Description of Conticribra tricircularis , a new genus and species of Thalassiosirales, with a discussion on its relationship to other continuous cribra species of Thalassiosira Cleve (Bacillariophyta) and its freshwater origin

Figures & data

Figs 1–6. Conticribra tricircularis sp. nov., SEM, external view of valve. Specimens from the Pliocene sediment from Trout Creek (Oregon, USA). Figs 1, 2. Whole valve surface with non-plicated valve face; valve surface with radiate areolation pattern. Note external openings of marginal fultoportulae forming three regular rings (arrow). Figs 3, 4. Valve surface with loculate areolae arranged in radiate rows, and external openings of marginal fultoportulae usually forming three rings (arrow). Figs 5, 6. Partly dissolved valves, with details of external openings of marginal fultoportulae (arrow) and network of ribs. Scale bars: 5 µm (Figs 1, 2): 2 µm (Figs 3, 4); 1 µm (Figs 5, 6).

Table 1.  Fossil record of extinct species in Conticribra and Thalassiosira s.l. with continuous cribra.

Species with continuous cribra	Age range	Locality	Habitat	Source
Non-plicated
T. inlandica	Pleistocene	Himalayan (Nepal)	Freshwater	Hayashi et al. (2007)
C. tricircularis	Pliocene	Oregon, USA	Freshwater	This paper
C. nevadica	Miocene (c. 11–12 Ma)	Nevada, USA	Freshwater	Khursevich & VanLandingham (1993)
Thalassiosira sp.	Neogene	the Balkan Peninsula	Freshwater	Ognjanova-Rumenova (unpublished)
T. manifesta	Miocene	N. Sakhalin, Russia	Marine	Makarova (1988)
Plicated
T. kamczatica	Late Miocene	Western Kamchatka (the Ermanovskaya suite), Russia	Freshwater	Lupikina & Khursevich (1992)
T. kilarskii	Miocene	The Harper Basin, Oregon, USA	Freshwater	Kaczmarska (1985); Kociolek & Khursevich (2001)
Thalassiosira sp. A	Miocene (c. 17 Ma)	Oidawara formation, Japan	Marine	Julius & Tanimura (2001)

Figs 7–14. Conticribra tricircularis sp. nov., SEM, internal view of valve. Specimens from the Pliocene sediment from Trout Creek (Oregon, USA). Figs 7, 8. Internal view of valve surface, showing non-plicated valve face, semi-continuous cribra and three regular rings of marginal fultoportulae. Note absence of valve face fultoportula(e). Figs 9–11. Processes (arrow R: rimoportula; arrow: marginal fultoportulae), semi-continuous cribra on valve face and continuous cribra on valve mantle. Fig. 9. Raised rimoportula replacing marginal fultoportulae, externally opened by elongated, spine-like tube, internally oriented along valve radius. Figs 10, 11. Details of marginal fultoportulae: slightly elongated central tube possessing satellite pore cover (arrow C) and four cowlings. Note hyaline area in process zone. Figs 12–14. Partly dissolved valves with processes (arrow R: rimoportula; arrow: marginal fultoportulae), semi-continuous cribra on valve face and continuous cribra on valve mantle. Scale bars: 5 µm (Figs 7, 8); 1 µm (Figs 9, 11, 12, 14); 0.5 µm (Figs 10, 13).

Fig. 15. Diagrams to show phylogenetic relationships and hypothesis of the freshwater origin of Conticribra. (a) Modified from Alverson et al. (2007, fig. 5) to include only species relevant to this paper; (b) phylogenetic relationships from a with addition of extinct species of Conticribra and Thalassiosira kilarskii (based on morphology). Abbreviations: mf: marginal fultoportulae; R: rimoportula; vf: valve face fultoportulae; +: extinct species. (c) Phylogenetic relationships from a with addition of extinct species of Conticribra and Thalassiosira kilarskii (based on morphology) showing freshwater origin of the genus and recent brackish water colonizations.

Table 2.  Comparison of morphological characters of species in Conticribra and two Thalassiosira species with continuous cribra and non-plicated valve face.

Morphological Character	C. tricircularis ^a	C. nevadica ^b	C. guillardii ^c,g,h	C. weissflogii ^d,g,h	T. inlandica ^e	T. pseudonana ^f–h
Cribra	(Semi) continuous	Semi-continuous	Continuous	Continuous	(Semi) continuous	Continuous
Marginal fultoportulae	• 3 rings	• 1 ring on v f/m junction	• 1 ring on v f/m junction	• Irregular?1 ring on mantle	• 1 ring on mantle	• 1 ring
	• 4 satellite pores	• 2(or4?) satellite pores	• 4 satellite pores	• 3(or 4?) satellite pores	• 3 satellite pores	• 3 satellite pores
	• Long external tubes	• Long external tubes	• Long external tubes	• Long external tubes	• Pore on conical thickenings	• External tube
Valve face fultoportulae	Absence	Absence	Absence-3, 2–4 satellite pores	Several, often make a circle, 4? satellite pores	Absence	Absence-2?, 2–3 satellite pores
Rimoportula
Number	1	1	1	1	Multiple	1
Location	On the mantle, replace mf	On the mantle replace mf?	On the mantle, replace mf	On the mantle, replace mf?	On the mantle, near mf	On the mantle, midway between mf
External opening	External tube, spine-like	External tube	External tube	External tube	Pore	External tube?
Abbreviations: mf: mantle fultoportula(e); v f/m: valve face/mantle.
References: ^athis paper; ^bKhursevich & Vanlandingham (1993); ^cHasle (1978); ^dFryxell & Hasle (1977); ^eHayashi et al. (2007); ^fHasle (1976); ^gMakarova (1988); ^hGlezer et al. (1988).

Hayashi , T , Tanimura , Y and Sakai , H . 2007 . A fossil freshwater Thalassiosira, T. inlandica sp. nov. (Bacillariophyta), with semicontinuous cribra and elongated marginal fultoportulae . Phycologia , 46 : 353 – 362 .

 Web of Science ®Google Scholar

Khursevich , GK and VanLandingham , S . 1993 . Frustular morphology of some diatom species from Miocene freshwater sedimentary rocks of western U.S.A. and Canada . Nova Hedwig. , 56 : 389 – 400 .

 Web of Science ®Google Scholar

Makarova, I.V. (1988). Diatomovie vodorosli morei SSSR: rod Thalassiosira Cl. USSR. Acad. Sci. “Nauka”, Leningrad. (In Russian)

 Google Scholar

Lupikina , EG and Khursevich , GK . 1992 . A new freshwater species of Thalassiosira (Bacillariophyta) from Miocene deposits of Kamchatka . Paleontologichesky Zhurnal , 1 : 136 – 138 .

 Google Scholar

Kaczmarska , I . 1985 . The diatom flora of Miocene lacustrine diatomites from the Harper Basin, Oregon, USA . Acta Paleobotanica , 25 : 33 – 100 .

 Google Scholar

Kociolek , JP and Khursevich , GK . 2001 . Valve ultrastructure of new and rare fossil freshwater species of Thalassiosira Cl. (Bacillariophyta) from China and USA . Int. J. Algae , 3 : 86 – 98 .

 Google Scholar

Julius , ML and Tanimura , Y . 2001 . Cladistic analysis of plicated Thalassiosira. (Bacillariophyceae) . Phycologia , 40 : 111 – 122 .

 Web of Science ®Google Scholar

Alverson , AJ , Jansen , RK and Theriot , EC . 2007 . Bridging the Rubicon: phylogenetic analysis reveals repeated colonizations of marine and fresh waters by thalassiosiroid diatoms . Mol. Phylogen. Evol. , 45 : 193 – 210 .

 PubMed Web of Science ®Google Scholar

Hasle , GR . 1978 . Some freshwater and brackish water species of the diatom genus Thalassiosira Cleve . Phycologia , 17 : 263 – 292 .

 Web of Science ®Google Scholar

Fryxell , GA and Hasle , GR . 1977 . The genus Thalassiosira: Some species with a modified ring of central strutted processes . Nova Hedwig., Beih. , 54 : 67 – 98 .

 Google Scholar

Hasle , GR . 1976 . Examination of diatom type material: Nitzschia delicatissima Cleve, Thalassiosira minuscula Krasske, and Cyclotella nana Hustedt . Br. Phycol. J. , 11 : 101 – 110 .

 Google Scholar

Glezer, Z.I., Makarova, I.V., Moisseeva, A.I., & Nikolaev, V.A. (1988). The diatoms of the USSR. Fossil and recent, 2(I): Pyxidiculaceae, Thalassiosiropsidaceae, Triceratiaceae, Thalassiosiracea. “Nauka”, Leningrad. (In Russian)

 Google Scholar