C 4 ). The enameloid crystallites are round or 387 subround (Fig 4C 4 ), and the EDJ is clear. Penetrating the enameloid layer there are 388 canals issuing from the dentine below (Fig 4C 2 ) nearly reaching the surface of the tooth 389 (Fig 4C 3 ). This organization is similar to the one that was found in the tooth plate of a 19 390 Carboniferous Helodus from England, although the enamloid layer in our tooth is thicker 391 [53].

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The 3D model of Helodus sp. tooth (Fig 6A 1 , A 4 , A 7 ) showing a complete vascular 393 system (Fig 6A 2 -A 3 ,A 5 -A 6 ,A 8 ) which consists of some major types of pore cavities 394 (white arrows Fig 6A 9 ). Most of the cavities are single and isolated from other cavities.

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Another type is paired, with a small shared basal part close to the junction between the 396 pore cavities and canal. A third type shares a larger basal portion and the two cavities  The most common osteichthyan fossils in the Kūmas quarry are represented by 419 their isolated scales and teeth (Fig 7-8). Rare in these assemblages are poorly 420 preserved bony fish vertebral centra, (Fig 7N) and possible otoliths (not pictured here) 421 and a single unidentified actinopterygian tooth plate (Fig 7O).   Scales of this type are located on the ventral flank sector [59] (Fig 7 F-G). It is generally 478 elongated ( Fig 7F); some scales have a square shaped (Fig 7G), thick, with slightly twisted These scales belong to the pectoral peduncle sector (Fig 7H-I). They are small, elongated, This type of scales belonged to undetermined fish body sector, because material is not 504 completely preserved (Fig 7L-M). Such samples composed out of the straight dorsal, 505 ventral and anterior margins while entire posterior margin is pectinated, has several 506 narrow serrations. The crown field is covered by ganoine tissue with tiny, roundish 507 microtubercles pattern. Some scales are smooth, showing flask-shaped pore cavities 508 visible through translucent ganoine layer (Fig 7M). Sometimes, the anterior margin is thick, 509 slightly massive with significant, wide foramen of vascular system (white arrow Fig 7L).

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The SEM analysis of the teeth (Fig 9A 1 , B 1 ) reveals an enameloid layer 564 surrendering a dentine core with a central pulp cavity (Fig 9A 2 ). The enameloid extends 565 only over the apical part of the tooth, attaining its maximum thickness at the base of the 566 cap where it reaches 70 µm (Fig 9A 3 ). Lingually and labially, the enameloid layer is 567 reduced to 5 µm (Fig 9A 4 -A 5 ). The hypermineralised layer is composed of single the layer is thicker, the enameloid crystallites are organised in loose bundles (Fig 9A 3 ) 573 that display a tendency to be oriented parallel to the tooth surface (Fig 9B 2 ). The teeth 574 do not show any kind of superficial layer, which may have been removed in the process 575 of tooth wear. The enameloid-dentine junction (EDJ) is clearly distinguishable and 576 regular.   The teeth have a rod-shaped, short cylinder outline with flat apex (note: tooth cap was 605 probably lost?) (Fig 8N-O 1 ). Some teeth have many vertical, convex ridges which 606 covered an entire lateral margin and with visible enameloid, dentine tissues inner 607 structure in the cross section in apical view (Fig 8N). While other teeth have only well-608 developed vertically elongate, narrow microtubercles in lateral view with smooth rubbed 609 apex (Fig 8O 2 ). The tooth reaches a maximum of 0.5 mm in diameter and 0.2 mm in 610 height. The results presented in this paper reveal a late Permian fish assemblage from