As a clergyman, he was later appointed Vicar Apostolic of Nordic Missions and Titular Bishop of Titopolis by Pope Innocent XI.

[16] Niels Steensen was born in Copenhagen on New Year's Day 1638 (Julian calendar), the son of a Lutheran goldsmith who worked regularly for King Christian IV of Denmark.

[18] After completing his university education, Steensen set out to travel through Europe; in fact, he would be on the move for the rest of his life.

At the urging of Thomas Bartholin, Steensen first travelled to Rostock, then to Amsterdam, where he studied anatomy under and lodged with Gerard Blasius, focusing on the lymphatic system.

Invited to Paris by Henri Louis Habert de Montmor and Pierre Bourdelot, he there met Ole Borch and Melchisédech Thévenot who were interested in new research and in demonstrations of his skills.

After travelling through France, he settled in Italy in 1666 – at first as professor of anatomy at the University of Padua and then in Florence as in-house physician of Grand Duke of Tuscany Ferdinando II de' Medici, who supported arts and science and whom Steensen had met in Pisa.

[23] During his stay in Amsterdam, Steensen discovered a previously undescribed structure, the "ductus Stenonis" (the duct of the parotid salivary gland) in sheep, dog and rabbit heads.

A dispute with Blasius over credit for the discovery arose, but Steensen's name remained associated with this structure known today as the Stensen's duct.

[citation needed] In October 1666, two fishermen caught a huge female shark near the town of Livorno, and Ferdinando II de' Medici, Grand Duke of Tuscany, ordered its head to be sent to Steensen.

He noted that the shark's teeth bore a striking resemblance to certain stony objects, found embedded within rock formations, that his learned contemporaries were calling glossopetrae or "tongue stones".

Steensen's contemporary Athanasius Kircher, for example, attributed fossils to a "lapidifying virtue diffused through the whole body of the geocosm", considered an inherent characteristic of the earth – an Aristotelian approach.

Fabio Colonna, however, had already shown by burning the material to show that glossopetrae were organic matter (limestone) rather than soil minerals,[30] in his treatise De glossopetris dissertatio published in 1616.

Steensen's ideas still form the basis of stratigraphy and were key in the development of James Hutton's theory of infinitely repeating cycles of seabed deposition, uplifting, erosion, and submersion.

[37] Steensen gave the first accurate observations on a type of crystal in his 1669 book De solido intra solidum naturaliter contento (the Dissertationis prodromus).

Steensen's seminal work paved the way for the law of rational indices of French mineralogist René-Just Haüy in 1801.

After making comparative theological studies, including reading the Church Fathers and by using his natural observational skills, he decided that Catholicism, rather than Lutheranism, provided more sustenance for his constant inquisitiveness.

[33] Upon request of Duke Johann Friedrich of Hanover, Pope Innocent XI made him Vicar Apostolic for the Nordic Missions on 21 August 1677.

[45] There he had talks with Gottfried Leibniz, the librarian; the two argued about Spinoza and his letter to Albert Burgh, then Steensen's pupil.

After John Frederick death's, Prince-Bishop of Paderborn Ferdinand of Fürstenberg appointed him as Auxiliary Bishop of Münster (Church Saint Liudger) on 7 October 1680.

Earlier, Augustus' wife, Sophia of Hanover, had made fun of Steensen's piousness; he had sold his bishop's ring and cross to help the needy.