Aquinnah landowners who have been battling for nearly 18 years to develop some 30 acres of land off Moshup Trail won a favorable decision this week from the state Court of Appeals, but the convoluted legal saga continues.

The latest ruling in a complex set of cases originally brought in 1997 by Bear Realty Trust, its principals, James M. Decoulos and Maria A. Kitras, and others relates to their effort to establish easements to landlocked parcels. If a property does not have legal access to a public road, it cannot be developed.

The Hon. Janis M. Berry, writing the majority opinion for a three-judge panel of the appellate court, ruled that a long tradition of common access to the land when it was owned by the Wampanoag tribe of Gay Head (Aquinnah) in effect created an implied easement that remains with the property to this day. The appeals court panel voted 2-1 to send the case back to land court to figure out what those easements are.

But in a dissent almost twice as long as the opinion, the Hon. Peter W. Agnes Jr. said his fellow judge’s reasoning was “an extraordinary alteration of traditional principles of Massachusetts law,” and suggested the Supreme Judicial Court should review it. An appeal to that body appears inevitable.

The case is one of several related cases that will ultimately decide the fate of a large swath of salt-blasted coastal heathland in a part of Aquinnah known as Zack’s Cliffs. In addition to the town of Aquinnah, other defendants in the long-running litigation include the Vineyard Conservation Society, the Martha’s Vineyard Land Bank, the state Department of Environmental Protection, and Caroline Kennedy and Edwin Schlossberg, who own property nearby. Issues relating to zoning are being litigated in other pending cases.

This case, which has bounced back and forth between the land court and the appeals court, draws on the land’s complicated tribal history and hangs on an arcane legal doctrine called easement by necessity.

Up until the late 19th century, much of the land in what is now Aquinnah was held in common by the Wampanoag tribe. In the 1860s and 1870s, the Massachusetts legislature took several actions designed to provide tribal members with the same rights as other citizens, including the incorporation of the town of Gay Head. A process called partition was undertaken in 1871 and 1878 in which commissioners appointed by a probate court judge split up the land into set-off lots and deeded them to tribal members.

In her opinion, Justice Berry found that tribal members had a long custom of sharing access to the land that was unchanged even after it was divided. Since the historical record shows no specific intention to negate that common access, the continued easement is implied, the judge ruled.

“We conclude that the ancient origins of that common access — dating back before the late eighteenth century — establish the equivalent of a chain of title, with access rights that would not yield landlocked parcels,” the judge wrote in a 14-page opinion released Wednesday. The Hon. R. Marc Kantrowicz was the concurring judge.

The decision reversed a land court ruling in August 2010 that came to the opposite conclusion. And in his dissent, Justice Agnes also took a different view, saying that under Massachusetts law, the landowners needed to prove there was an affirmative intent to grant access easements when the property changed hands.

“It may be that a presumption should exist that when land previously held in common by members of a Native American tribe is partitioned pursuant to an act of the Legislature, preexisting tribal rights are perpetuated and become binding on the successor grantees in perpetuity,” he wrote. “However, to date there is no such presumption under the law.”